Thyroid nodule workup involves ultrasound assessment of nodule features followed by fine needle aspiration cytology classified using the Bethesda system (categories I–VI) to stratify malignancy risk and guide management.

Exam domain	One-glance essentials
Definition / diagnosis	A thyroid nodule is a discrete lesion radiologically distinct from surrounding thyroid parenchyma ^[1] Workup answers one key question: benign or malignant ^[1] Most nodules are benign; malignancy is found in approximately 10–15% ^[1]
Epidemiology / risks	Prior head-and-neck irradiation, especially childhood exposure, raises papillary carcinoma risk ^[1] Family history of thyroid cancer or MEN2/RET mutation suggests inherited risk ^[1] Age extremes, iodine deficiency, Hashimoto thyroiditis and long-standing MNG increase concern ^[1]
Core mechanism	TSH drives thyroid growth; a suppressed TSH suggests autonomous hormone production ^[1] Hot nodules are rarely malignant because differentiated tissue retains iodine-trapping function ^[1] FNAC samples cells, not capsular architecture, so invasion cannot be assessed ^[1]
Clinical picture	Assess a solitary or dominant nodule for firmness, fixation, rapid growth, cervical nodes and compressive symptoms ^[1] Hoarseness suggests recurrent laryngeal nerve involvement; dysphagia or stridor suggests local compression or invasion ^[1] Rapid enlargement in an older patient is a red flag for anaplastic carcinoma or lymphoma ^[1]
Investigations	*Check TSH first: low TSH → scintigraphy; normal/high TSH → thyroid and neck USS* ^[1] USS assesses composition, echogenicity, margins, shape, calcification, extrathyroidal extension and cervical nodes ^[1] Suspicious features include solid hypoechogenicity, microcalcifications, irregular margins and taller-than-wide shape ^[1] US-guided FNAC is performed when the nodule meets the ATA pattern and size threshold ^[1]
Management	Bethesda I: repeat US-guided FNAC; Bethesda II: observation with clinical and ultrasound follow-up ^[1] Bethesda III: repeat FNAC or molecular testing; persistent atypia may require diagnostic surgery ^[1] Bethesda IV: hemithyroidectomy because adenoma and carcinoma require capsular or vascular invasion assessment ^[1] Nodules > 4 cm, gross invasion or confirmed nodal metastasis may proceed directly to surgery ^[1]
Complications	FNAC may cause pain, bleeding, a non-diagnostic sample or false-negative cytology from sampling error ^[1] After thyroidectomy, neck haematoma can rapidly compromise the airway and requires immediate wound release ^[1] Recurrent laryngeal nerve injury causes hoarseness; bilateral injury causes stridor and airway obstruction ^[1]
Prevention / follow-up	Avoid unnecessary FNAC by following the TSH → scintigraphy or USS → risk-stratified FNAC pathway ^[1] After benign cytology, repeat assessment if ultrasound shows > 20% diameter growth or > 50% volume growth ^[1] Before surgery, document vocal-cord function and correct thyrotoxicosis to reduce airway and thyroid-storm risk ^[1]
Exam traps	Do not order scintigraphy when TSH is normal or high; it is useful when TSH is suppressed ^[1] A multinodular goitre does not exclude cancer: assess each nodule and biopsy the suspicious one ^[1] FNAC cannot distinguish follicular adenoma from carcinoma; invasion requires histology ^[1] Purely cystic nodules are usually benign, but target any suspicious solid component ^[1]

References

^[1] Lecture slides: GC 177. A thyroid nodule benign thyroid nodules; thyroid cancer.pdf (p5, p10, p12)

Thyroid Nodule Workup (USS + Bethesda FNAC)

A thyroid nodule is a discrete lesion within the thyroid gland that is radiologically distinct from the surrounding thyroid parenchyma ^[1]^[2]. The word "nodule" comes from Latin nodulus = "small knot." In practical terms, it is a focal area of thyroid tissue that behaves differently (structurally and sometimes functionally) from the rest of the gland.

The "workup" of a thyroid nodule refers to the systematic clinical, biochemical, ultrasonographic, and cytological evaluation aimed at answering one critical question: Is this nodule malignant or benign?

Why does this matter?

Thyroid nodules are extraordinarily common — detectable in 3–7% of people by palpation, but > 30% if you do ultrasound or autopsy studies ^[2]^[3]. The vast majority (> 85–90%) are benign. The entire workup exists to identify the ~5–15% that harbour malignancy while avoiding unnecessary surgery for the rest.

2. Epidemiology

Method of detection	Prevalence
Palpation	3–7% ^[2]^[3]
USG / autopsy	> 30% ^[2]^[3]

Prevalence increases with age, female sex, iodine deficiency, and prior ionizing radiation exposure ^[2]^[3].
Incidental thyroid nodules ("thyroid incidentalomas") are increasingly discovered on imaging done for other reasons — CT, MRI, PET, carotid Doppler — and now account for the majority of newly discovered nodules ^[3].

Female >> Male (~4:1 for nodule prevalence) ^[2]^[4].
However, a nodule in a male is more likely to be malignant — thyroid nodules are less common in males but carry a higher per-nodule malignancy risk ^[3]^[5].
Age < 14 years or > 70 years carries higher malignancy risk; nodules in the 3rd–6th decade are usually benign ^[3]^[5].

Hong Kong is generally iodine-sufficient (historically borderline); iodine deficiency shifts the histological spectrum toward follicular carcinoma and multinodular goitre (MNG).
Nasopharyngeal carcinoma (NPC) — extremely common in southern China/HK — is treated with head-and-neck radiotherapy, which is a major risk factor for subsequent papillary thyroid carcinoma. Always ask about history of H&N cancer, especially NPC ^[3]^[5].

Understanding risk factors matters because they guide how aggressively you pursue investigation.

Risk factor	Mechanism / Explanation
Female sex	Higher nodule prevalence (oestrogen-mediated thyroid cell proliferation), but per-nodule malignancy risk is actually higher in males ^[4]^[5]
Age extremes (< 14y or > 70y)	Childhood thyroid tissue is more radiosensitive; elderly nodules may harbour anaplastic transformation ^[3]^[5]
Head & neck irradiation	DNA double-strand breaks → RET/PTC rearrangements → papillary CA. Includes: brain irradiation for childhood leukaemia, TBI for bone marrow transplant, environmental radiation (e.g. Chernobyl, Fukushima) ^[4]^[5]
Family history of thyroid cancer	~5% of papillary CA is familial; ~20% of medullary CA is familial (MEN2) ^[3]^[4]^[5]
Familial syndromes	MEN2A/2B (RET mutation → medullary thyroid carcinoma); FAP (APC mutation → papillary CA); Cowden syndrome (PTEN → follicular CA); Carney complex; Werner syndrome ^[4]
History of autoimmune thyroid disease	Hashimoto's thyroiditis → ↑ risk of thyroid lymphoma (chronic antigenic stimulation → MALT lymphoma transformation) ^[3]^[5]
Iodine deficiency	↑ TSH drive → follicular hyperplasia → ↑ risk of follicular CA ^[4]
Prior thyroid disease / long-standing MNG	Long-standing goitre can harbour de-differentiation → anaplastic CA ^[3]
Smoking	Weak but documented association ^[3]

MEN2 — Must Know

Type	Gene	Features
MEN1	MEN1 (menin)	Pancreatic endocrine tumour, Pituitary tumour (Prolactinoma), Parathyroid hyperplasia — "3 Ps"
MEN2A	RET	Medullary thyroid carcinoma + Phaeochromocytoma + Parathyroid hyperplasia
MEN2B	RET	Medullary thyroid carcinoma + Phaeochromocytoma + Mucosal neuromas / intestinal ganglioneuromas

Prophylactic total thyroidectomy is indicated for all MEN2 carriers because virtually 100% develop clinically apparent MTC ^[4].

4. Anatomy and Function (Relevant to Workup)

Location: Anterior neck, overlying the 2nd–4th tracheal rings, wrapped around the anterolateral trachea.
Lobes: Right and left lobes connected by an isthmus; a pyramidal lobe (embryological remnant of the thyroglossal duct) is present in ~50%.
Blood supply: Superior thyroid artery (from external carotid) and inferior thyroid artery (from thyrocervical trunk). Occasionally an ima artery from the aortic arch/brachiocephalic trunk.
Venous drainage: Superior, middle, and inferior thyroid veins → internal jugular and brachiocephalic veins.
Lymphatic drainage: First echelon = Level VI (central compartment) nodes — this is the first site of metastasis for differentiated thyroid carcinoma ^[5]. Then lateral neck levels II–V.

Structure	Clinical relevance
Recurrent laryngeal nerve (RLN)	Runs in the tracheo-oesophageal groove; damage → vocal cord palsy → hoarseness (unilateral) or airway compromise (bilateral). Assessed pre-operatively by direct laryngoscopy ^[3]
External branch of superior laryngeal nerve (EBSLN)	Innervates cricothyroid muscle; damage → loss of high-pitched voice
Parathyroid glands (×4)	Sit on posterior thyroid capsule; at risk during thyroidectomy → hypocalcaemia
Trachea	Large goitres cause tracheal deviation/compression → stridor, dyspnoea
Oesophagus	Posterior compression → dysphagia

Cell type	Hormone	Relevance to nodules
Follicular cells	T3, T4 (from thyroglobulin)	Give rise to papillary CA, follicular CA, anaplastic CA
Parafollicular C cells	Calcitonin	Give rise to medullary thyroid carcinoma

Thyroid hormone synthesis depends on TSH stimulation via the HPT axis: hypothalamus (TRH) → anterior pituitary (TSH) → thyroid (T3/T4) → negative feedback on TRH/TSH.

A suppressed TSH (i.e. hyperthyroid state) suggests the nodule may be autonomously functioning ("hot") — these are rarely malignant (< 1%) ^[2]^[3].
A normal or elevated TSH means the nodule is likely non-functioning — proceed with USS ± FNAC as per ATA guidelines ^[1]^[6].

This is why TSH is the first-line blood test in any thyroid nodule workup — it determines the subsequent pathway.

5. Etiology / Pathology of Thyroid Nodules

The pathological breakdown of thyroid nodules is as follows ^[1]:

Pathology	Approximate %
Nodular goitre (colloid / haemorrhagic cystic / complex / hyperplastic / adenomatous nodule)	70%
Benign follicular adenoma (mainly non-toxic)	15%
Well-differentiated thyroid carcinoma	10%
Miscellaneous (other thyroid malignancies, thyroiditis)	5%

5.1 Benign nodules (~85–90%)

Pathophysiology: Recurrent cycles of TSH-driven hyperplasia → involution over years/decades → heterogeneous nodules growing at varying rates, some with cystic degeneration, haemorrhage, fibrosis, or calcification ^[3].
Some nodules may acquire autonomous function (somatic activating mutations in TSH receptor or Gsα) → toxic MNG (Plummer disease).
Note: Follicular adenoma is NOT a risk factor for follicular carcinoma — they are biologically distinct entities ^[5].

Benign, encapsulated, clonal neoplasm of follicular cells.
Cannot be distinguished from follicular carcinoma on FNAC alone — requires histological demonstration of capsular or vascular invasion (this is a fundamental limitation of cytology) ^[2]^[3].

True simple cysts: Rare (< 2%); lined by epithelium, almost always benign.
Complex/colloid cysts: Partially cystic nodules with solid components — need FNAC of solid component.
Haemorrhagic cysts: Arise from bleeding into a pre-existing nodule → sudden painful enlargement.

Hashimoto's thyroiditis can present as a pseudo-nodule (focal lymphocytic infiltration).
Subacute (de Quervain's) thyroiditis: painful, tender gland post-viral illness.

Type	% of thyroid CA	Age	Cell of origin	Key pathological features	Spread pattern
Papillary	85%	Young adult	Well-differentiated follicular cells	Multifocal, non-encapsulated; papillae; psammoma bodies (microcalcifications); Orphan-Annie nuclei (nuclear clearing, grooves, pseudo-inclusions)	Lymphatic (to Level VI first) ^[5]
Follicular	10–15%	Middle age (40–60y)	Well-differentiated follicular cells	Focal, encapsulated; diagnosed by capsular/vascular invasion (cf. adenoma); Hürthle cell variant has worse prognosis	Haematogenous (bone, lung) ^[5]
Anaplastic	1–3%	Old (> 60y)	Undifferentiated follicular cells (de novo or de-differentiation from papillary/follicular CA)	Small blue round cells; very aggressive	Lymphatic + haematogenous ^[5]
Medullary	3–7%	Sporadic > 50y; Familial < 30y	Parafollicular C cells	Amyloid deposits (Congo red stain); 25% genetic (RET oncogene); tumour markers: calcitonin (95%), CEA (80%)	Lymphatic ^[5]
Other rare	< 1%	—	—	Lymphoma (requires core biopsy), SCC, poorly differentiated CA, metastatic (RCC most common)	— ^[5]

6. Classification Systems

The ACR TI-RADS (American College of Radiology Thyroid Imaging Reporting and Data System) is the current standard for classifying thyroid nodules on USS. It uses a points-based system across 5 ultrasound categories:

Category	Options (points)
Composition	Cystic (0), Spongiform (0), Mixed cystic-solid (1), Solid or almost completely solid (2)
Echogenicity	Anechoic (0), Hyperechoic/isoechoic (1), Hypoechoic (2), Very hypoechoic (3)
Shape	Wider than tall (0), Taller than wide (3)
Margin	Smooth (0), Ill-defined (0), Lobulated/irregular (2), Extra-thyroidal extension (3)
Echogenic foci	None/large comet-tail artefacts (0), Macrocalcifications (1), Peripheral (rim) calcification (2), Punctate echogenic foci / microcalcifications (3)

TI-RADS Level	Points	Interpretation	FNAC threshold
TR1	0	Benign	No FNA
TR2	2	Not suspicious	No FNA
TR3	3	Mildly suspicious	≥ 2.5 cm FNA; ≥ 1.5 cm follow-up
TR4	4–6	Moderately suspicious	≥ 1.5 cm FNA; ≥ 1.0 cm follow-up
TR5	≥ 7	Highly suspicious	≥ 1.0 cm FNA; ≥ 0.5 cm follow-up

The ATA 2015 guidelines ^[1]^[6] use a descriptive pattern-based approach:

ATA Sonographic Pattern	Description	Estimated malignancy risk	FNA size threshold
High suspicion	Solid hypoechoic nodule (or solid hypoechoic component of partially cystic nodule) with ≥ 1 of: irregular margins, microcalcifications, taller-than-wide, rim calcification with extrusive soft tissue, extrathyroidal extension	> 70–90%	≥ 1 cm
Intermediate suspicion	Hypoechoic solid nodule with smooth margins, WITHOUT above features	10–20%	≥ 1 cm
Low suspicion	Isoechoic or hyperechoic solid nodule, or partially cystic with eccentric solid component, WITHOUT above features	5–10%	≥ 1.5 cm
Very low suspicion	Spongiform or partially cystic without any suspicious features	< 3%	≥ 2 cm
Benign	Purely cystic (no solid component)	< 1%	No FNA (consider aspiration if symptomatic)

Key Point — ATA Flowchart

If TSH is normal or elevated → Thyroid/Neck USS → Classify nodule by ATA sonographic pattern → Apply size thresholds for FNAC → Report cytology using Bethesda system ^[1]^[6]. This is the backbone of the entire workup.

From the senior notes, the mnemonic "SHIT CME" helps recall suspicious sonographic features ^[2]^[3]:

Letter	Feature	Explanation
S	Solid	Solid nodules have higher malignancy risk than cystic
H	Hypoechoic	Lower echogenicity than surrounding thyroid parenchyma = suspicious
I	Irregular margin / shape	Suggests infiltrative growth
T	Taller than wide	Growth perpendicular to tissue planes = aggressive; normal benign nodules grow along tissue planes (wider than tall)
C	Calcification (microcalcification)	< 0.2mm punctate echogenic foci representing psammoma bodies of papillary CA
M	Missing halo (absent/incomplete perilesional halo)	Halo = compressed normal thyroid tissue around a well-demarcated nodule; absence suggests invasion
E	Extra-thyroidal Extension / abnormal lymph nodes / intranodular vascularity	Direct invasion into strap muscles; suspicious cervical LN features (absent hilum, microcalcification, round shape, peripheral vascularity)

The most important features are solid and hypoechoic ^[2]^[3].

