• Location: C5–C7 vertebral level ^[5]
• Butterfly-shaped gland, composed of two lateral lobes connected by an isthmus
• Enclosed by:
  • True capsule (thin fibrous capsule adherent to the gland)
  • False capsule (formed by the pretracheal layer of the deep cervical fascia) ^[5]
• A pyramidal lobe may be present (embryological remnant of the thyroglossal duct), extending superiorly from the isthmus ^[5]

• Superior thyroid artery (first branch of the external carotid artery)
• Inferior thyroid artery (from the thyrocervical trunk of the subclavian artery)
• Extensive anastomosis within and on the surface of the gland — provides collateral circulation ^[5]
• Thyroid ima artery (inconstant, directly from the aortic arch or brachiocephalic artery — important in tracheostomy!)

• Recurrent laryngeal nerve (RLN): runs in the tracheoesophageal groove; supplies all intrinsic laryngeal muscles except cricothyroid. Injury → vocal cord paralysis → hoarseness
• External branch of the superior laryngeal nerve (EBSLN): runs close to the superior thyroid artery; supplies the cricothyroid muscle. Injury → loss of high-pitched voice

• Lobule is the functional unit → each lobule contains 20–40 follicles ^[5]
• Each follicle consists of:
  • Follicular cells (thyrocytes): synthesise and secrete thyroid hormones (T3, T4)
  • Colloid (intraluminal): stores thyroglobulin (the precursor molecule for T3/T4)
  • Parafollicular C cells: secrete calcitonin (important in medullary thyroid carcinoma)

1. Iodide trapping: Follicular cells actively transport iodide from blood via the sodium-iodide symporter (NIS) on the basolateral membrane
2. Oxidation and organification: Iodide is oxidised by thyroid peroxidase (TPO) and attached to tyrosine residues on thyroglobulin → MIT (monoiodotyrosine), DIT (diiodotyrosine)
3. Coupling: MIT + DIT → T3; DIT + DIT → T4 (all still attached to thyroglobulin in the colloid)
4. Secretion: Thyroglobulin is endocytosed back into the follicular cell → lysosomal proteolysis → free T3 and T4 released into blood
5. Peripheral conversion: ~80% of circulating T3 comes from peripheral deiodination of T4 (mainly in the liver and kidneys)

This physiology is key to understanding why autonomous nodules in TMNG produce excess hormone: they trap iodine and produce T3/T4 independently of TSH stimulation.

This is a continuum, and understanding the progression is essential:

1. Simple (diffuse) goitre: A diffuse, non-neoplastic, non-inflammatory thyroid enlargement. Causes include iodine deficiency, goitrogens, increased physiological demand (puberty, pregnancy). The gland is uniformly hyperplastic ^[2][3].

2. Non-toxic multinodular goitre: Over years to decades, the diffuse goitre undergoes recurrent episodes of hyperplasia and involution (due to unknown or fluctuating stimuli). This results in hyperplastic nodules growing at varying rates — some become colloid-rich, some undergo haemorrhage/degeneration, some become cystic ^[2][3].

3. Toxic multinodular goitre (Plummer's disease): Eventually, some nodules acquire somatic mutations that confer autonomous function — they produce thyroid hormone independently of TSH. When the autonomous output exceeds the body's needs, the patient becomes thyrotoxic ^[2][3].

The key molecular events that lead to autonomous hormone production include:

The fundamental point: these are somatic (acquired) mutations, not germline. They accumulate over time. This explains why TMNG is a disease of the elderly — it takes decades for enough autonomous clones to develop.

Think of it as a "threshold" model:

• In non-toxic MNG, small amounts of autonomous hormone production are present but are counterbalanced by TSH suppression of the remaining normal thyroid tissue (negative feedback)
• As autonomous tissue mass grows (more nodules, bigger nodules, more mutations), the total autonomous output exceeds the body's requirement
• The normal thyroid tissue is now maximally suppressed (TSH is undetectable) but the autonomous nodules keep producing → overt thyrotoxicosis
• An iodine load (e.g., CT contrast, amiodarone) can tip a subclinically toxic MNG into overt toxicosis because it provides substrate for the autonomous nodules to ramp up production — this is the Jod-Basedow phenomenon (Jod = iodine in German; Basedow = German eponym for Graves'-like thyrotoxicosis)

Goitre is classified as ^[1]:

The pathophysiology of TMNG centres on autonomous, TSH-independent thyroid hormone synthesis and secretion by one or more nodules within a multinodular goitre.

This is a commonly tested concept:

• In Graves' disease, the entire gland is stimulated by TRAb → intense, diffuse hormone overproduction
• In TMNG, only the autonomous nodules are producing excess hormone; the non-autonomous tissue is suppressed by the low TSH → the total output is generally lower than in Graves'
• Therefore, TMNG typically presents with milder thyrotoxicosis — often subclinical first, progressing slowly to overt disease
• T3-thyrotoxicosis is common in TMNG (elevated T3 with normal T4) — because autonomous nodules preferentially secrete T3 (it is biologically more active), and the peripheral conversion of T4→T3 is preserved

Thyroid hormones (primarily T3) act on virtually every organ system. The effects are mediated by:

1. Nuclear thyroid hormone receptors → altered gene transcription → protein synthesis
2. Increased basal metabolic rate (BMR) — T3 increases mitochondrial oxidative phosphorylation and thermogenesis
3. Sensitisation to catecholamines — T3 upregulates β-adrenergic receptors, amplifying sympathetic effects

Unlike Graves' disease (which causes a diffuse, moderately enlarged gland), TMNG is characterised by large, asymmetric, nodular goitres that may extend retrosternally. This is because the goitre has been growing for years/decades before becoming toxic. Compression occurs on surrounding structures:

This is the first and most important investigation ^[1][2][3][4].

What to order: Ultrasensitive TSH + fT4 as the initial screen ^[3][4]

Why TSH first?

• TSH is the most sensitive indicator of thyroid function ^[4][6] — this is because of the log-linear relationship between TSH and fT4. A small change in fT4 produces a large, amplified change in TSH. So TSH becomes abnormal before fT4 moves outside the reference range. This is why subclinical disease (where fT4 is still normal) is picked up by TSH first.

How to interpret:

Additional TFT considerations:

• Sick euthyroidism: In acutely ill patients, systemic illness causes ↓peripheral conversion of T4→T3, altered binding protein levels, and ↓TSH secretion. The pattern: TSH low/low-normal, fT4 low/normal/high, T3 usually low ^[3][4]. Key point: T3 should be checked if suspected hyperthyroidism with concurrent illness — in sick euthyroidism T3 is low, whereas in true hyperthyroidism T3 is elevated ^[3][4].

• Other causes of ↓TSH (without hyperthyroidism): central hypothyroidism (pituitary/hypothalamic insufficiency), systemic illness, pregnancy (hCG cross-reacts with TSH receptor in first trimester) ^[3][4].

Routine for ALL patients with goitre/palpable nodules ^[1][2][3].

Why is USG essential in TMNG? USG serves multiple purposes simultaneously:

Technical details: 7.5 or 10 MHz probes, B-mode ^[3]. Readily available, non-invasive, ↑sensitivity but ↓specificity ^[3]. Used as an extension of physical examination to guide (not confirm) diagnosis ^[3].

USG features to report:

The nodule itself ^[3][6]:

Surrounding tissues ^[3]:

• Other nodules: presence of multiple nodules suggests MNG → generally reassuring (but dominant/atypical nodules still need evaluation)
• Parenchymal abnormalities: heterogeneous, hypoechoic parenchyma may suggest underlying thyroiditis
• Lymph nodes: absent hilum, microcalcification, round shape, peripheral vascularity, hyperechoic → more likely malignant ^[3]

Mnemonic for suspicious USG features: "SHIT CME" — Solid, Hypoechoic, Irregular margins, Taller than wide, Calcification (micro), Microcalcification, Extrathyroidal extension — most important are solid & hypoechoic ^[5].

Sonographic criteria for FNA (ATA 2015 Guidelines) ^[6]:

This is the key investigation for determining the aetiology of thyrotoxicosis and the functional status of nodules ^[1][2][3][8].