Feature	Why suspicious
Loss of fatty hilum	Normal LNs have an echogenic fatty hilum; metastatic replacement obliterates it
Microcalcification within LN	Pathognomonic for papillary CA metastasis (psammoma bodies deposited in LN)
Round shape (L/S ratio approaching 1)	Normal LNs are elongated; metastatic LNs become round due to expansile growth
Peripheral / chaotic vascularity	Normal LN vascularity is hilar; neoangiogenesis in metastatic LNs is peripheral
Hyperechoic	Thyroid tissue within LN (colloid/thyroglobulin)
Cystic change	Necrosis within metastatic deposit

7. Pathophysiology — Connecting It All

Thyroid nodules result from clonal or polyclonal proliferation of thyroid follicular cells driven by:

TSH stimulation: Chronic ↑ TSH (e.g. iodine deficiency, Hashimoto's) → diffuse hyperplasia → focal nodular transformation (some clones grow faster → nodularity).
Somatic mutations: Gain-of-function mutations in TSH receptor or Gsα → constitutive cAMP pathway activation → autonomous growth ± hormone secretion ("toxic adenoma").
Oncogenic mutations: RET/PTC rearrangements, BRAF V600E, RAS mutations, PAX8-PPARγ fusions → malignant transformation.
Radiation damage: DNA double-strand breaks → chromosomal rearrangements (especially RET/PTC in papillary CA).

This is a fundamental concept. Follicular carcinoma is defined by capsular or vascular invasion — a feature that requires assessment of the tumour-capsule interface on histological sections, not individual cells aspirated through a needle. On cytology, follicular adenoma and follicular carcinoma cells look identical. This is why Bethesda Category IV ("follicular neoplasm") requires surgical excision (hemithyroidectomy) for definitive diagnosis ^[1]^[2]^[3].

A "hot" nodule on scintigraphy has upregulated iodine-trapping and organification — it is doing what normal thyroid cells do, just excessively. Malignant thyroid cells, by contrast, tend to de-differentiate and lose the ability to efficiently trap iodine (lose NIS expression). Therefore, most cancers are "cold" (non-functioning) on scintigraphy. The malignancy risk of a hot nodule is < 1% ^[2]^[3].

Psammoma bodies are concentric lamellated calcified structures formed within papillary thyroid carcinoma. They result from dystrophic calcification of infarcted tips of papillae (tumour outgrows its blood supply → tip necrosis → calcium deposition). These tiny calcifications (< 0.2 mm) appear as punctate hyperechoic foci without posterior acoustic shadowing on USS — highly specific for papillary CA ^[2]^[3].

Benign nodules tend to grow along tissue planes (horizontally) → wider than tall on transverse USS. Malignant nodules grow perpendicular to tissue planes (invading through normal tissue architecture) → taller than wide (anteroposterior dimension exceeds transverse dimension). This feature is relatively specific for malignancy.

8.1 Overview — The Workup Algorithm

Routine investigations for ALL patients with a thyroid nodule/goitre ^[2]^[3]:

TFT (ultrasensitive TSH ± fT4)
Thyroid + neck USS
FNAC (if indicated by USS)

Selective investigations ^[2]^[3]:

Investigation	Indication
Thyroid scintigraphy	Only if TSH is suppressed + nodule present (to determine if hot vs cold)
CT scan	Only for (1) retrosternal goitre (cannot be visualized by USS; needed for surgical planning) or (2) locally advanced thyroid cancer (delineation of cervical fascia structures)
PET scan	No diagnostic role at all
Calcitonin	Clinical suspicion of MTC or MEN2
ESR, anti-thyroid antibodies	Suspected thyroiditis
Direct laryngoscopy	Assess RLN palsy pre-operatively
CXR / Flow-volume loop	Retrosternal goitre / upper airway obstruction

Why CT and not just USS for retrosternal goitre?

Retrosternal goitre requires CT because: (1) USS cannot visualise the mediastinum (sound waves don't penetrate bone/air well), (2) surgical planning requires knowledge of extent, relationship to great vessels, tracheal deviation, (3) retrosternal goitre may be malignant ^[2].

Caution: Iodinated CT contrast can interfere with subsequent radioactive iodine (RAI) scanning/therapy for 6–8 weeks — important to consider if thyroid cancer is suspected ^[3].

9. Clinical Features

Symptom	Pathophysiological basis
Neck swelling / palpable lump	Direct enlargement of thyroid tissue (nodular growth)
Gradual painless enlargement	Typical of benign nodular goitre or well-differentiated carcinoma (slow growth)
Sudden painful enlargement	Haemorrhage into a cyst/necrotic nodule (rupture of thin-walled vessels within the nodule → acute distension of capsule → pain) OR subacute thyroiditis OR anaplastic carcinoma (rapid growth with necrosis) ^[3]^[5]
Dysphagia	Posterior compression/invasion of oesophagus by large goitre or locally advanced cancer
Dyspnoea / stridor	Tracheal compression (large goitre, especially retrosternal) or tracheal invasion (anaplastic CA). Stridor indicates > 50% reduction in tracheal lumen
Dysphonia (hoarseness of voice)	Recurrent laryngeal nerve invasion or compression → vocal cord paralysis. A red flag for malignancy (especially if progressive). Note: can be absent in well-differentiated CA ^[3]^[5]
Thyrotoxic symptoms	Autonomous hormone secretion: weight loss despite ↑ appetite, heat intolerance, sweating, palpitations, tremor, diarrhoea, irritability
Hypothyroid symptoms	Hashimoto's thyroiditis presenting as a nodule: fatigue, weight gain, cold intolerance, constipation, bradycardia
Incidental finding	Majority of thyroid nodules are found incidentally on physical exam or imaging (USS, CT, PET) ^[3]

Sign	Pathophysiological basis
Solitary or dominant nodule	More likely to be malignant than multiple nodules (though MNG can harbour malignancy in any nodule) ^[3]^[5]
Firm / hard consistency	Suggests malignancy — fibrosis/desmoplasia within carcinoma makes it hard (cf. soft, rubbery benign nodules)
Fixation to surrounding tissues	Invasion beyond thyroid capsule into strap muscles, trachea, oesophagus, or carotid sheath — hallmark of advanced malignancy
Moves with swallowing	Pre-tracheal fascia encloses thyroid → thyroid moves with laryngeal elevation during swallowing. This confirms the lump is thyroid in origin
Does NOT move with tongue protrusion	Distinguishes thyroid nodule from thyroglossal duct cyst (which moves up with tongue protrusion due to attachment to foramen caecum via thyroglossal duct remnant)
Cervical lymphadenopathy	Metastatic spread, especially Level VI nodes (central compartment, first echelon drainage) ^[3]^[5]. Also lateral levels II–V
Tracheal deviation	Large goitre pushing trachea to contralateral side
Pemberton's sign	Raising arms above head → facial plethora, distended neck veins, cyanosis. Indicates thoracic inlet obstruction from retrosternal goitre compressing SVC/brachiocephalic veins
Vocal cord palsy (on laryngoscopy)	RLN invasion/compression → immobile vocal cord
Signs of thyrotoxicosis	Tremor, tachycardia, lid retraction, lid lag, warm moist palms, AF, hyperreflexia
Signs of hypothyroidism	Dry skin, periorbital oedema, delayed relaxation of reflexes, bradycardia, myxoedema
Eye signs (Graves')	Lid retraction, exophthalmos, chemosis, proptosis, ophthalmoplegia — specific to Graves' disease (autoimmune stimulation of orbital fibroblasts expressing TSH receptor) ^[2]

Inspection:

Surgical scars (previous thyroidectomy?)
Swallowing test — ask patient to swallow; thyroid lump rises with swallowing
Tongue protrusion test (tongue tug test) — ask patient to protrude tongue; thyroglossal duct cyst rises, thyroid nodule does not

Voice assessment: Hoarseness?

Palpation (from behind the patient):

Diffuse vs solitary nodule vs MNG vs dominant nodule in MNG
Size
Consistency (soft / firm / hard)
Location (which lobe? isthmus?)
Lower border palpable? (if not → suspect retrosternal extension)
Cervical lymph nodes (systematic palpation of all levels, especially Level VI)
Trachea (central or deviated?)

General examination — thyroid status:

Eyes: Lid retraction, exophthalmos, chemosis, proptosis, lid lag, ophthalmoplegia
Hands: Tremor, sweating, tachycardia (radial pulse), thyroid acropachy, palmar erythema
Lower limbs: Proximal muscle weakness (thyrotoxic myopathy), pretibial myxoedema (Graves'), ankle jerk relaxation time (delayed in hypothyroidism)

This is the critical checklist that determines how aggressively you pursue FNAC:

Category	Features
Demographics	Male sex; age < 14y or > 70y
Nodule characteristics	Solitary or dominant nodule; slow but progressive growth (weeks–months); firm/hard; fixed to surrounding tissues
Pressure symptoms / RLN palsy	Indicates rapid growth with invasion (though can be absent in well-differentiated CA)
Cervical lymphadenopathy	Especially Level VI
PMHx	Neck irradiation (NPC Tx in HK context!)
FHx	Thyroid CA (especially medullary CA, MEN2, papillary CA, FAP)
USS features	"SHIT CME": Solid, Hypoechoic, Irregular, Taller-than-wide, Calcification (micro), Missing halo, Extrathyroidal extension / abnormal LN

Red Flags for Thyroid Malignancy

Do NOT miss these in a clinical exam or OSCE:

Hard, fixed, painless nodule
Hoarseness of voice (RLN palsy)
Cervical lymphadenopathy
History of neck irradiation
Family history of MEN2/medullary CA
Rapid growth in an elderly patient (anaplastic CA)

11. Ultrasound of the Thyroid — Deep Dive

Probe: 7.5 or 10 MHz linear probe, B-mode ^[2]^[3]
Advantages: Readily available, non-invasive, no radiation, high sensitivity
Disadvantage: High sensitivity but LOW specificity — many benign nodules have "suspicious" features → this is why USS is used to guide (not confirm) diagnosis ^[2]^[3]
Indication: For ALL patients with goitre or palpable nodules
NOT a screening test for healthy subjects (too sensitive → too many false positives → unnecessary anxiety and procedures) ^[2]^[3]

Domain	Features to assess
The nodule itself	Composition (solid/cystic/mixed), echogenicity, shape (taller vs wider), margins, calcifications, vascularity, halo
Surrounding thyroid	Other nodules (MNG → slightly reassuring), parenchymal abnormalities (Hashimoto's = diffuse heterogeneous hypoechogenicity)
Cervical lymph nodes	Especially Level VI (deep, not palpable clinically) — USS is essential to detect these
Retrosternal extension	Lower pole of thyroid descending behind sternum

FNAC is the single most important investigation for a thyroid nodule when TSH is not suppressed ^[2]^[3].

Indications based on ATA sonographic pattern:

Pattern	FNAC size threshold
High suspicion	≥ 1 cm
Intermediate suspicion	≥ 1 cm (some guidelines say ≥ 1.5 cm)
Low suspicion	≥ 1.5 cm
Very low suspicion	≥ 2 cm
Benign (purely cystic)	No FNAC (consider aspiration if symptomatic)

Additional indications for FNAC ^[2]^[4]:

Hypofunctioning ("cold") nodules on scintigraphy (10–20% malignancy risk)
Dominant or atypical nodule in MNG
Nodules associated with abnormal cervical lymph nodes
Complex or recurrent cystic nodules
Symptomatic / large cysts (therapeutic aspiration)

Can proceed directly to total thyroidectomy (bypass FNAC) if ^[2]^[3]:

Nodule > 4 cm
Gross extrathyroidal invasion
Cervical lymph node metastasis confirmed

Technical aspects:

Trans-isthmic approach ± USS guidance
USS guidance advantages: Confirm presence of nodule, target biopsy to the most suspicious region (e.g. solid component of a mixed cystic-solid nodule), avoid vascular structures ^[2]^[3]
Accuracy: 90–95% ^[2]

Malignancy risk is much lower overall in multinodular glands
USS: assess EACH nodule separately
FNAC strategy:
- If no suspicious nodules → FNA the largest nodule
- If any suspicious nodules → FNA ALL suspicious nodules

12. Bethesda System for Reporting Thyroid Cytopathology

The Bethesda System is the standardized international classification for thyroid FNAC cytology. "Bethesda" refers to Bethesda, Maryland, USA, where the NCI hosted the 2007 consensus conference that established it. It has been updated (most recently in 2023, 3rd edition) but the 6-tier structure remains.

Bethesda Category	Diagnostic Category	Risk of Malignancy (%)	Usual Management
I	Non-diagnostic / Unsatisfactory	1–4 (up to 5–10%)	Repeat FNA (or surgery if radiologically suspicious / high clinical risk) ^[1]^[2]^[5]
II	Benign	0–3	Clinical follow-up (USS monitoring) ^[1]^[2]^[5]
III	AUS (Atypia of Undetermined Significance) / FLUS (Follicular Lesion of Undetermined Significance)	5–15 (up to 6–18%)	Repeat FNA (low risk) OR molecular testing OR hemithyroidectomy if AUS × 2 or high clinical risk ^[1]^[2]^[5]
IV	Follicular Neoplasm / Suspicious for Follicular Neoplasm / Hürthle cell neoplasm	15–30 (up to 10–40%)	Hemithyroidectomy (lobectomy) ± molecular testing ^[1]^[2]^[5]
V	Suspicious for Malignancy	60–75 (up to 45–60%)	Hemithyroidectomy + frozen section → completion total thyroidectomy (or direct TT) ^[1]^[2]^[5]
VI	Malignant	97–99	Total thyroidectomy ^[1]^[2]^[5]

12.3 Deep Dive — Each Category Explained

The aspirate has insufficient cells for cytological interpretation (typically < 6 groups of 10 well-preserved follicular cells).
Causes: Too much blood (haemodiluted), too few cells (fibrotic nodule), cystic fluid only with no follicular cells.
Management: Repeat FNA in 4–6 weeks (to allow haematoma resolution). If still non-diagnostic:
- Low clinical risk → observation
- High clinical risk → hemithyroidectomy
High-risk features for proceeding to surgery ^[5]:
- Suspicious USS features
- Clinical risk factors: FHx of thyroid CA, prior neck radiation
- P/E: voice hoarseness, hard/irregular/fixed mass, cervical LN positive
- Growing nodule

Includes colloid nodule, hyperplastic nodule, lymphocytic thyroiditis (Hashimoto's), granulomatous thyroiditis (de Quervain's).
Still a small risk of malignancy (0–3%) due to sampling error ^[5].
Management: Observation with clinical + USS monitoring.
Indications for surgery in a benign nodule ^[5]:
1. Pressure symptoms (dysphagia, dyspnoea, dysphonia)
2. Growing size
3. Cosmetic concern
4. Patient's worry
Thyroxine suppression therapy is mostly obsoleted ^[5]:
- Controversial benefits
- Works in < 20% of patients
- Significant side effects (osteoporosis, AF from iatrogenic subclinical hyperthyroidism)
- Thyroid gland regrows after cessation

Bethesda II — Don't forget!

A "benign" FNAC result does NOT mean you can forget the patient. There is still a 0–3% false-negative rate. Patients need clinical and USS follow-up. If the nodule grows significantly (> 20% increase in diameter or > 50% increase in volume), repeat FNAC.

AUS = Atypia of Undetermined Significance; FLUS = Follicular Lesion of Undetermined Significance.
"Atypia" here is a morphological description rather than a premalignant lesion ^[5] — it simply means the cells look a bit "off" but don't meet criteria for follicular neoplasm or malignancy.
This is the "grey zone" — the cytopathologist is essentially saying "I'm not sure."
Management:
- Repeat FNA (most will be reclassified as Bethesda II on repeat)
- Molecular testing (e.g. ThyroSeq, Afirma GSC) — can help "rule in" or "rule out" malignancy
- Hemithyroidectomy if AUS × 2 or if high clinical risk ^[2]^[3]^[5]

The cytology shows a follicular-patterned lesion — microfollicular architecture with scant colloid.
Critical concept: FNAC CANNOT distinguish follicular adenoma from follicular carcinoma — this requires histological assessment of the tumour capsule for capsular or vascular invasion ^[1]^[2]^[3].
After resection, the majority turn out to be papillary CA (with psammoma bodies) — some lesions initially classified as "follicular neoplasm" on cytology are actually follicular variant of papillary CA on final histology ^[5].
Hürthle cell (oncocytic) neoplasm: A variant with abundant mitochondria-rich eosinophilic cytoplasm; worse prognosis; not amenable to RAI (Hürthle cells lose NIS expression) → requires surgical treatment ^[5].
Management: Hemithyroidectomy (lobectomy) ^[1]^[2]^[5]
- If histology shows benign (adenoma / minimally invasive with < 5 vessel invasion + no wide invasion) → lobectomy is curative ^[3]
- If malignant (widely invasive, > 5 vessel invasion) → completion total thyroidectomy + RAI ablation (due to ↑ risk of distant metastases) ^[3]

Frozen Section in Bethesda IV — Is It Helpful?

Frozen section (FS) is NOT helpful during hemithyroidectomy for follicular neoplasm. It only gives diagnostic information in 13% of cases, modifies surgical procedure in 3.3%, and causes misguided intervention in 5%. One should wait for the final histology report after lobectomy ^[3].

Cytological features are highly suggestive but not definitively diagnostic of malignancy (e.g. some but not all features of papillary CA).
Management: Hemithyroidectomy + frozen section → if positive, proceed to completion total thyroidectomy ^[1]^[2].
Alternatively, some centres proceed directly to total thyroidectomy if clinical/USS suspicion is very high.

Definitive cytological diagnosis of malignancy (e.g. classic papillary CA with nuclear grooves, pseudoinclusions, psammoma bodies; or medullary CA with amyloid + positive calcitonin staining; or anaplastic CA).
Management: Total thyroidectomy ^[1]^[2]^[5].

Veracyte Afirma Genomic Sequencing Classifier (GSC), ThyroSeq v3 — multi-gene panels that can reclassify Bethesda III/IV nodules as "benign" (with high NPV, potentially avoiding surgery) or "suspicious" ^[2]^[3].
Currently expensive, no universal standards, and not readily available — especially in HK ^[2]^[3].
Genes tested include BRAF V600E, RAS, RET/PTC, PAX8-PPARγ, TERT promoter mutations.

13. Thyroid Scintigraphy — When and Why

Radioactive iodine is handled in the same manner as normal iodine by thyroid follicular cells ^[7].
Radiopharmaceuticals: ⁹⁹ᵐTc-pertechnetate (iodine trapping only) or ¹²³I / ¹³¹I (trapping + organification) ^[7].
- ⁹⁹ᵐTc-pertechnetate has a similar ionic size to iodide → taken up by the sodium-iodide symporter (NIS) but NOT organified.
Level of uptake = metabolic activity → detected by gamma camera.
Images obtained at anterior, LAO, and RAO views ^[7].

Used in patients with nodule/MNG + suppressed TSH ^[2]^[3] — to determine if the nodule is the source of hyperthyroidism.
"Hot" nodule: Uptake greater than surrounding thyroid → autonomous function → rarely malignant (< 1%) → does NOT require FNAC ^[2]^[4].
"Cold" nodule: Uptake less than surrounding thyroid → 10–20% malignancy risk → requires FNAC (provided sonographic criteria are met) ^[2]^[4].
"Warm" / "Indeterminate" nodule: Uptake similar to surrounding thyroid → intermediate risk → FNAC if USS suspicious.

Radiation exposure, expensive, low specificity/sensitivity ^[3].
Most cancers are cold, but most cold nodules are benign — hence scintigraphy alone cannot diagnose malignancy.
Limited spatial resolution — cannot characterise nodules < 1 cm well.

The ATA 2015 guidelines ^[1]^[6] provide the definitive algorithm:

Suspected thyroid nodule (palpated or incidental)
Step 1: TFT (ultrasensitive TSH)
- If TSH is low (suppressed) → Thyroid scintigraphy
  - Hot nodule → Manage as toxic nodule (no FNAC needed)
  - Cold/indeterminate nodule → Proceed to USS + FNAC
- If TSH is normal or elevated → Thyroid + neck USS
Step 2: USS — classify by ATA sonographic pattern
- No nodule found or nodule does not meet FNA size cutoff → Observation/follow-up
- Nodule meets criteria → USS-guided FNAC
Step 3: FNAC — report using Bethesda system
Step 4: Management based on Bethesda category (see table above)

14.1 Summary — Overview of Management by Presentation [5]

	Solitary	Multinodular
Euthyroid	Observe; hemithyroidectomy if 4C's (cancer, compression, cosmesis, concern)	Observe; total thyroidectomy if 4C's
Hyperthyroid	Hemithyroidectomy	Total thyroidectomy

Category	Differential
Thyroid enlargement	Solitary nodule, MNG, diffuse goitre (Graves', Hashimoto's, simple goitre), thyroid cyst, thyroid carcinoma
Lymphadenopathy	Reactive, infective (TB in HK!), metastatic (NPC, thyroid CA, other H&N cancers), lymphoma
Skin lumps	Sebaceous cyst, lipoma, dermoid cyst
Midline developmental	Thyroglossal duct cyst (moves with tongue protrusion — remnant of descent of thyroid from foramen caecum)
Lateral developmental	Branchial cyst (anterior border of SCM, smooth fluctuant)
Vascular	Carotid body tumour (pulsatile, at carotid bifurcation)
Other	Laryngocoele, pharyngeal pouch

High Yield Summary

Thyroid nodules are extremely common (> 30% on USS) but only ~5–15% are malignant. The workup exists to identify cancer while avoiding unnecessary surgery.
First step is ALWAYS TSH: Suppressed TSH → scintigraphy (hot nodules are rarely malignant, cold nodules need FNAC). Normal/elevated TSH → USS → FNAC.
USS suspicious features ("SHIT CME"): Solid, Hypoechoic, Irregular margin, Taller-than-wide, Calcification (micro), Missing halo, Extrathyroidal extension / abnormal LN. Most important = solid + hypoechoic.
ATA 2015 sonographic patterns determine FNAC size thresholds: High suspicion ≥ 1 cm, Intermediate ≥ 1 cm, Low ≥ 1.5 cm, Very low ≥ 2 cm, Benign (purely cystic) → no FNA.
Bethesda classification (6 categories): I = Non-diagnostic (repeat); II = Benign (follow-up); III = AUS/FLUS (repeat FNA or molecular testing); IV = Follicular neoplasm (hemithyroidectomy — FNAC cannot distinguish adenoma from carcinoma); V = Suspicious (hemiT + FS → TT); VI = Malignant (TT).
FNAC cannot distinguish follicular adenoma from follicular carcinoma — requires histological capsular/vascular invasion assessment.
Frozen section is NOT helpful for Bethesda IV (follicular neoplasm) — wait for final histology.
Risk factors for malignancy: Male sex, extremes of age, neck irradiation (NPC Tx in HK!), FHx (MEN2, FAP), solitary/dominant nodule, hard/fixed, hoarseness, cervical LN.
CT is NOT routine — only for retrosternal goitre or locally advanced cancer. PET has no diagnostic role. Scintigraphy only if TSH is suppressed.
Thyroxine suppression therapy for benign nodules is mostly obsoleted — controversial, works in < 20%, significant side effects, regrows after cessation.

Active Recall - Thyroid Nodule Workup (USS + Bethesda FNAC)

^[1] Lecture slides: GC 177. A thyroid nodule benign thyroid nodules; thyroid cancer.pdf (p5, p10, p12) ^[2] Senior notes: Ryan Ho Endocrine.pdf (p17–p21, p31–p32, p38) ^[3] Senior notes: Ryan Ho Fundamentals.pdf (p425–p428) ^[4] Senior notes: felixlai.md (Etiology, FNA, Bethesda sections) ^[5] Senior notes: maxim.md (Bethesda classification, Approach to multiple nodules, Thyroid cancer overview) ^[6] Lecture slides: Management of differentiated thyroid carcinoma.pdf (p2) ^[7] Senior notes: Ryan Ho Diagnostic Radiology.pdf (p59) ^[8] Lecture slides: GC 218. I have a swelling in the neck Neck mass.pdf

Differential Diagnosis of a Thyroid Nodule

When a patient presents with a thyroid nodule (or, more broadly, an anterior neck lump), the differential diagnosis must be constructed systematically. The most efficient approach is to think in layers:

Is the lump actually thyroid? (vs non-thyroid anterior neck lump)
If thyroid — what is the morphological pattern? (solitary nodule vs multiple nodules vs diffuse enlargement)
What is the thyroid functional status? (hyperthyroid, euthyroid, hypothyroid) — because this dramatically narrows the differential
What is the pathological nature? (non-neoplastic vs benign neoplasm vs malignant)

The entire purpose of the USS + Bethesda FNAC workup is to navigate this differential and arrive at a tissue diagnosis — or at least a risk-stratified management plan.

Before you even think "thyroid nodule," you need to confirm the lump is thyroid in origin. The D/dx of an anterior neck lump ^[2]^[3]^[5]:

Category	Differentials	How to distinguish
Thyroid enlargement	Solitary nodule, MNG, diffuse goitre, thyroid cyst, thyroid carcinoma	Moves with swallowing (thyroid is enclosed in pretracheal fascia, which is attached to the laryngeal skeleton)
Thyroglossal duct cyst	Midline developmental cyst (embryological remnant of thyroid descent from foramen caecum → tongue base to neck)	Moves with tongue protrusion (attached to foramen caecum via fibrous thyroglossal duct remnant) AND moves with swallowing
Lymphadenopathy	Reactive, infective (TB — common in HK), metastatic (NPC, thyroid CA, other H&N cancers), lymphoma	Does NOT move with swallowing; location often lateral to midline; may be multiple
Branchial cyst	Lateral developmental cyst (remnant of 2nd branchial cleft) — more common in paediatric/young adults	Anterior border of SCM, smooth, fluctuant; does NOT move with swallowing
Skin lumps	Sebaceous (epidermoid) cyst, lipoma, dermoid cyst	Superficial to strap muscles; moves with skin; NOT deep
Vascular	Carotid body tumour (paraganglioma at carotid bifurcation)	Pulsatile, mobile side-to-side but NOT vertically (Fontaine sign); at level of carotid bifurcation
Other	Laryngocoele, pharyngeal pouch, cervical rib	Rare; specific clinical/radiological features

Exam Tip — Swallowing Test vs Tongue Protrusion Test

Both thyroid lumps AND thyroglossal duct cysts move with swallowing (both are attached to pretracheal structures). The tongue protrusion test differentiates them: only a thyroglossal duct cyst moves upward on tongue protrusion (because the duct remnant connects to the foramen caecum at the tongue base). A thyroid nodule does NOT move with tongue protrusion.

2. Differential Diagnosis of a Thyroid Nodule — By Morphological Pattern

Once you have confirmed the lump is thyroid, classify by what you feel on examination and see on USS ^[2]^[5]:

Pathology	Explanation / Why it presents as a solitary nodule
Dominant nodule in MNG	The most common "solitary" nodule is actually a dominant nodule in a MNG that is not yet clinically apparent — USS often reveals additional smaller nodules. The remaining nodules are too small to palpate ^[5]
Cyst	True simple cyst (rare, < 2%), colloid nodule with cystic degeneration (common) — fluid accumulation within a follicle or from haemorrhage into a pre-existing nodule
Benign follicular adenoma	Clonal, encapsulated neoplasm of follicular cells; usually non-toxic (does not secrete excess thyroid hormone). Uncommonly, somatic gain-of-function TSH-receptor mutation → toxic adenoma (autonomous T3/T4 secretion → suppressed TSH → "hot" on scintigraphy) ^[5]
Carcinoma	Papillary (most common), follicular, medullary, anaplastic, or rarely metastatic. A solitary nodule is more likely to be malignant than a nodule in a multinodular gland ^[2]^[3]

Key point: Around 10–15% of thyroid nodules are malignant ^[5]. The differential of a solitary nodule therefore always includes carcinoma until proven otherwise by FNAC.

Pathology	Explanation
Multinodular goitre (MNG)	Hyperplastic/adenomatous nodules with varying degree of cystic degeneration — result of recurrent cycles of TSH-driven hyperplasia and involution ^[2]^[3]. Some nodules may become autonomous → toxic MNG (Plummer disease) ^[5]
Multiple cysts	Multiple colloid cysts or haemorrhagic cysts within a background of MNG
Multiple adenomas	Rare; multiple independent clonal neoplasms
Malignancy within MNG	A MNG does NOT protect against cancer — any suspicious nodule within a MNG must be biopsied. Malignancy risk is lower overall in multinodular glands, but USS must assess EACH nodule separately ^[5]

Thyroid status	Differential	Pathophysiology
Hypothyroid	Hashimoto's thyroiditis	Autoimmune destruction of follicular cells (anti-TPO, anti-Tg) → compensatory TSH-driven hypertrophy → diffuse firm goitre. Eventually progresses to atrophic gland with fibrosis
Euthyroid	Simple diffuse goitre (physiological: pregnancy, puberty; iodine deficiency; goitrogen exposure)	↑ Thyroid hormone demand or ↓ synthesis → ↑ TSH → diffuse hyperplasia. Often self-limiting (e.g. puberty)
	Early MNG	Before individual nodules become clinically palpable
	Infiltrative disease (e.g. lymphoma)	Lymphoid proliferation expanding the gland diffusely
	Treated Graves' disease	After antithyroid drug/RAI → gland remains enlarged but function normalised
Hyperthyroid	Graves' disease	TSH-receptor stimulating antibodies (TRAb) → constitutive TSH-receptor activation → diffuse hyperplasia + excess T3/T4 secretion → diffuse, smooth, symmetrical goitre with bruit
Mixed / fluctuating	Destructive thyroiditis (subacute/de Quervain's, postpartum, painless)	Thyrotoxic phase (stored hormone release from damaged follicles) → hypothyroid phase (depleted stores + damaged cells) → recovery

This is clinically the most useful framework because TFT is the first investigation and immediately narrows the differential ^[2]^[3]:

Why does scintigraphy come BEFORE USS/FNAC when TSH is low?

If TSH is suppressed, the nodule might be a hot (autonomously functioning) nodule. Hot nodules are almost never malignant (< 1%) because the very features that make a cell malignant (de-differentiation) also make it lose the ability to trap iodine efficiently. If the nodule is confirmed "hot" on scintigraphy, you save the patient an unnecessary and uncomfortable FNAC. That's why the ATA algorithm directs you to scintigraphy first when TSH is low, and USS/FNAC first when TSH is normal or elevated ^[1]^[6].