When to order it:

Thyroid scintigraphy is indicated in patients with thyroid nodule(s) + ↓TSH ^[1][2][3][6] — i.e., when there is biochemical evidence of hyperthyroidism AND nodularity. It is also indicated when the clinical picture does not clearly point to a specific aetiology.

It is NOT recommended for routine diagnostic use ^[6] — only for specific indications:

Radiopharmaceuticals ^[8]:

Principle: Radioactive iodine is handled in the same manner as normal iodine ^[8]. The level of uptake reflects metabolic activity — areas producing more hormone trap more tracer and appear "hot"; areas that are inactive appear "cold" ^[8].

Images: Obtained at anterior, left anterior oblique (LAO), and right anterior oblique (RAO) views ^[8].

Interpretation — this is extremely high yield ^[1][2][3][4]:

Radio-isotope scintigraphy (I¹²³ or Tc⁹⁹ᵐ): for diagnosis of malignancy it has low sensitivity and specificity; its main role is functional assessment in thyrotoxic patients ^[1].

Why NOT use scintigraphy routinely (i.e., in euthyroid patients)?

• Scintigraphy should NOT be used if TSH is normal ^[3] — because if the patient is euthyroid, the nodule is not hyperfunctioning. Most cold nodules are benign anyway, and the test would lead to unnecessary biopsies. In euthyroid patients, USG + FNAC is the appropriate pathway.

TRAb: Sensitivity 97%, Specificity 99% with newer assays ^[2][3][4].

When to order: When the aetiology of thyrotoxicosis is not clinically apparent ^[2][3]. In TMNG, TRAb is used primarily to exclude Graves' disease:

Antibody prevalence across conditions ^[6]:

Note that 10–20% of MNG patients can have low-titre TRAb — this does not mean they have Graves'. Context matters.

FNAC is the single most important investigation for thyroid nodules if TSH is not depressed ^[2][3]. In TMNG specifically, FNAC is used to rule out coexistent malignancy in suspicious nodules.

Technique: Trans-isthmic approach ± USG guidance ^[2][3]. USG guidance is preferred because it confirms the presence of the nodule and targets the biopsy to the most suspicious region ^[3].

Indications for FNAC in TMNG ^[6]:

• Dominant or atypical nodule in multinodular goitre
• Hypofunctioning (cold) nodules on scintigraphy
• Nodules meeting sonographic size criteria (see ATA 2015 criteria above)
• Nodules associated with abnormal cervical lymph nodes
• Complex or recurrent cystic nodules

NOT indicated for:

• Hot (hyperfunctioning) nodules — these are almost never malignant ^[6]

Reporting: The Bethesda Classification ^[3][6]:

Limitations: Histological demonstration of capsular or vascular invasion is required to diagnose whether a follicular lesion is benign or malignant ^[3] — FNA cannot distinguish follicular adenoma from follicular carcinoma. This is why Bethesda IV (follicular neoplasm) requires surgical excision for definitive diagnosis.

Newer technology: Molecular diagnostics (e.g., Veracyte Afirma) — based on multi-gene expression panels to help reclassify indeterminate nodules (Bethesda III/IV), but currently expensive, no universal standards, and not readily available ^[3].

Non-selective short-acting β-blocker (e.g., propranolol, nadolol) for short-term alleviation of symptoms ^[2][3][4].

Why beta-blockers? Thyroid hormone excess sensitises tissues to catecholamines by upregulating β-adrenergic receptor expression. Beta-blockers directly antagonise these receptors → rapid relief of adrenergic symptoms (tachycardia, palpitations, tremor, anxiety, sweating) within hours, even before thyroid hormone levels normalise.

Role: Symptom bridge only — beta-blockers do NOT address the underlying hormone overproduction. They are used while awaiting the effect of ATDs or definitive treatment.

Contraindications: Asthma (non-selective β-blockers cause bronchospasm), severe bradycardia, decompensated heart failure, second/third-degree heart block. In these patients, consider diltiazem (CCB) as alternative ^[6][9].