4. Differential Diagnosis — By Pathological Nature

This is the framework that FNAC (Bethesda system) addresses directly. For completeness, let's lay it out with the pathophysiology of each entity ^[1]^[2]^[3]^[5]:

Entity	Pathophysiology	Key features
Colloid nodule	Focal accumulation of colloid (thyroglobulin-rich material) within enlarged follicles — often within a MNG	USS: isoechoic/hyperechoic, partially cystic, "comet-tail" artefact (from colloid crystals). FNAC: abundant colloid, scant follicular cells → Bethesda II
Hyperplastic / adenomatous nodule	Focal hyperplasia of follicular cells driven by TSH or local growth factors; NOT a true neoplasm (polyclonal)	Part of the MNG spectrum. Can undergo cystic degeneration, haemorrhage, calcification
Haemorrhagic cyst	Bleeding into a pre-existing nodule (vessels within the nodule rupture) → sudden painful enlargement	History of acute onset pain; USS: complex cystic lesion with internal echoes (blood). FNAC: haemosiderin-laden macrophages, old blood
Simple cyst	True epithelial-lined cyst; very rare (< 2% of all thyroid nodules)	USS: purely anechoic, thin smooth wall, posterior acoustic enhancement. Purely cystic nodules are almost always benign (< 1% malignancy risk) → no FNAC needed ^[4]
Thyroiditis presenting as pseudo-nodule	Focal lymphocytic infiltration in Hashimoto's, or focal inflammation in subacute thyroiditis, mimicking a discrete nodule on palpation	USS: ill-defined hypoechoic area within diffusely heterogeneous gland. FNAC: lymphocytes and Hürthle cells (Hashimoto's); granulomatous inflammation with giant cells (de Quervain's) → Bethesda II

Entity	Pathophysiology	Key features
Follicular adenoma (non-toxic)	Monoclonal proliferation of follicular cells → encapsulated, well-circumscribed nodule. Benign by definition (no capsular/vascular invasion)	USS: well-circumscribed, complete halo, isoechoic. FNAC: microfollicular pattern, scant colloid → Bethesda IV ("follicular neoplasm") — FNAC cannot distinguish from follicular carcinoma ^[2]^[3]
Toxic adenoma	Somatic activating mutation of TSH-receptor or Gsα → autonomous cAMP activation → excess T3/T4 production independent of TSH	Suppressed TSH; "hot" on scintigraphy; < 1% malignancy risk; does NOT require FNAC ^[4]. Managed with hemithyroidectomy or RAI
Hürthle cell (oncocytic) adenoma	Variant with mitochondria-rich eosinophilic follicular cells. Benign but cannot be distinguished from Hürthle cell carcinoma on FNAC	Not amenable to RAI (Hürthle cells lose NIS expression → cannot trap iodine) → requires surgical treatment ^[5]

Type	%	Typical age	Pathophysiology / Mutations	Key distinguishing features
Papillary CA	85%	Young adult	RET/PTC rearrangements, BRAF V600E, RAS → well-differentiated follicular cell origin	Multifocal, non-encapsulated; Orphan-Annie nuclei; psammoma bodies (microcalcifications on USS); lymphatic spread → Level VI nodes
Follicular CA	10–15%	40–60y	RAS, PAX8-PPARγ fusion → well-differentiated follicular cell origin	Focal, encapsulated; diagnosed ONLY by capsular/vascular invasion on histology; haematogenous spread → bone, lung
Medullary CA (MTC)	3–7%	Sporadic > 50y; Familial < 30y	RET proto-oncogene mutation (germline in MEN2, somatic in sporadic) → parafollicular C-cell origin	Amyloid deposits (Congo red); calcitonin (95%); CEA (80%); 25% genetic (MEN2A/2B); lymphatic spread
Anaplastic CA	1–3%	> 60y	TP53, TERT promoter mutations; often de-differentiation from prior papillary/follicular CA or longstanding goitre	Rapidly enlarging hard mass over weeks; undifferentiated small blue round cells; almost uniformly fatal (median survival < 6 months)
Primary thyroid lymphoma	Rare	Elderly with Hashimoto's	Chronic antigenic stimulation in Hashimoto's → MALT lymphoma → DLBCL transformation	Rapidly enlarging goitre; requires core biopsy (FNAC insufficient — need tissue architecture for lymphoma subtyping) ^[5]

Primary site	Notes
Renal cell carcinoma (most common metastasis to thyroid)	RCC is notoriously vascular and has a propensity for unusual metastatic sites; thyroid has rich blood supply → haematogenous seeding
Colorectal, lung, breast, uterine	Less common; usually in context of widely disseminated disease

Other thyroid malignancies: SCC, poorly differentiated carcinoma
Thyroiditis presenting as a nodule (overlap with 4A above)

This table, adapted from the senior notes ^[5], is the high-yield exam summary:

Presentation	Differentials (with thyroid status)
Solitary nodule	Dominant nodule in MNG; Cyst (true simple cyst, colloid nodule); Neoplastic: adenoma, toxic adenoma, carcinoma
Multiple nodules	MNG (hyperplastic/adenomatous nodules with varying cystic degeneration), toxic MNG; Multiple cysts; Neoplastic: multiple adenoma
Diffuse	Graves' disease; Physiological (pregnancy, puberty); Hashimoto's thyroiditis; De Quervain's/subacute thyroiditis

Around 10–15% of nodules are malignant ^[5].

Understanding which investigation eliminates which differential is the key to the workup:

Investigation	What it tells you	Which differentials it separates
TFT (TSH)	Functional status	Toxic adenoma/toxic MNG (↓ TSH) vs euthyroid nodule vs Hashimoto's (↑ TSH). Determines whether scintigraphy or USS/FNAC comes next ^[1]^[6]
Thyroid scintigraphy	Functional status of individual nodule	Hot nodule (< 1% cancer → no FNAC) vs cold nodule (10–20% cancer → FNAC) ^[4]^[7]
USS	Morphology + risk stratification	Classifies nodule by ATA sonographic pattern → determines whether FNAC is needed and at what size threshold. Evaluates cervical LN (especially Level VI — not palpable clinically). Detects retrosternal extension ^[2]^[3]
FNAC (Bethesda)	Cytological nature	Separates benign (II) from malignant (VI), with intermediate categories (III–V) requiring further action. Cannot distinguish follicular adenoma from follicular carcinoma ^[2]^[3]
CT	Anatomical extent	Only for retrosternal goitre (USS cannot see mediastinum) or locally advanced CA (delineation of cervical fascia structures) ^[5]. PET has NO diagnostic role ^[5]
Calcitonin	C-cell origin	Elevated calcitonin → medullary thyroid carcinoma (95% sensitivity). Must be ordered if clinical suspicion of MTC or MEN2 ^[2]^[3]
Anti-TPO / Anti-Tg Abs	Autoimmune thyroiditis	Elevated in Hashimoto's → explains diffuse goitre or pseudo-nodule
Molecular testing	Genetic mutations	BRAF V600E (papillary CA), RAS, RET/PTC, PAX8-PPARγ → helps reclassify Bethesda III/IV nodules ^[2]^[3]

This is the "clinical reasoning" layer. When you take a history and examine the patient, certain constellations of features point you strongly toward specific diagnoses:

Clinical scenario	Most likely diagnosis	Why
Young woman + painless solitary nodule + euthyroid + microcalcifications on USS	Papillary thyroid carcinoma	Most common thyroid CA; young adults; psammoma bodies cause microcalcifications
Middle-aged woman + solitary nodule + cold on scintigraphy + USS shows encapsulated lesion with complete halo	Follicular neoplasm (adenoma or carcinoma — need excision to tell)	Encapsulated follicular lesion; cold because not functioning normally; requires histology for definitive dx
Elderly + rapidly enlarging hard neck mass over weeks + dysphagia/stridor/hoarseness	Anaplastic carcinoma	Aggressive, undifferentiated; rapid growth; locally invasive; median survival < 6 months ^[2]^[5]
Elderly + AF + large MNG	Toxic MNG (Plummer disease)	Classically AF + multinodular goitre in elderly ^[3]; autonomous nodules produce excess T3/T4 → AF
Solitary nodule + suppressed TSH + hot on scintigraphy	Toxic adenoma	Autonomous T3/T4 secretion; < 1% malignancy risk
Firm nodule + FHx of MEN2 + elevated calcitonin	Medullary thyroid carcinoma	Parafollicular C cells; 25% genetic (RET mutation); calcitonin is the tumour marker ^[5]
Painful thyroid swelling post-viral illness + fever + ↑ ESR + fluctuating thyroid function	Subacute (de Quervain's) thyroiditis	Viral-triggered granulomatous inflammation; thyrotoxic → hypothyroid → recovery
Rapidly enlarging goitre in elderly with known Hashimoto's	Primary thyroid lymphoma	Chronic Hashimoto's → MALT → DLBCL transformation; requires core biopsy (not FNAC) ^[5]
Sudden painful enlargement of a previously stable nodule	Haemorrhage into cyst/necrotic nodule	Vessel rupture within nodule → acute distension
History of NPC with prior neck radiotherapy + now thyroid nodule	Papillary carcinoma (radiation-induced)	Radiation causes RET/PTC rearrangements; ask about H&N cancer history, especially NPC in HK ^[3]

Overall malignancy risk is lower in a multinodular gland than a solitary nodule — but it is NOT zero.
USS must assess EACH nodule separately — each nodule gets its own risk stratification.
FNAC strategy:
- If no suspicious nodules → FNA the largest nodule
- If any suspicious nodules → FNA ALL suspicious nodules
Do NOT assume that because the gland is multinodular, all nodules are benign. A classic mistake is to dismiss a suspicious-looking nodule within a MNG.

Common Exam Pitfall

Students often assume that a multinodular goitre = benign. While the overall per-nodule risk is lower, each nodule must be assessed individually by USS. A suspicious nodule in a MNG gets the same FNAC workup as a solitary suspicious nodule.

Mimicker	How to distinguish
Parathyroid adenoma	Usually posterior to thyroid; associated with hypercalcaemia + ↑ PTH; USS may show a well-defined hypoechoic nodule posterior to the thyroid lobe; sestamibi scan is localising
Ectopic thyroid	Lingual thyroid (at foramen caecum), sublingual thyroid; scintigraphy shows uptake in ectopic location with absent/diminished uptake in normal thyroid bed
Thyroglossal duct cyst	Midline; moves with tongue protrusion; USS: well-defined anechoic/hypoechoic cystic lesion in midline anterior to strap muscles
Prominent pyramidal lobe	Normal variant; can be mistaken for isthmus nodule

High Yield Summary — Differential Diagnosis

Always confirm the lump is thyroid first (moves with swallowing, does NOT move with tongue protrusion).
D/dx of anterior neck lump: thyroid enlargement, lymphadenopathy, thyroglossal duct cyst, branchial cyst, skin lumps, carotid body tumour.
Pathological breakdown of thyroid nodules: Non-neoplastic (70%) > Benign adenoma (15%) > Well-differentiated carcinoma (10%) > Miscellaneous (5%).
TFT is the first branch point: Low TSH → scintigraphy (hot = safe; cold = FNAC). Normal/high TSH → USS → FNAC per ATA pattern.
Solitary nodule is more likely malignant than multiple nodules — but each nodule in a MNG must be assessed individually.
FNAC cannot distinguish follicular adenoma from follicular carcinoma — requires surgical excision for histological assessment of capsule/vascular invasion.
Hot nodules are rarely malignant (< 1%) because malignant cells de-differentiate and lose iodine-trapping ability.
Thyroid lymphoma requires core biopsy — FNAC cannot provide tissue architecture needed for lymphoma subtyping.
In Hong Kong, always ask about NPC/prior head-and-neck radiotherapy — major risk factor for papillary CA.
Red flag differentials: Rapidly enlarging mass in elderly → anaplastic CA or lymphoma; hard fixed nodule + hoarseness → invasive carcinoma; FHx MEN2 + elevated calcitonin → medullary CA.

Active Recall - Differential Diagnosis of Thyroid Nodule

References

^[1] Lecture slides: GC 177. A thyroid nodule benign thyroid nodules; thyroid cancer.pdf (p5, p9, p10, p12, p13) ^[2] Senior notes: Ryan Ho Endocrine.pdf (p17–p20, p32, p38) ^[3] Senior notes: Ryan Ho Fundamentals.pdf (p425–p428) ^[4] Senior notes: felixlai.md (USS criteria, FNA indications, Bethesda classification, Scintigraphy sections) ^[5] Senior notes: maxim.md (Differential diagnosis table, Bethesda classification, Approach to multiple nodules, Thyroid cancer overview) ^[6] Lecture slides: Management of differentiated thyroid carcinoma.pdf (p2) ^[7] Senior notes: Ryan Ho Diagnostic Radiology.pdf (p59)

Diagnostic Criteria, Algorithm & Investigation Modalities

Unlike many medical conditions that have neat "diagnostic criteria" (e.g. rheumatoid arthritis — ACR/EULAR criteria), a thyroid nodule is not a diagnosis in itself — it is a finding that triggers a risk-stratification workup. The "diagnosis" we seek is the answer to: Is this nodule benign or malignant? And if malignant, what type?

There is no single test that gives a definitive answer for every nodule. Instead, the diagnostic process is a sequential, branching algorithm where each investigation narrows the differential and determines the next step. The three pillars are:

TFT (functional triage)
USS (morphological risk stratification)
FNAC with Bethesda reporting (cytological diagnosis)

Everything else is selective and complementary.

This is the ATA 2015 guideline algorithm ^[1]^[6], the single most important framework to learn:

Why this sequence and not another?

The algorithm is designed to be cost-effective and minimally invasive. TSH comes first because it is cheap, fast, and determines whether scintigraphy (an expensive nuclear medicine test) is even needed. USS comes before FNAC because it determines whether FNAC is indicated and where to target the needle. FNAC is the most invasive step in the initial workup and is reserved for nodules that meet specific size and morphology thresholds. This tiered approach avoids unnecessary procedures — which matters enormously given that > 85% of nodules are benign ^[1]^[2]^[3].

2. Investigation Modalities — Detailed Breakdown

2A. Routine Investigations (For ALL Patients)

These three are routine for all patients with a thyroid nodule or goitre ^[1]^[2]^[3]^[5]:

Investigation	Routine?
History + Physical examination	✓
Thyroid function test (TFT)	✓
USG thyroid ± FNAC	✓

What to order: Ultrasensitive TSH ± fT4 ^[1]^[2]^[3]

Why TSH first?

TSH has the shortest half-life and is the most sensitive indicator of thyroid functional status because of the log-linear relationship between TSH and free T4 — tiny changes in fT4 cause large swings in TSH ^[4].
It is the gatekeeper that determines the next branch of the algorithm.

Interpretation and next steps:

TSH result	Interpretation	Next step	Rationale
↓ TSH (suppressed)	Overt or subclinical hyperthyroidism → nodule may be autonomously functioning	Thyroid scintigraphy ^[1]^[2]^[4]	Need to determine if the nodule is "hot" (autonomous) — hot nodules are almost never malignant (< 1%) and do NOT need FNAC
Normal TSH	Euthyroid — most common scenario	USS → FNAC per ATA pattern ^[1]^[6]	Cannot be a hot nodule → proceed directly to morphological assessment
↑ TSH	Hypothyroidism (likely Hashimoto's or iodine deficiency)	USS → FNAC per ATA pattern + anti-TPO/Tg antibodies ^[2]^[3]	The nodule is definitely not hyperfunctioning. ↑ TSH also mildly ↑ thyroid cancer risk (chronic TSH stimulation is a growth promoter). Check for autoimmune thyroiditis

Exam Pitfall

Scintigraphy is only indicated when TSH is LOW + nodule is present ^[2]^[3]^[4]. If TSH is normal or high, scintigraphy adds nothing — the nodule cannot be "hot" because TSH is not suppressed, so the result will not change management. Ordering scintigraphy with normal TSH wastes money and exposes the patient to radiation.

USG of thyroid: 7.5 or 10 MHz linear probe, B-mode ^[2]^[3]

Why USS for everyone?

Readily available, non-invasive, no radiation, high sensitivity ^[2]^[3]
Functions as an extension of the physical examination — can detect non-palpable nodules, characterise nodule morphology, and evaluate cervical lymph nodes (especially Level VI nodes which are too deep to palpate) ^[2]^[5]
Guides FNAC to the most suspicious area within a nodule ^[2]^[3]

What USS is NOT:

NOT a screening test for healthy asymptomatic subjects — it is too sensitive (would detect clinically insignificant nodules in > 30% of the population), leading to unnecessary anxiety and procedures ^[2]^[3]
NOT a confirmatory diagnostic test — it guides (not confirms) the diagnosis ^[2]^[3]. The definitive answer comes from cytology/histology.

What to systematically assess on USS:

Domain	Features to evaluate	What each feature means
The nodule itself
— Echogenicity	Hypoechoic vs isoechoic vs hyperechoic	Hypoechoic = suspicious (less organised tissue reflects less sound back); hyperechoic/isoechoic = more reassuring ^[2]^[4]
— Composition	Solid vs cystic vs mixed	Solid = higher risk; purely cystic = almost always benign (< 1%); spongiform (> 50% microcystic component) = very low risk ^[1]^[4]
— Shape	Taller than wide vs wider than tall	Taller than wide (AP > transverse) = suspicious — malignant nodules grow perpendicular to tissue planes (infiltrative growth pattern) ^[2]^[4]^[5]
— Margins	Smooth vs irregular/lobulated vs ill-defined	Irregular/microlobulated = suspicious — suggests infiltrative growth without a clear capsule ^[4]
— Calcifications	Microcalcifications vs macrocalcifications vs rim calcification	Microcalcifications (< 0.2 mm, punctate echogenic foci without shadowing) = highly suspicious — represent psammoma bodies of papillary CA ^[2]^[3]^[4]. Macrocalcifications and comet-tail artefacts are reassuring (represent degenerative changes/colloid crystals)
— Perilesional halo	Complete vs incomplete/absent	Complete halo = compressed normal thyroid tissue forming a pseudocapsule → reassuring (well-demarcated benign nodule). Absent/incomplete halo = suspicious (loss of demarcation suggests invasion) ^[2]^[3]
— Vascularity	Intranodular (central) vs peripheral	Intranodular/chaotic central vascularity = suspicious (neoangiogenesis within tumour). Peripheral vascularity is more typical of benign compressed rim ^[4]^[5]
— Local invasion	Into strap muscles, trachea, oesophagus, carotid	Direct evidence of malignancy — extrathyroidal extension (ETE) ^[2]^[4]
Surrounding thyroid	Other nodules, parenchymal abnormalities	Multiple nodules suggest MNG (slightly reassuring for per-nodule risk). Diffuse heterogeneous hypoechogenicity → Hashimoto's ^[2]^[3]
Cervical lymph nodes	See table below	Critical — Level VI nodes cannot be palpated; USS is the only way to detect them pre-operatively ^[2]^[5]
Retrosternal extension	Lower pole extending behind sternum	If present, will need CT for full assessment (USS cannot see mediastinum) ^[5]

Suspicious lymph node features on USS:

Feature	Why it suggests malignancy
Loss of fatty hilum	Normal LN has echogenic fatty hilum; metastatic tumour replaces it
Microcalcification within LN	Pathognomonic for papillary CA metastasis (psammoma bodies deposited in LN)
Round shape (L:S ratio → 1)	Normal LN = oval/elongated; metastatic expansion makes it round
Peripheral / chaotic vascularity	Normal hilar flow pattern destroyed by tumour neoangiogenesis
Hyperechoic	Thyroglobulin/colloid within metastatic thyroid tissue
Cystic change	Necrosis within large metastatic deposit
Size > 2 cm	Large nodes more likely pathological ^[4]

Mnemonic for suspicious USS features of the nodule: "SHIT CME" — Solid, Hypoechoic, Irregular margin, Taller-than-wide, Chaotic central vascularity, Microcalcifications, Extrathyroidal extension. Most important are solid & hypoechoic ^[5].

High vs Low risk USS features — Summary Table ^[4]:

Feature	High risk of cancer	Low risk of cancer
Echogenicity	Hypoechoic	Hyperechoic
Calcifications	Microcalcifications	Large coarse calcifications
Shape	Taller than wide	Wider than tall
Margins	Irregular margins, incomplete halo	Spongiform appearance, comet-tail shadowing
Vascularity	Central (intranodular)	Peripheral

The ATA 2015 guidelines ^[1]^[6] classify nodules into 5 sonographic patterns, each with a specific estimated malignancy risk and FNA size cutoff:

ATA Pattern	USS Description	Risk of Malignancy	FNA Size Cutoff
High suspicion	Solid hypoechoic nodule (or solid hypoechoic component of partially cystic nodule) + ≥ 1 of: microcalcifications, rim calcification with extrusive soft tissue, taller-than-wide, irregular margins, extrathyroidal extension	> 70–90%	≥ 1 cm
Intermediate suspicion	Hypoechoic solid nodule with smooth margins WITHOUT microcalcifications, taller-than-wide, or ETE	10–20%	≥ 1 cm
Low suspicion	Isoechoic or hyperechoic solid nodule, or partially cystic with eccentric solid area WITHOUT suspicious features	5–10%	≥ 1.5 cm
Very low suspicion	Spongiform or partially cystic without any suspicious features	< 3%	≥ 2 cm (or observe)
Benign	Purely cystic nodule (no solid component)	< 1%	No FNA

Why these specific size cutoffs?

The size thresholds exist because tiny nodules — even if they look suspicious — have a very low absolute risk of clinically significant cancer. A 5 mm papillary microcarcinoma, for example, is almost universally indolent and may never cause harm. The cutoffs balance the risk of missing a clinically significant cancer against the risk of over-investigating incidental findings. As suspicion decreases, the size threshold increases because you need a larger nodule to justify the invasiveness of FNAC.

FNAC is the single most important investigation for a thyroid nodule when TSH is not suppressed ^[2]^[3].

The name tells you the technique: "Fine needle" = 23–27G needle (thin, minimises bleeding); "aspiration" = suction applied to collect cells; "cytology" = examination of individual cells (not tissue architecture).

Technical details:

Process: trans-isthmic approach ± USG guidance ^[2]^[3]
- The needle passes through the isthmus into the target nodule — this provides a stable trajectory and avoids lateral neck structures (carotid, jugular)
- USG guidance advantages: Confirms nodule presence, targets the most suspicious region (e.g. solid component of a mixed cystic-solid nodule), avoids cystic areas (which yield non-diagnostic aspirates) ^[2]^[3]
Accuracy: 90–95% ^[2]^[3] — can avoid unnecessary diagnostic thyroidectomy
Not a core needle biopsy — core needle biopsy is NOT performed on the thyroid because the gland is extremely vascular → risk of massive bleeding. FNAC is sufficient because it is very accurate at identifying thyroid cancer types ^[4]

Why FNAC cannot distinguish follicular adenoma from follicular carcinoma:

Follicular carcinoma is defined by capsular or vascular invasion — a histological diagnosis requiring assessment of the tumour–capsule interface on tissue sections ^[2]^[3]
FNAC aspirates individual cells, not tissue architecture → cannot see the capsule → cannot assess invasion
This is why Bethesda IV ("follicular neoplasm") requires surgical excision (hemithyroidectomy) for definitive diagnosis ^[1]^[2]^[3]

Indications for FNAC ^[1]^[2]^[3]^[4]:

Indication	Explanation
Nodule meeting ATA sonographic criteria (see table above)	Risk-stratified approach
Hypofunctioning ("cold") nodule on scintigraphy	10–20% malignancy risk ^[4]
Dominant or atypical nodule in MNG	Cannot assume all nodules in MNG are benign ^[4]^[5]
Nodules associated with abnormal cervical LN	Suspicious for metastatic thyroid cancer ^[4]
Complex or recurrent cystic nodules	May harbour papillary CA in the solid component ^[4]
Symptomatic / large cysts	Therapeutic aspiration (decompression) ^[2]^[3]

When you can bypass FNAC and proceed directly to surgery ^[2]^[3]:

Nodule > 4 cm — very large nodules have higher malignancy risk and are usually excised regardless of cytology
Gross extrathyroidal invasion on imaging
Cervical LN metastasis confirmed (e.g. by FNA of LN with thyroglobulin washout)

FNAC complications: Pain, bleeding (usually self-limiting), false negative (~3%), non-diagnostic aspirate ^[5]

Standard reporting system for thyroid FNAC ^[1]^[2]^[3]^[4]^[5]. The following table integrates information from both the lecture slides and senior notes:

Bethesda Category	Diagnostic Category	Risk of Ca (%)	Usual Management	Key Cytological Features
I	Non-diagnostic / Unsatisfactory	1–4	Repeat FNA; or OT if radiologically suspicious ^[1]^[2]^[3]	< 6 groups of 10 well-preserved follicular cells; excessive blood; cyst fluid only
II	Benign	0–3	Clinical follow-up ^[1]^[2]^[3]	Colloid nodule (abundant colloid, benign follicular cells); Hashimoto's (lymphocytes + Hürthle cells); granulomatous thyroiditis
III	AUS or FLUS	5–15	Repeat FNA; molecular testing; HemiT if AUS ×2 ^[1]^[2]^[3]	Atypical cells that don't meet criteria for categories IV–VI; nuclear atypia or architectural atypia of uncertain significance
IV	Follicular neoplasm / Hürthle cell neoplasm	15–30	Hemithyroidectomy; ± molecular testing ^[1]^[2]^[3]^[6]	Microfollicular pattern, scant colloid, cellular specimen; CANNOT distinguish adenoma from carcinoma
V	Suspicious for malignancy	60–75	HemiT + frozen section → completion TT ^[1]^[2]^[3]	Features suggestive but not definitive: e.g. some but not all papillary CA nuclear features
VI	Malignant	97–99	Total thyroidectomy ^[1]^[2]^[3]	Definitive papillary CA (Orphan-Annie nuclei, nuclear grooves, pseudoinclusions, psammoma bodies); medullary CA (amyloid + calcitonin); anaplastic CA (pleomorphic bizarre cells)

Deep dive — Category-specific management nuances:

Bethesda I (Non-diagnostic):

Repeat FNA in 4–6 weeks (allows haematoma to resolve) ^[5]
If still non-diagnostic after repeat: Decision depends on risk profile ^[5]:
- Low clinical risk → observation
- High clinical risk → hemithyroidectomy
- High risk features: Suspicious USS features; FHx of thyroid CA / prior neck radiation; voice hoarseness / hard-irregular-fixed mass / cervical LN positive; growing nodule ^[5]

Bethesda IV (Follicular neoplasm) — Frozen section:

Frozen section is NOT routinely performed during hemithyroidectomy for follicular neoplasm ^[2]^[3]^[6]
Diagnostic information in only 13% of cases
Modifies surgical procedure in only 3.3%
Misguided intervention in 5% (i.e. leads to unnecessary total thyroidectomy)
The correct approach: Wait for final histology after lobectomy ^[2]^[3]^[6]
- If encapsulated and minimally invasive (< 5 vessel invasion + no wide invasion) → lobectomy is curative
- If widely invasive → completion total thyroidectomy + RAI ablation (due to ↑ risk of distant metastases) ^[2]^[3]

Must Know — Frozen Section in FN

Frozen section is NOT helpful in hemithyroidectomy for follicular neoplasm (Bethesda IV). This is directly stated in both the lecture slides and senior notes ^[2]^[3]^[6]. One should wait for the final histology report. Frozen section IS used in Bethesda V (suspicious for malignancy) to determine whether to proceed to completion total thyroidectomy intraoperatively.

Bethesda III (AUS/FLUS) — The grey zone:

"Atypia" here is a morphological description, NOT a premalignant lesion ^[5] — the cytopathologist is saying "these cells look a bit off, but I can't commit to a more specific diagnosis"
Repeat FNA reclassifies the majority to Bethesda II (benign) ^[2]^[3]
Molecular testing (ThyroSeq, Afirma GSC) can help — currently expensive, no universal standards, not readily available ^[2]^[3]

2B. Selective Investigations (NOT Routine)

Investigation	Routine?	Indication	What it tells you
Thyroid scintigraphy	✗	Only if TSH is LOW + nodule present ^[1]^[2]^[3]	Functional status of individual nodules (hot vs cold)
CT scan	✗	Only for: (1) Retrosternal goitre, (2) Locally advanced thyroid cancer ^[5]	Anatomical extent, relationship to great vessels, tracheal compression, surgical planning
PET scan	✗	No diagnostic role at all ^[5]	—
CXR	✗	Retrosternal goitre, thoracic inlet assessment	Tracheal deviation, mediastinal mass
Flow-volume loop (spirometry)	✗	Suspected upper airway obstruction from large goitre	UAO results in a blunted flow-volume loop (plateau on both inspiratory and expiratory limbs → fixed obstruction; or variable if extrathoracic) ^[2]^[3]
Direct laryngoscopy	✗	Pre-operative assessment; suspected RLN palsy (hoarseness)	Documents vocal cord mobility — essential before thyroidectomy ^[2]^[3]
OGD	✗	Suspected oesophageal involvement	Direct visualisation of oesophageal compression/invasion ^[2]^[3]

Radio-isotope scintigraphy (¹²³I or ⁹⁹ᵐTc) ^[1]^[7]

Principle:

Radioactive iodine is handled in the same manner as normal iodine by the thyroid ^[7]
Radiopharmaceuticals:
- ⁹⁹ᵐTc-pertechnetate: Taken up by the sodium-iodide symporter (NIS) due to similar ionic size to iodide — iodine trapping only (not organified) ^[7]
- ¹²³I or ¹³¹I: Trapping + organification — more physiological but more expensive and involves higher radiation ^[7]
Level of uptake reflects metabolic activity → detected by gamma camera
Images obtained at anterior, left anterior oblique (LAO), and right anterior oblique (RAO) views ^[7]

Clinical indications ^[1]^[7]:

Assessment of thyroid nodules, goitre, thyroid cancer
Evaluation of ectopic thyroid
Diagnosis of causes of thyrotoxicosis or hypothyroidism
Post-surgery/radiotherapy assessment of residual thyroid gland

Interpretation:

Finding	Definition	Malignancy risk	Next step
Hot nodule	Uptake > surrounding thyroid tissue	< 1%	No FNAC needed; manage as toxic nodule ^[4]
Cold nodule	Uptake < surrounding thyroid tissue	10–20%	Requires FNAC (if sonographic criteria met) ^[4]
Warm / Indeterminate	Uptake ≈ surrounding tissue	Intermediate	FNAC if USS suspicious

Classic scintigraphy patterns ^[1]:

Condition	Scintigraphy appearance	Why
Graves' disease	Diffuse, homogeneous, increased uptake throughout the gland	TRAb stimulates all follicular cells equally → global ↑ iodine trapping
Toxic adenoma	Single hot focus with suppressed uptake in rest of gland	Autonomous nodule produces excess T4 → ↓ TSH → rest of gland suppressed
Toxic multinodular goitre	Multiple hot and cold areas, patchy uptake	Heterogeneous autonomous function across different nodules
Cold nodule	Photopenic defect (reduced/absent uptake in nodule area)	Non-functioning tissue (could be benign or malignant)

Limitations ^[3]:

Radiation exposure, expensive, low specificity and sensitivity for malignancy ^[1]
Most cold nodules are benign (only 10–20% are cancer) → low positive predictive value
Cannot characterise nodules < 1 cm well (limited spatial resolution)
Should NOT be used if TSH is normal — most cold nodules are benign, and the result would only lead to unnecessary biopsies ^[3]

Not routine — only for specific indications ^[2]^[3]^[5]:

Indication	Why CT/MRI is needed
Retrosternal goitre	1. Cannot be visualised by USS (ultrasound cannot penetrate sternum/mediastinum); 2. Surgical planning (need to know extent, relationship to great vessels); 3. Retrosternal goitre may be malignant ^[5]
Locally advanced thyroid cancer	Important structures within cervical fascia (carotid, jugular, trachea, oesophagus, prevertebral fascia) need better delineation for surgical planning ^[5]

Iodinated contrast warning

The use of iodinated CT contrast may affect post-operative radioactive iodine (RAI) body scan and therapy for 6–8 weeks because the iodine load saturates the thyroid's iodine-trapping capacity ^[2]. If thyroid cancer is suspected and RAI is anticipated, consider non-contrast CT or MRI instead.

Test	Indication	Interpretation
ESR, anti-thyroid antibodies (anti-TPO, anti-Tg)	Suspected thyroiditis (Hashimoto's, de Quervain's) ^[2]^[3]	↑ ESR in subacute thyroiditis; ↑ anti-TPO/Tg in Hashimoto's
Calcitonin	Hx or clinical suspicion of medullary thyroid carcinoma or MEN2 ^[2]^[3]^[5]	95% of MTC produces calcitonin; if > 500 pg/mL → likely metastatic → CT T+A+P + bone scan ^[5]
CEA	Suspected MTC (baseline tumour marker) ^[5]	80% of MTC produces CEA; used alongside calcitonin for follow-up
Thyroglobulin (Tg)	Baseline tumour marker for differentiated thyroid carcinoma (papillary/follicular) ^[4]^[5]	Used post-thyroidectomy for recurrence surveillance. NOT useful for initial diagnosis (elevated in many benign conditions)
Anti-thyroglobulin (anti-Tg) antibodies	Must be measured whenever thyroglobulin is measured ^[4]	Anti-Tg antibodies interfere with Tg assays → can cause falsely low Tg readings. If anti-Tg is positive, Tg cannot be reliably used as a tumour marker
Ca²⁺, PO₄³⁻	Pre-operative parathyroid function assessment ^[4]^[5]	Baseline for detecting post-operative hypoparathyroidism
24h urine metanephrines	Rule out phaeochromocytoma in suspected MEN2 ^[5]	Must exclude phaeo BEFORE thyroidectomy (risk of hypertensive crisis under anaesthesia)
RET proto-oncogene mutation	All patients with MTC ^[4]^[5]	Identifies familial MTC/MEN2 → screen relatives → prophylactic thyroidectomy
Molecular testing (Afirma, ThyroSeq)	Bethesda III/IV — to help reclassify indeterminate cytology ^[2]^[3]	Currently expensive, no universal standards, not readily available ^[2]^[3]

Test	Indication	What it shows
Direct laryngoscopy	Pre-operative vocal cord assessment; suspected RLN palsy ^[2]^[3]	Documents vocal cord mobility. Essential before any thyroidectomy — a pre-existing vocal cord palsy has major implications for surgical approach (contralateral RLN must be preserved at all costs)
OGD	Suspected oesophageal involvement by locally advanced cancer ^[2]^[3]	Direct visualisation of mucosal invasion

Thyroidectomy can be both diagnostic AND therapeutic ^[2]^[3]:

Diagnostic: Bethesda IV (follicular neoplasm) → hemithyroidectomy → final histology determines whether it is adenoma or carcinoma
Therapeutic: Bethesda V–VI → definitive surgical treatment of confirmed/suspected malignancy

If MTC is suspected or confirmed, a specific additional workup is required ^[5]:

Rule out familial disease (25% of MTC is familial):
- Family history
- 24h urine metanephrines (to exclude phaeochromocytoma — MUST be done before surgery!)
- Ca²⁺ and PTH (to exclude parathyroid hyperplasia in MEN2A)
Tumour markers:
- Calcitonin (if > 500 → suspect metastatic disease → staging CT T+A+P + bone scan)
- CEA
RET proto-oncogene mutation analysis
If calcitonin > 500: CT thorax + abdomen + pelvis and bone scan for metastatic workup ^[5]

Why exclude phaeochromocytoma BEFORE thyroidectomy in MEN2?