Summary table for ATDs in toxic MNG ^[5]:

Pharmacology of Thionamides

The thionamides — carbimazole, methimazole, and propylthiouracil (PTU) — are the ATD drugs used ^{[2][4][6][10]}:

Mechanism of action ^[6][10]:

• Inhibit thyroid peroxidase (TPO) → block organification (iodination of tyrosine residues on thyroglobulin) and coupling of iodotyrosines → ↓T3 and T4 synthesis
• PTU only: additionally inhibits peripheral conversion of T4→T3 (by inhibiting type 1 deiodinase) — this is why PTU is preferred in thyroid storm
• Immunosuppressive effects (relevant mainly in Graves'): ↓serum TRAb levels ^[10]

Why is there a delay in onset? Onset of euthyroid state takes 3–4 weeks because the thyroid gland has a large storage of pre-formed hormones (weeks' worth of T3/T4 stored in the colloid as thyroglobulin) ^[6]. The ATDs block NEW hormone synthesis but do NOT affect the release of already-stored hormone. The existing stores must be depleted before clinical effect is seen.

Choice of ATD ^[10]:

• Prefer carbimazole/methimazole (over PTU) for maintenance because:
  • Achieves euthyroid more rapidly
  • Once-daily dosing (better compliance)
  • ↓hepatotoxicity (PTU causes hepatocellular injury; carbimazole causes cholestatic injury — the former is more dangerous)
  • Little or no effect on subsequent RAI success
• Prefer PTU only in: (1) first trimester of pregnancy (↓teratogenicity), (2) thyroid storm (↓peripheral T4→T3 conversion), (3) minor reactions to carbimazole ^[10]

Dosing ^[10]:

• Initiation: Carbimazole 15–60 mg/day in 2–3 divided doses (depends on severity of thyrotoxicosis)
• Monitoring: TFT ± CBC/LFT every 4–6 weeks until euthyroid
• Maintenance: Titrate down gradually to 5–15 mg/day
• Baseline bloods before starting: CBC and LFT ^[10]

Regimen types ^[10]:

• Titrating regimen: Start high → titrate down guided by TSH
• Block and replace: High-dose ATD + levothyroxine replacement — useful where control is difficult

Adverse effects of ATDs ^[6][10]:

RAI is preferred for toxic MNG if there are no indications for surgery (no "4C") ^[5].

How does RAI work? ^[6]:

• ¹³¹I is taken up and processed by the thyroid gland in the same way as normal iodide — via the sodium-iodide symporter (NIS)
• The specificity to the thyroid is due to preferential thyroid uptake via the NIS
• ¹³¹I becomes incorporated into thyroglobulin within the follicular cells
• It then emits β-radiation (short range, ~0.5–2 mm) within the thyroid gland → destruction of thyroid follicular cells by necrosis and radiation-induced DNA damage ^[6]
• Autonomous ("hot") nodules preferentially take up more ¹³¹I (because they are metabolically active) → they receive the highest radiation dose and are preferentially destroyed

Advantages ^[2][3]:

• Non-surgical, outpatient procedure
• ↓cost
• Can be repeated if needed
• Good efficacy for small to moderate-sized toxic goitres

Disadvantages ^[2][3]:

• Hypothyroidism: the most important long-term consequence — transient in 3.5–28%; permanent in 10–15% in first 2 years and ~3%/year thereafter ^[6] → requires lifelong T4 replacement monitoring
• Slow response — takes weeks to months for full effect
• Restricted proximity to other persons (especially children, pregnant women) due to radiation exposure ^[3]
• Risk of radiation thyroiditis (~3%) — transient painful swelling ^[3]
• Risk of transition to Graves' disease (~5%) — RAI can trigger new autoimmune response ^[3]
• May be less effective for very large goitres (insufficient RAI dose relative to gland volume; may need repeated doses)
• Does NOT address compressive symptoms — the goitre shrinks slowly and incompletely

Contraindications ^[6][9]:

• Pregnancy and lactation — ¹³¹I crosses the placenta and is concentrated by the fetal thyroid (from ~12 weeks gestation) → damages fetal thyroid gland; secreted in breast milk
• Children and adolescents — avoid potential long-term radiation effects in young age ^[9]
• Active moderate-to-severe Graves' ophthalmopathy — RAI can worsen GO (not directly relevant to TMNG, but important if Graves' co-exists) ^[10][11]
• Suspicion of thyroid malignancy — RAI treats the functional problem but does NOT address the cancer; surgery is needed
• Large goitre with significant compression — RAI is too slow and insufficient to relieve compressive symptoms urgently