A patient with an undiagnosed phaeochromocytoma can develop a lethal hypertensive crisis during anaesthesia (catecholamine surge triggered by intubation, surgical manipulation, or stress). If 24h urine metanephrines are elevated, the phaeo must be resected FIRST (or at least alpha-blocked with phenoxybenzamine before surgery).

Once cytology or histology confirms thyroid cancer:

Investigation	Purpose
CT neck and chest	Staging for locally advanced disease / bulky LN; lung metastases (especially follicular CA) ^[5]
Bone scan	If MTC with calcitonin > 500; or follicular CA with bone pain
Whole-body RAI scan	Post-thyroidectomy for differentiated CA (papillary/follicular) — identifies residual/metastatic tissue that takes up iodine

TNM Staging (AJCC 8th edition) ^[5]:

Stage	Description
T staging	T1a: ≤ 1 cm; T1b: 1–2 cm; T2: 2–4 cm; T3: > 4 cm limited to thyroid; T4: extrathyroidal extension
N staging	N1a: Level VI/VII nodes; N1b: Level I–V nodes
Age-dependent staging for differentiated CA	Age < 55: Stage I if M0, Stage II if M1 — never Stage III/IV (because prognosis is excellent in young patients regardless of T/N); Age ≥ 55: Up to Stage IVB
Anaplastic CA	Stage IV automatically (regardless of T, N, or M — because prognosis is uniformly dismal)

The key insight is that investigations are ordered sequentially, not simultaneously. Each test answers a specific question that determines the next test:

Step	Investigation	Question it answers	Determines
1	TFT (TSH)	Is the nodule functioning?	Whether to do scintigraphy (TSH low) or USS (TSH normal/high)
2a	Scintigraphy (if TSH low)	Is the nodule hot or cold?	Whether FNAC is needed (cold → yes; hot → no)
2b	USS (if TSH normal/high, or cold nodule)	What does the nodule look like?	ATA pattern → whether FNAC is needed and at what size threshold
3	FNAC (if USS criteria met)	What are the cells?	Bethesda category → management (observe / repeat / surgery)
4	Selective bloods, CT, laryngoscopy	Staging, pre-op assessment, specific cancer workup	Extent of surgery, need for RAI, MEN2 screening

High Yield Summary — Diagnostic Workup

Routine for ALL patients: History + P/E, TFT (ultrasensitive TSH), USS thyroid/neck ± FNAC.
TSH is the gatekeeper: Low → scintigraphy first; Normal/High → USS + FNAC per ATA pattern.
USS features suspicious for malignancy ("SHIT CME"): Solid, Hypoechoic (most important pair), Irregular, Taller-than-wide, Chaotic central vascularity, Microcalcifications, Extrathyroidal extension.
ATA sonographic patterns with FNAC thresholds: High/Intermediate ≥ 1 cm; Low ≥ 1.5 cm; Very low ≥ 2 cm; Benign (purely cystic) → no FNA.
Bethesda classification: I–VI with escalating malignancy risk (1–4% → 97–99%) and management (repeat FNA → total thyroidectomy).
FNAC is the single most important Ix when TSH is not suppressed; accuracy 90–95%; cannot distinguish follicular adenoma from carcinoma.
Frozen section NOT helpful for Bethesda IV (follicular neoplasm) — wait for final histology.
Scintigraphy: ONLY when TSH low + nodule. Hot = safe (< 1% cancer). Cold = needs FNAC (10–20% cancer).
CT only for retrosternal goitre or locally advanced cancer. PET has NO diagnostic role.
For suspected MTC: Calcitonin, CEA, 24h urine metanephrines (rule out phaeo before surgery!), Ca/PTH, RET mutation analysis.

Active Recall - Diagnostic Criteria and Algorithm

References

^[1] Lecture slides: GC 177. A thyroid nodule benign thyroid nodules; thyroid cancer.pdf (p7, p9, p10, p12, p13) ^[2] Senior notes: Ryan Ho Endocrine.pdf (p17, p19–p20, p32) ^[3] Senior notes: Ryan Ho Fundamentals.pdf (p427–p429) ^[4] Senior notes: felixlai.md (USS features, sonographic criteria for FNA, Bethesda classification, scintigraphy sections) ^[5] Senior notes: maxim.md (Investigations table, SHIT CME, Bethesda classification, approach to multiple nodules, staging, MTC workup) ^[6] Lecture slides: Management of differentiated thyroid carcinoma.pdf (p2, p21) ^[7] Senior notes: Ryan Ho Diagnostic Radiology.pdf (p59–p60)

Management Algorithm & Treatment Modalities

Management of a thyroid nodule is driven entirely by what the workup tells you — specifically the Bethesda category from FNAC and, if malignancy is confirmed, the histological type, staging, and patient risk profile. The key philosophical tension in management is:

Under-treatment = missing a cancer that could have been cured by surgery
Over-treatment = performing a total thyroidectomy (with its lifelong consequences) for what turns out to be a benign lesion

The entire management algorithm is designed to navigate this tension.

2. Management by Clinical Scenario

2A. Bethesda II — Benign Nodule / Non-Suspicious Goitre

This is the most common outcome of FNAC (~60–70% of aspirates). The nodule is cytologically benign — but remember there is still a 0–3% false-negative rate due to sampling error ^[1]^[2]^[5].

Monitor: TFT + neck exam + USS every 12 months for asymptomatic, stable goitre (< 80 mL) ^[2]^[3]
Most benign nodules remain stable or grow very slowly and never need intervention
Repeat FNAC if: > 20% increase in diameter or > 50% increase in volume on follow-up USS

Indications for thyroidectomy in benign thyroid disease ("3Cs" + Cancer) ^[2]^[5]:

Indication	Explanation
Cancer: Confirmed CA or suspicious FNAC (Bethesda IV–VI)	Self-explanatory — primary reason for surgery
Compression: Dysphagia, dysphonia, dyspnoea, retrosternal goitre	Large goitre compresses trachea, oesophagus, or RLN → functional impairment
Cannot be treated medically: Frequent relapses of thyrotoxicosis, when RAI unsuitable or large goitre > 80 g	MNG that recurs after ATD cessation (unlike Graves', MNG does not undergo immunological remission) ^[2]
Cosmesis / Patient's worry	Patient preference for a visible, concerning lump

These are also listed in the lecture slides as indications of treatment for benign thyroid nodules ^[1]:

Symptomatic (size of goitre/nodule)
Increase in goitre size
Trachea compression or deviation
Retrosternal extension
Suspected malignancy
Cosmetic considerations / patient wish

Thyroxine suppression therapy is mostly obsoleted ^[5]:

MoA: Exogenous T4 → ↓ TSH via negative feedback → ↓ TSH-driven thyroid growth → ↓ goitre size
Problems:
1. Controversial benefits
2. Works in < 20% of patients
3. Significant side effects: Iatrogenic subclinical hyperthyroidism → osteoporosis (↑ bone resorption), AF (cardiac risk)
4. Thyroid gland regrows after cessation of thyroxine
Only role: May be considered if ↑ TSH is documented (e.g. Hashimoto's thyroiditis) where replacement is indicated anyway ^[2]^[3]

Why doesn't T4 suppression work well?

In a euthyroid patient, TSH is already normal — so suppressing it further provides minimal additional benefit to goitre size. The nodule's growth may not be TSH-dependent at all (many nodules have autonomous growth driven by somatic mutations). Meanwhile, you are giving the patient all the downsides of subclinical hyperthyroidism.

	Solitary	Multinodular
Euthyroid	Observe; Hemi if 4Cs	Observe; Total if 4Cs
Hyperthyroid	Hemi	Total

Why this pattern?

Solitary + euthyroid: The nodule is the only problem → hemithyroidectomy suffices (removes the disease, preserves contralateral function)
Multinodular + euthyroid: If surgery is needed, total thyroidectomy prevents recurrence in the remaining lobe (recurrence rate with hemiT = 8.4% vs 0.2% with TT) ^[2]
Solitary + hyperthyroid (toxic adenoma): Hemithyroidectomy removes the autonomous nodule; contralateral lobe resumes normal function
Multinodular + hyperthyroid (toxic MNG): Multiple autonomous nodules → must remove entire gland

3. Surgical Treatment Modalities

Type	What is removed	Typical indications	Key considerations
Hemithyroidectomy (lobectomy + isthmectomy)	One lobe + isthmus + pyramidal lobe ^[5]	Bethesda IV (follicular neoplasm); Bethesda V (suspicious); uninodular benign goitre; toxic adenoma ^[1]	Safe, minimal morbidity; future reoperation on contralateral lobe without difficulty; ~10–20% chance of hypothyroidism ^[1]
Total thyroidectomy	Both lobes + isthmus + pyramidal lobe ^[5]	Bethesda VI (malignant); symptomatic MNG; bilateral disease; toxic MNG; completion after hemiT showing malignancy	No recurrence; but lifelong T4 replacement + ↑ risk of hypoparathyroidism (1–2%) ^[1]^[2]
Near-total thyroidectomy	Entire gland except small rim of tissue near RLN/parathyroids on one side	Graves' disease; some differentiated cancers	Reduces risk of bilateral RLN injury while achieving near-complete resection
Subtotal thyroidectomy (bilateral)	> 50% of both lobes + isthmus	Rarely indicated ^[1]	Higher recurrence rate; largely replaced by total thyroidectomy

Lecture slide summary ^[1]:

Unilateral lobectomy (hemithyroidectomy):

For uninodular goitre

Safe, minimal morbidities, diagnosis and cure

Future reoperation on contralateral lobe without difficulty

Around 10–20% chance of hypothyroidism

Total/near-total thyroidectomy (bilateral thyroidectomy):

For symptomatic multinodular goitre

No recurrence/need of reoperation

Additional surgical risk (hypoparathyroidism)

Needs long-term thyroxine replacement

Partial or bilateral subtotal thyroidectomy:

Rarely indicated

Hemi vs Total — The Trade-off

Hemithyroidectomy preserves one functioning lobe → only 10–20% develop hypothyroidism (the remaining lobe compensates). But there is a recurrence rate of 8.4% (disease can develop in the contralateral lobe later, requiring a second operation — which is technically harder due to scar tissue). Total thyroidectomy has a recurrence rate of only 0.2% but guarantees lifelong thyroxine replacement and carries 1–2% risk of permanent hypoparathyroidism ^[2]^[3].

Term	Definition
Total thyroidectomy	Resection of both lobes + isthmus + pyramidal lobe
Subtotal thyroidectomy	Resection of > 50% of both lobes + isthmus
Hemithyroidectomy	Resection of one lobe + isthmus
Lobectomy	Resection of one lobe (isthmus preserved)

Bilateral axillo-breast approach (BABA) — scarless in neck
Transoral vestibular approach
Retro-auricular trans-hairline approach (RATH)
These are cosmetic alternatives to conventional cervical incision; oncological outcomes are equivalent for selected patients

3D. Pre-operative Preparation

Maintain biochemically euthyroid — critical to prevent intra-operative thyroid storm
Vocal cord function assessment by laryngoscopy — documents baseline; if pre-existing unilateral cord palsy, the surgeon must preserve the contralateral RLN at all costs (bilateral palsy = airway emergency)
Monitor Ca²⁺ and vitamin D levels — supplement accordingly (risk of post-op hypoparathyroidism / hungry bone syndrome)

Antithyroid medications until euthyroid → prevents thyroid storm
β-blockers for 2 weeks → controls symptoms until biochemically euthyroid
Lugol's iodine solution for 10 days prior to surgery → mechanism: ↓ thyroid hormone secretion AND ↓ vascularity and size of thyroid gland (Wolff-Chaikoff effect — high iodine load transiently inhibits organification) ^[2]

Why Lugol's solution before surgery?

The thyroid in Graves' disease is extremely vascular (stimulated by TRAb). Lugol's iodine exploits the Wolff-Chaikoff effect — massive iodine load temporarily blocks thyroid hormone synthesis (↓ organification) AND causes involution of the hyperplastic gland, making it smaller, firmer, and less bloody. This makes surgery technically easier and safer. The effect is temporary (the thyroid "escapes" after ~10 days), so surgery must be timed within this window.

These are for benign nodules only — not for primary thyroid cancer treatment ^[1]^[2]^[3]:

Modality	Mechanism	Indication	Notes
Ethanol injection (PEI)	Cytoplasmic protein dehydration / coagulation necrosis ^[1]	Cystic or predominantly cystic nodules ^[1]	Most effective for simple cysts; high recurrence for solid nodules
Radiofrequency ablation (RFA)	Thermal coagulation necrosis (alternating current → tissue heating) ^[1]	Complex or solid benign nodules ^[1]	Under local anaesthesia or sedation; clinic or day procedure ^[1]; not 100% curative ^[2]^[3]
High-intensity focused ultrasound (HIFU)	Thermal coagulation necrosis (focused ultrasound energy) ^[1]	Benign nodule < 5 cm ^[2]	Non-invasive (no needle insertion); under LA or sedation
Percutaneous laser ablation (LA)	Thermal coagulation necrosis (laser energy via fibre) ^[1]	Solid benign nodules	Similar to RFA
Microwave ablation (MWA)	Thermal coagulation necrosis (microwave energy) ^[1]	Solid benign nodules	Similar to RFA

From the lecture slide: Ethanol injection → cytoplasmic protein dehydration/coagulation necrosis → for cystic/predominantly cystic nodules. Thermal ablation (HIFU, RFA, LA, MWA) → thermal coagulation necrosis → for complex or solid nodules. Under LA or sedation; clinic or day procedure ^[1].

Why can't you use ablation for thyroid cancer?

Non-surgical ablation treats tissue in situ without removing it — you cannot assess surgical margins, cannot perform lymph node dissection, and cannot confirm complete tumour destruction. For malignancy, surgical excision with histological margin assessment remains the standard. Ablative techniques are reserved for benign nodules causing symptoms or cosmetic concern where the patient is not a surgical candidate or declines surgery.

5. Radioactive Iodine (RAI, ¹³¹I)

RAI serves different roles depending on the clinical scenario:

Indication: Alternative to surgery for toxic MNG, toxic adenoma, or large goitre in patients with high surgical risk ^[2]^[3]
MoA: Oral ¹³¹I → trapped and organified like normal iodine → emits β radiation → necrosis of follicular cells with fibrosis + disappearance of colloid → ↓ T4 secretion; also ↓ replication of non-destroyed cells ^[2]
Advantages: ↓ cost, ↓ subject to side effects, can be repeated if needed ^[3]
Disadvantages ^[3]:
- Restricted proximity to other persons (especially children at home) — radiation safety precautions
- Slow response (takes weeks–months for full effect)
- Risk of radiation thyroiditis (3%)
- Transition to Graves' disease (5%) — release of thyroid antigens → stimulation of autoimmunity
- Hypothyroidism (15–20%) — progressive destruction of functioning tissue

Used after total thyroidectomy for differentiated thyroid carcinoma (papillary/follicular) to:
1. Ablate residual normal thyroid tissue (remnant ablation) → allows thyroglobulin to be used as a tumour marker
2. Treat microscopic residual cancer / metastases
3. Facilitate post-ablation whole-body scan (diagnostic)
NOT effective for medullary CA (C cells do not trap iodine) or anaplastic CA (undifferentiated cells lose NIS)
Hürthle cell neoplasm is NOT amenable to RAI (Hürthle cells lose NIS expression) → requires surgical treatment ^[5]

Contraindication	Reason
Pregnancy	¹³¹I crosses placenta + concentrated by fetal thyroid (after 12 weeks' gestation) → fetal thyroid destruction
Breastfeeding	¹³¹I secreted in breast milk → radiation exposure to infant
Contemplating pregnancy within 6 months	Transient radiation-induced changes in gametes → avoid conception for 4–6 months post-RAI
Large goitre > 50 mL / large nodules	RAI may worsen goitre acutely (radiation thyroiditis → swelling) → risk of airway obstruction; also need histological assessment
Moderate-to-severe Graves' orbitopathy	RAI can worsen eye disease (release of thyroid antigens → ↑ TRAb → immune stimulation of orbital fibroblasts)

6. Management by Thyroid Cancer Type

6A. Differentiated Thyroid Carcinoma (Papillary / Follicular)

Scenario	Surgery
Low-risk papillary CA (unifocal, < 4 cm, no ETE, N0, M0)	Hemithyroidectomy (lobectomy) may be sufficient
Higher-risk differentiated CA (any of: > 4 cm, multifocal, ETE, N1, M1, aggressive histology)	Total thyroidectomy ± central neck dissection (Level VI)

Indications for total thyroidectomy in differentiated CA ^[5]:

Tumour > 4 cm
Bilateral/multifocal disease
ETE (T4)
N1 or M1 disease
Aggressive histological variants: tall cell, columnar cell, diffuse sclerosing, poorly differentiated papillary CA
Need for post-operative RAI (requires near-total thyroid removal for RAI to work — any residual normal thyroid tissue would preferentially trap the RAI instead of cancer cells)

Indications for total thyroidectomy in differentiated CA — may also accept hemithyroidectomy ^[5]:

1–4 cm unifocal papillary CA without aggressive features
Minimally invasive follicular CA (encapsulated, < 5 vessel invasion, no wide invasion)

From the lecture slide ^[6]:

FNAC → follicular lesion → Hemithyroidectomy → Frozen section not routinely performed

Diagnostic information in 13%

Surgical procedure modified in 3.3%

Misguided intervention in 5%

Wait for final histology:

If encapsulated and minimally invasive (< 5 vessel invasion + no wide invasion) → lobectomy is curative ^[2]^[3]
If widely invasive → completion total thyroidectomy + RAI ablation (due to ↑ risk of distant metastases) ^[2]^[3]

1. RAI ablation (see section 5B above) — for intermediate-to-high-risk patients

2. Thyroxine — Replacement ± Suppression ^[5]

Risk group	Features	TSH target
Low risk	None of the high/intermediate risk features	No TSH suppression (0.5–2.0 mIU/L) — replacement dose only
Intermediate risk	T3, N1, aggressive histology, vascular invasion positive	Low TSH suppression (0.1–0.5 mIU/L)
High risk	T4, M1, incomplete resection	High TSH suppression (< 0.1 mIU/L)

Why TSH suppression? TSH is a growth factor for differentiated thyroid cancer cells (they retain TSH receptors). High TSH stimulates residual tumour growth → suppressing TSH with supraphysiological T4 doses reduces recurrence risk. However, TSH suppression carries risks of subclinical hyperthyroidism (AF, osteoporosis), so the degree of suppression is risk-adapted — low-risk patients do NOT need suppression ^[5].

Neck USS every 6 months
Bloods: TSH and thyroglobulin (on thyroxine suppression) every 3 months
- After total thyroidectomy: Tg < 0.2 ng/mL (virtually undetectable — any residual Tg suggests residual disease)
- After hemithyroidectomy: Tg < 30 ng/mL (remaining lobe produces some Tg)
Anti-thyroglobulin antibodies must be measured alongside Tg (anti-Tg Ab can cause falsely low Tg → unreliable marker)

Total thyroidectomy (even for apparently unilateral disease — high rate of bilateral/multifocal involvement)
Routine central ± lateral neck dissection (depending on calcitonin level and imaging evidence of nodal metastases)
Genetic analysis for RET proto-oncogene → screen asymptomatic relatives → prophylactic thyroidectomy (best done < 5–10 years of age) ^[2]
No good adjuvant therapy — MTC does NOT respond to RAI (C cells don't trap iodine) or TSH suppression (C cells don't have TSH receptors)
Follow-up: T4 replacement to keep euthyroid (NOT TSH suppression); serum calcitonin and CEA at 6 months post-op → ↑ calcitonin → screen for residual/metastatic disease ^[2]
Prognosis: 5-year survival 60–70% ^[2]

Palliative intent — uniformly lethal, median survival < 6 months ^[2]
Chemoirradiation ± surgical debulking
Palliative tracheostomy if airway compromise
NOT responsive to RAI (undifferentiated cells lose NIS expression)
Stage IV automatically regardless of T, N, or M ^[5]

Requires core biopsy (NOT FNAC — need tissue architecture for subtyping)
Treatment: chemotherapy ± radiotherapy (NOT surgical — lymphoma is a systemic disease)
Usually DLBCL or MALT → treat per haematological lymphoma protocols

	Graves' disease	Toxic MNG (Plummer's)	Toxic adenoma
Antithyroid drugs	1st line	Ineffective (recurs upon discontinuation); prolonged use if patient declines RAI/surgery	—
Radioactive iodine	2nd line	Preferred if no 4Cs	Preferred (upfront RAI over ATD because of ↑ iodine uptake in autonomous nodule)
Surgery	2nd line	Preferred if 4Cs	Hemithyroidectomy (if no contralateral nodules)

Why ATD is 1st line for Graves' but not for toxic MNG? ^[2]

In Graves' disease, the underlying autoimmune process may subside after 18 months of ATD therapy (spontaneous immunological remission). Therefore a trial of ATD makes sense before committing to definitive treatment.
In toxic MNG, the autonomous nodules are caused by somatic mutations (not autoimmune) → they will never remit. ATD only controls symptoms while you take it; the moment you stop, the thyrotoxicosis returns. Hence early definitive treatment (RAI or surgery) is preferred ^[2].

Modality	Benign nodule	Toxic nodule	Thyroid cancer	Key advantage	Key limitation
Observation	✓ (stable, asymptomatic)	✗	✗	Non-invasive	Risk of missing slow growth/change
T4 suppression	Mostly obsoleted ^[5]	✗	✓ (post-op, risk-adapted)	Simple, oral medication	Controversial in euthyroid; S/E of subclinical hyperthyroidism
Ethanol injection	✓ (cystic)	✗	✗	Effective for cysts	High recurrence for solid nodules
Thermal ablation (RFA/HIFU/LA/MWA)	✓ (solid/complex, < 5 cm)	✗	✗	Minimally invasive, day procedure	Not 100% curative; no histology ^[2]^[3]
RAI	✓ (high surgical risk)	✓ (preferred for toxic MNG without 4Cs)	✓ (post-TT for differentiated CA)	No surgery, can be repeated	Slow, hypothyroidism, radiation precautions, C/I in pregnancy ^[2]^[3]
Surgery	✓ (4Cs present)	✓ (preferred if 4Cs)	✓ (primary treatment)	Definitive, allows histology	Invasive, RLN/PTH risk

High Yield Summary — Management

Bethesda-guided management: I = Repeat; II = Observe; III = Repeat/Molecular/HemiT if persistent; IV = HemiT (no routine frozen section); V = HemiT + FS → TT; VI = TT.
Indications for thyroidectomy in benign disease (4Cs): Cancer (suspicious FNAC), Compression, Cannot treat medically, Cosmesis.
HemiT vs TT: HemiT for solitary/euthyroid/unilateral disease (10–20% hypothyroidism, 8.4% recurrence). TT for bilateral/multinodular/malignant (lifelong T4, 0.2% recurrence, 1–2% hypoparathyroidism).
Frozen section NOT routinely performed for Bethesda IV (follicular neoplasm) — diagnostic info in only 13%, misguided intervention in 5%. Wait for final histology.
Thyroxine post-cancer: Low risk → no suppression (TSH 0.5–2.0); Intermediate → mild suppression (TSH 0.1–0.5); High risk → aggressive suppression (TSH < 0.1).
Non-surgical ablation (PEI, RFA, HIFU, LA, MWA): For benign nodules only. PEI for cystic; thermal for solid. Day procedure under LA/sedation. Not for cancer.
RAI contraindicated in: Pregnancy, breastfeeding, large goitre > 50 mL, moderate-severe Graves' orbitopathy.
MTC: TT + neck dissection + RET testing + screen relatives. NO RAI (C cells don't trap iodine). NO TSH suppression (C cells don't have TSH receptors).
Anaplastic CA: Palliative only. Chemo-RT ± debulking ± tracheostomy. Stage IV automatically.
Pre-op thyrotoxic patients: ATD until euthyroid + β-blocker 2 weeks + Lugol's iodine 10 days → ↓ vascularity, ↓ size, prevent thyroid storm.

Active Recall - Management of Thyroid Nodules

References

^[1] Lecture slides: GC 177. A thyroid nodule benign thyroid nodules; thyroid cancer.pdf (p7, p10, p12, p14, p15, p16) ^[2] Senior notes: Ryan Ho Endocrine.pdf (p19, p20, p21, p25, p32) ^[3] Senior notes: Ryan Ho Fundamentals.pdf (p427–p429) ^[4] Senior notes: felixlai.md (Bethesda classification, scintigraphy sections) ^[5] Senior notes: maxim.md (Bethesda classification, overview of management, thyroidectomy indications, thyroxine suppression, staging, MTC/anaplastic management, thyrotoxicosis indications) ^[6] Lecture slides: Management of differentiated thyroid carcinoma.pdf (p2, p21)

Complications

Complications in the context of thyroid nodule workup and management fall into three broad categories:

Complications of the thyroid nodule/goitre itself (if left untreated)
Complications of FNAC (the diagnostic procedure)
Complications of thyroidectomy (the definitive treatment) — this is the most exam-relevant category

The surgical complications are overwhelmingly the highest-yield for exams because they involve critical anatomy (RLN, parathyroids, major vessels) and are a favourite viva/OSCE topic.

These are essentially the reasons you intervene (the "4Cs"). If a benign nodule is left alone and grows, or if cancer is missed, the following can occur:

Complication	Mechanism
Tracheal compression / deviation	Large goitre (especially retrosternal) compresses the trachea. Stridor occurs when tracheal lumen is reduced by > 50%. Retrosternal goitres are particularly dangerous because the thoracic inlet is a rigid bony ring — a goitre growing inferiorly has no room to expand except inward against the trachea
Oesophageal compression → dysphagia	Posterior displacement of oesophagus by large goitre or invasive cancer
RLN compression/invasion → dysphonia	Cancer (especially anaplastic or locally advanced papillary) invading the tracheo-oesophageal groove where the RLN runs
Thoracic inlet obstruction (Pemberton's sign)	Retrosternal goitre compresses brachiocephalic veins → venous congestion of head/neck/arms → facial plethora, distended neck veins, cyanosis when arms raised
Haemorrhage into nodule/cyst	Rupture of thin-walled vessels within a nodule → sudden painful enlargement → can cause acute airway compromise if large enough
Malignant transformation / missed malignancy	Long-standing MNG can harbour de-differentiation (especially to anaplastic CA); inadequate follow-up of a "benign" FNAC (0–3% false-negative rate) can miss cancer
Thyrotoxicosis and its complications	Autonomous nodule(s) → excess T3/T4 → AF, osteoporosis, high-output heart failure, thyroid storm

FNAC is a very safe procedure, but not zero-risk ^[5]:

Complication	Mechanism / Explanation
Pain	Needle traverses skin, strap muscles, and thyroid parenchyma — local nerve endings are stimulated. Usually mild and self-limiting
Bleeding / haematoma	Thyroid is a highly vascular organ (supplied by superior and inferior thyroid arteries). A 23–27G needle minimises this risk, which is why core needle biopsy is NOT performed on the thyroid — it would lead to massive bleeding ^[4]
False negative	Sampling error — the needle may miss the malignant area (especially in large nodules with heterogeneous composition, or when the solid component of a mixed cyst is not targeted). This is why USS guidance improves accuracy and why Bethesda II still carries a 0–3% malignancy risk ^[2]^[3]
Non-diagnostic aspirate (Bethesda I)	Insufficient cellularity (< 6 groups of 10 follicular cells) — occurs in ~10–15% of aspirates. Repeat in 4–6 weeks
Rare: needle-track seeding	Extremely rare with fine-needle technique; more of a concern with core biopsy

3. Complications of Thyroidectomy — The Main Event

This is what examiners love. Think of it as: what structures are at risk during surgery, and what happens when they are damaged?

Timing	Complication
Immediate (intraoperative / < 24 hours)	Haemorrhage / haematoma; RLN injury; SLN injury; Vagus nerve injury; Oesophageal injury; Tracheal injury; Thyroid storm; Tracheomalacia
Intermediate (1 day – 1 month)	Wound infection; Seroma; Hypocalcaemia / hypoparathyroidism; Dysphagia
Late (> 1 month)	Hypothyroidism; Permanent hypoparathyroidism; Recurrence; Hypertrophic scar / keloid

3B. Detailed Complication-by-Complication Breakdown

Incidence: ~1.25% ^[2]

Anatomy/Mechanism: The thyroid bed is in the paratracheal region beneath the strap muscles. Post-operative bleeding (usually from a small arterial or venous bleeder) accumulates in this confined space → venous obstruction → acute laryngeal oedema → airway compromise ^[2]^[5].

Why is this so dangerous? The neck is a closed fascial compartment. Even a small haematoma (50–100 mL) can compress the jugular veins → venous congestion of the larynx → massive oedema of the supraglottic structures → the patient cannot breathe. This can kill within minutes.

Signs: Large, tense, firm, immobile neck swelling + dyspnoea ^[2]. The swelling develops rapidly (usually within 6–12 hours post-op). The patient looks distressed, may have stridor, and the wound is bulging.

Emergency Management ^[2]^[4]^[5]:

Cut subcuticular stitches AND stitches holding strap muscles — this is done at the bedside immediately, not waiting for an operating theatre
Evacuate all blood (scoop out clots manually)
Call seniors for intubation — patient may need re-intubation or emergency tracheostomy
Return to operating theatre for definitive haemostasis

Exam Favourite — Bedside Wound Opening

If you are an F1/F2 on the ward and a post-thyroidectomy patient develops a rapidly expanding neck swelling with stridor — do NOT wait for senior review. Open the wound at the bedside by cutting the skin and strap muscle stitches. This is a life-saving manoeuvre that buys time for the airway team to arrive. Every surgical trainee must know this.

Seroma: Superficial, soft, mobile, fluctuant collection. Self-limiting — no emergency. Distinguished from haematoma by being non-tense and developing more gradually ^[2].

Incidence: < 1% for transient; permanent injury is rarer ^[2]

Anatomy: The RLN runs in the tracheo-oesophageal groove on each side, close to the posterior aspect of the thyroid lobe and the ligament of Berry. It enters the larynx at the cricothyroid joint. The RLN supplies all intrinsic muscles of the larynx EXCEPT the cricothyroid (which is supplied by the external branch of the SLN) ^[4].

Types of injury:

Transient (tractional) — due to stretching, handling, or oedema from heat (diathermy). Typically recovers over weeks to months
Permanent (transection) — irreversible

Clinical presentation depends on whether the injury is unilateral or bilateral ^[4]^[5]:

	Unilateral RLN Injury	Bilateral RLN Injury
Mechanism	One vocal cord paralysed	Both vocal cords paralysed
Vocal cord position	Paramedian (cadaveric position) — denervated cord drifts to midline	Both cords in paramedian/adducted position (6 adductors overpower 2 abductors when both RLNs are damaged) ^[5]
Symptoms	Hoarseness, breathy voice, ineffective cough ^[4]	Stridor, dyspnoea, airway obstruction ^[4]^[5]
Why?	One cord is immobile → incomplete glottic closure → air leak during phonation (hoarse voice) + weak cough (cannot build subglottic pressure)	Both cords adducted → narrow glottis → cannot abduct for inspiration → airway obstruction
Risk of aspiration	↑ Risk of aspiration pneumonia (incomplete glottic closure during swallowing) ^[4]	Aspiration risk + airway compromise
Immediate Mx	Voice therapy; if persistent > 12 months → medialization procedures (e.g. injection thyroplasty — inject fat/Teflon into paralysed cord to improve apposition with contralateral cord; or open thyroplasty with Gore-Tex implant) ^[5]	Require immediate re-intubation ± tracheostomy upon extubation ^[2]

Why '6 adductors > 2 abductors' in bilateral RLN injury?

The larynx has more adductor muscles (lateral cricoarytenoid, thyroarytenoid, interarytenoid — 3 pairs = 6 muscles) than abductors (posterior cricoarytenoid — 1 pair = 2 muscles). When the RLN is damaged, ALL intrinsic muscles except cricothyroid lose their nerve supply. However, the adductors have greater bulk and resting tone, so the cords default to a near-midline (adducted) position — fine for phonation (voice may be surprisingly normal) but terrible for breathing, because the airway is now a narrow slit. This is why bilateral RLN injury causes stridor and airway obstruction rather than aphonia ^[5].

Anatomy: The EBSLN runs with the superior thyroid artery near the upper pole of the thyroid. It innervates the cricothyroid muscle, which tenses (lengthens) the vocal cord to produce high-pitched sound ^[4]^[5].

Clinical presentation ^[4]^[5]:

Vocal fatigue
Loss of high-pitched voice (cannot sing high notes)
Poor volume
Easy voice fatigue

Clinical significance: This injury is more subtle than RLN injury — the patient can still speak, but cannot project or sing. Important to ask pre-operatively if the patient is a professional singer or public speaker ^[2] — the surgical approach to the upper pole must be modified to protect the EBSLN.

Incidence ^[2]^[5]:

Transient: 10–20% (especially from ischaemia in benign surgery)
Permanent: 1–4% (especially in cancer surgery requiring extensive dissection)

Anatomy/Mechanism: The 4 parathyroid glands sit on the posterior capsule of the thyroid lobes, supplied by the inferior thyroid artery ^[5]. During thyroidectomy, they can be:

Devascularised — ligation of the inferior thyroid artery disrupts their blood supply → ischaemic necrosis
Inadvertently removed — small glands embedded in thyroid tissue may be excised with the specimen
Bruised/contused — surgical handling causes swelling and temporary dysfunction

This results in ↓ PTH → ↓ serum calcium (PTH normally maintains calcium by stimulating renal reabsorption, bone resorption, and vitamin D activation).

Only occurs in total/subtotal thyroidectomy — hemithyroidectomy preserves the contralateral parathyroid glands ^[5].

Clinical features — Mnemonic: CATS GO NUMB ^[2]:

Letter	Feature	Mechanism
C	Convulsions	Hypocalcaemia → ↓ threshold for neuronal firing → seizures
A	Arrhythmia	Hypocalcaemia → prolonged QT interval → risk of Torsades de Pointes
T	Tetany	Hypocalcaemia → ↑ neuromuscular excitability → sustained muscle contraction
S	Laryngospasm	Hypocalcaemia → spasm of laryngeal muscles → acute upper airway obstruction. Severe hypoCa can require emergency intubation or surgical airway ^[5]
G	Go	—
N	Numbness — perioral and acral (distal extremity) paraesthesia	Hypocalcaemia → ↑ excitability of sensory nerve fibres → spontaneous firing → tingling around the mouth and in the fingers/toes. This is the earliest symptom ^[4]^[5]
U	—	—
M	Muscle spasms / cramps, carpopedal spasm	Hypocalcaemia → spontaneous contraction of hand/foot muscles (main d'accoucheur = obstetrician's hand)
B	—	—

Clinical signs:

Chvostek's sign: Tap over the facial nerve (anterior to ear, below zygomatic arch) → ipsilateral facial muscle twitching. Positive in ~10% of normocalcaemic individuals (not very specific)
Trousseau's sign: Inflate BP cuff above systolic pressure for 3 minutes → carpopedal spasm (main d'accoucheur). More specific than Chvostek's

Investigations ^[2]^[4]:

Serum corrected Ca²⁺ (or ionised Ca²⁺) — check routinely post-operatively (Day 1)
Serum PTH level — if low/undetectable, confirms hypoparathyroidism
ECG: Prolonged QT interval ± arrhythmia

Management ^[4]^[5]:

Severity	Treatment	Rationale
Acute / symptomatic	IV 10–20 mL of 10% calcium gluconate over 10 minutes (slow bolus) ^[4]	Rapidly raises ionised calcium; must be slow to avoid cardiac arrhythmia. Calcium gluconate preferred over calcium chloride (less tissue necrosis if extravasated)
Ongoing replacement	Oral calcium carbonate + Calcitriol (active vitamin D) ^[4]^[5]	Calcium for direct supplementation; calcitriol (1,25-dihydroxyvitamin D₃) because PTH normally activates the 1α-hydroxylase enzyme in the kidney that converts 25-OH-D to active calcitriol. Without PTH, the patient cannot activate vitamin D endogenously → must give the active form directly

When: Post-thyroidectomy in patients with pre-operative hyperthyroidism (especially longstanding Graves' or toxic MNG) ^[2]^[4]^[5]

Mechanism: Hyperthyroidism causes chronic ↑ bone turnover (thyroid hormone stimulates both osteoblasts and osteoclasts, but net effect is bone resorption → osteoporosis). After thyroidectomy, there is a sudden drop in thyroid hormone AND PTH (the latter from surgical manipulation of parathyroids) → osteoblastic activity suddenly predominates → massive calcium deposition into demineralised bone → rapid, profound, refractory hypocalcaemia ^[2]^[4]^[5].

Key difference from hypoparathyroidism: In hungry bone syndrome, the hypocalcaemia can be very severe and difficult to correct even with IV calcium because bone is acting as a "calcium sink." PTH may actually be elevated (appropriate response to hypocalcaemia) — unlike hypoparathyroidism where PTH is low/undetectable.

Management: Aggressive calcium + vitamin D supplementation; may require prolonged IV calcium infusion ^[5].

Mechanism: In patients with longstanding untreated or inadequately treated hyperthyroidism, surgical manipulation of the thyroid gland causes massive release of stored T3/T4 into the bloodstream → amplified sympathetic response (thyroid hormones have a permissive effect on catecholamine sensitivity — they upregulate β-adrenergic receptors) ^[4].

Clinical features ^[4]:

Hyperpyrexia (temperature > 40°C)
Tachycardia (often > 140 bpm) → can progress to atrial fibrillation
Hypertension (initially) → followed by hypotension and heart failure
Arrhythmia
Agitation, confusion, delirium, seizures
Multi-organ failure if untreated

Prevention: This is why all thyrotoxic patients must be rendered euthyroid pre-operatively with antithyroid drugs + β-blockers + Lugol's iodine ^[2].

Management: β-blockers (propranolol — high dose IV), cooling blankets, IV fluids, hydrocortisone (blocks T4 → T3 conversion), antithyroid drugs (PTU preferred in storm as it also blocks peripheral T4 → T3 conversion), supportive ICU care.

Mechanism: A long-standing, large goitre chronically compresses the trachea → degeneration of tracheal cartilage rings → loss of structural rigidity. When the compressing goitre is suddenly removed at surgery, the weakened tracheal wall collapses inward → airway obstruction ^[2]^[4]^[5].

Presentation: Stridor and dyspnoea upon extubation (or shortly after), despite patent vocal cords.

Management: Re-intubation; tracheostomy may be required temporarily. The tracheal wall may regain rigidity over weeks if the cartilage was not irreversibly destroyed.

Complication	Notes
Wound infection	Uncommon in clean surgical fields; thyroidectomy is a clean (Class I) wound ^[5]. Note: one source states wound infection is NOT a recognised complication ^[5] because of the clean surgical field — examiners may challenge you on this
Seroma	Superficial fluid collection; self-limiting; usually does not need drainage
Oesophageal injury	Rare; from posterior dissection; can cause mediastinitis if not recognised
Tracheal injury	Rare; from aggressive retraction or in locally advanced cancer
Hypothyroidism (late)	Guaranteed after total thyroidectomy → lifelong T4 replacement. After hemithyroidectomy: 10–20% ^[1]. Early post-operative hypothyroidism can also occur transiently
Recurrence	After hemithyroidectomy for benign disease: 8.4%. After total thyroidectomy: 0.2% ^[2]^[3]. After hemithyroidectomy for cancer: may require completion thyroidectomy
Hypertrophic scar / keloid	Anterior neck incision (Kocher incision) in a cosmetically sensitive area; more common in patients with keloid tendency
Dysphagia (post-operative)	Reason unclear; usually resolves spontaneously ^[2]

Complication	Mechanism
Hypothyroidism	Progressive destruction of functioning thyroid tissue. Permanent in 10–15% in first 2 years, then 3% per year ^[2]
Radiation thyroiditis (3%)	Acute inflammation from radiation damage → painful, tender thyroid; self-limiting
Transition to Graves' disease (5%)	Release of thyroid antigens from damaged follicles → stimulation of autoimmunity → ↑ TRAb → can worsen pre-existing orbitopathy ^[2]
Fetal risk	¹³¹I crosses placenta; concentrated by fetal thyroid after 12 weeks gestation → fetal thyroid destruction. Absolutely contraindicated in pregnancy ^[2]
Radiation exposure to others	Requires isolation, restricted proximity to children/pregnant women for several days post-dose
Sialadenitis (with higher therapeutic doses)	Salivary glands also express NIS → concentrate ¹³¹I → inflammation → dry mouth, pain, swelling

If a patient develops dyspnoea after thyroidectomy, you must rapidly differentiate between causes because some are immediately life-threatening:

Cause	Mechanism	Key distinguishing features
Haematoma	Bleeding → venous obstruction → laryngeal oedema	Tense neck swelling; rapid onset (hours); open wound at bedside immediately
Bilateral RLN injury	Both cords adducted → airway obstruction	Stridor upon extubation; no neck swelling; diagnosed by laryngoscopy
Laryngospasm from hypocalcaemia	↓ Ca²⁺ → ↑ neuromuscular excitability → spasm of laryngeal muscles	Perioral/acral numbness preceding; positive Chvostek's/Trousseau's; check Ca²⁺
Tracheal injury / pneumothorax	Direct surgical trauma	Subcutaneous emphysema; CXR shows pneumothorax
Tracheomalacia	Weakened tracheal cartilage collapses after goitre removal	Stridor; history of long-standing large goitre; no swelling; may need re-intubation

High Yield Summary — Complications

Post-operative haematoma is uncommon (1.25%) but potentially fatal → airway compromise from venous obstruction → laryngeal oedema. Immediate bedside management: cut subcuticular and strap muscle stitches, evacuate blood, call for intubation.
RLN injury (< 1%): Unilateral → hoarseness, weak cough, aspiration risk; managed with cord medialization. Bilateral → stridor, airway obstruction (6 adductors > 2 abductors); requires re-intubation ± tracheostomy.
EBSLN injury: Loss of high-pitched voice, vocal fatigue — ask pre-op if patient is a singer/speaker.
Hypocalcaemia/hypoparathyroidism: Most common complication of total thyroidectomy (transient 10–20%, permanent 1–4%). S/S: CATS GO NUMB. Earliest symptom = perioral numbness. Severe → laryngospasm (emergency!). Mx: IV calcium gluconate (acute); oral calcium + calcitriol (ongoing).
Hungry bone syndrome: Severe refractory hypocalcaemia post-op in hyperthyroid patients — sudden ↓ thyroid hormone → bone avidly takes up calcium. PTH may be elevated (cf. hypoparathyroidism where PTH is low).
Thyroid storm: Prevented by pre-op euthyroidism (ATD + β-blocker + Lugol's iodine). Features: hyperpyrexia, tachycardia, hypertension → heart failure.
Tracheomalacia: Tracheal cartilage weakened by long-standing goitre → collapses when goitre removed.
Post-op dyspnoea DDx: Haematoma, bilateral RLN injury, laryngospasm (hypoCa), tracheal injury/pneumothorax, tracheomalacia.
FNAC complications: Pain, bleeding (why core biopsy is NOT done — thyroid is highly vascular), false negative.
RAI complications: Hypothyroidism (progressive), radiation thyroiditis, worsening Graves' orbitopathy, fetal risk (C/I in pregnancy).

Active Recall - Complications of Thyroid Nodule Workup and Management

References

^[1] Lecture slides: GC 177. A thyroid nodule benign thyroid nodules; thyroid cancer.pdf (p10, p12, p15, p17) ^[2] Senior notes: Ryan Ho Endocrine.pdf (p19, p20, p21, p22) ^[3] Senior notes: Ryan Ho Fundamentals.pdf (p427–p429) ^[4] Senior notes: felixlai.md (Complications of thyroidectomy table, FNAC section) ^[5] Senior notes: maxim.md (Post-thyroidectomy complications, RLN/EBSLN injury, parathyroid injury, post-op dyspnoea DDx)

Thyroid Nodule Workup (uss + Bethesda Fnac)

1. Definition

2. Epidemiology

2.1 Prevalence

2.2 Demographics

2.3 Hong Kong context

3. Risk Factors for Thyroid Malignancy

4. Anatomy and Function (Relevant to Workup)

4.1 Thyroid gland anatomy

4.2 Critical adjacent structures

4.3 Thyroid cell types

4.4 Thyroid function — why we check TSH first

5. Etiology / Pathology of Thyroid Nodules

5.1 Benign nodules (~85–90%)

Nodular / Multinodular goitre (MNG)

Follicular adenoma

Cysts

Thyroiditis

5.2 Malignant nodules (~10–15%)

6. Classification Systems

6.1 ACR TI-RADS (Thyroid Imaging Reporting and Data System)

6.2 ATA (American Thyroid Association) 2015 Sonographic Pattern Classification

6.3 "SHIT CME" Mnemonic for Suspicious USS Features

6.4 Suspicious Lymph Node Features on USS

7. Pathophysiology — Connecting It All

7.1 Why do thyroid nodules form?

7.2 Why can't FNAC distinguish follicular adenoma from follicular carcinoma?

7.3 Why are "hot" nodules rarely malignant?

7.4 Why do psammoma bodies cause microcalcifications on USS?

7.5 Why "taller than wide"?

8. Clinical Approach to a Thyroid Nodule

9. Clinical Features

9.1 Symptoms

9.2 Signs

9.3 Physical examination — systematic approach [2]

10. Clinical Features Suggesting ↑ Risk of Malignancy — Summary [3][5]

11. Ultrasound of the Thyroid — Deep Dive

11.1 Technical aspects

11.2 What to look for

11.3 USS-guided FNAC — Indications (ATA 2015) [1][6]

11.4 Approach to Multiple Nodules [5]

12. Bethesda System for Reporting Thyroid Cytopathology

12.1 Overview

12.2 The Six Categories

12.3 Deep Dive — Each Category Explained

Bethesda I: Non-diagnostic

Bethesda II: Benign

Bethesda III: AUS / FLUS

Bethesda IV: Follicular Neoplasm (FN) / Hürthle Cell Neoplasm

Bethesda V: Suspicious for Malignancy

Bethesda VI: Malignant

12.4 Newer Adjuncts — Molecular Testing

13. Thyroid Scintigraphy — When and Why

13.1 Principle

13.2 Clinical Use

13.3 Limitations

14. Putting It All Together — The Complete Workup Algorithm

15. Differential Diagnosis of an Anterior Neck Lump [3][8]

Active Recall - Thyroid Nodule Workup (USS + Bethesda FNAC)

References

Organising Framework

1. Differential Diagnosis of an Anterior Neck Lump

2. Differential Diagnosis of a Thyroid Nodule — By Morphological Pattern

2A. Solitary Thyroid Nodule

2B. Multiple Thyroid Nodules

2C. Diffuse Thyroid Enlargement (Goitre)

3. Differential Diagnosis — By Thyroid Functional Status

4. Differential Diagnosis — By Pathological Nature

4A. Non-Neoplastic Nodules (~70%) [1]

4B. Benign Neoplasms (~15%) [1]

4C. Primary Thyroid Malignancies (~10%) [1][5]

4D. Metastatic Thyroid Disease (Rare) [5]

4E. Miscellaneous (~5%) [1]

5. The "Rapid-Fire" Differential Diagnosis Table — By Presentation Pattern

6. How Each Investigation Narrows the Differential

7. Specific Clues — Matching History/Exam to Differential

8. Approach to Multiple Nodules — Special Considerations [5]

9. Things That Look Like Thyroid Nodules But Aren't

Active Recall - Differential Diagnosis of Thyroid Nodule

Preamble — What Are We Diagnosing?