Pre-RAI Preparation ^[9]:

• Discussion of treatment options and informed consent
• Avoid iodine-containing food, medicine (e.g., cough suppressants), or radiological contrast for ≥ 4 weeks before ¹³¹I therapy — because excess iodine competes with RAI uptake and reduces efficacy
• Stop antithyroid medications ≥ 4 weeks before ¹³¹I therapy — ATDs reduce iodine organification and may lower RAI uptake/efficacy (though some centres stop only 3–7 days before)
• Symptomatic control with propranolol during the ATD-free period
• Pregnancy test for patients with child-bearing potential ^[9]

Post-RAI Care ^[9]:

• Symptomatic control of hyperthyroidism with propranolol (thyrotoxicosis may transiently worsen as damaged follicles release stored hormone)
• Discharge home immediately; avoid close contact with others (radiation protection)
• Safe contraception for ≥ 6 months — avoid pregnancy and breastfeeding for ≥ 6 months ^[9]
• Monitor TFT regularly — hypothyroidism may develop weeks to years later

Subclinical thyrotoxicosis (↓TSH, normal fT3/fT4) is the most common initial biochemical presentation of MNG. Management depends on the degree of TSH suppression and patient risk profile ^[2][3][7]:

Treatment in these cases follows the same paradigm: definitive therapy (RAI or surgery) is preferred, with ATD as a bridge or for patients refusing definitive treatment.

This is relevant because TMNG evolves from non-toxic MNG, and many exam questions ask about the transition:

Although thyroid storm can complicate any cause of thyrotoxicosis, it is an important management scenario for TMNG, especially in the perioperative setting or when an acute illness precipitates crisis in a previously undertreated patient ^[3][4][6][9].

Thyroid storm: rare but life-threatening (10% mortality, medical emergency) ^[3].

Setting/Precipitants ^[3][4][9]:

• Longstanding untreated/undertreated hyperthyroidism
• Acute infection, thyroid or non-thyroid surgery, trauma, childbirth
• Withdrawal of antithyroid drugs
• Shortly after treatment procedures (subtotal thyroidectomy or RAI)
• Acute iodine load (e.g., amiodarone) ^[9]

Clinical features ^[3]:

• CVS: tachycardia > 140/min, AF, high-output failure
• Hyperpyrexia: may reach > 40°C
• CNS disturbance: agitation, anxiety, delirium, psychosis, stupor, coma

Diagnosis: Burch and Wartofsky scoring system (sensitive but not specific): ≥ 45 = highly suggestive; 25–44 = supports diagnosis; < 25 = unlikely ^[3]

Treatment (the order and rationale matter enormously):

Subsequent management ^[9]:

• Stop iodine and taper steroids once clinical improvement is evident
• Titrate thionamide (switch to methimazole) to maintain euthyroidism
• Plan for definitive treatment (RAI or surgery) once stabilised

• Pathophysiology: T3 directly stimulates osteoclastic activity by upregulating RANKL expression on osteoblasts → ↑bone resorption. Simultaneously, the shortened bone remodelling cycle means that bone formation cannot keep pace with destruction → net bone loss → osteoporosis with ↑bone resorption and ↓bone density ^[2]
• At-risk population: Postmenopausal women with TMNG are at double jeopardy — oestrogen deficiency + thyroid hormone excess both promote bone loss
• Clinical consequence: Vertebral and hip fractures; may be the presenting feature of previously undiagnosed subclinical TMNG in an elderly woman

• Particularly relevant in Asian populations (including Hong Kong)
• Pathophysiology: Thyroid hormone excess enhances Na⁺/K⁺-ATPase activity (driving K⁺ into cells) → hypokalemia → sudden onset of paralysis, typically proximal and lower limbs, often precipitated by carbohydrate-rich meals or exertion
• Presents with sudden-onset flaccid paralysis + hypokalemia + thyrotoxicosis
• Management: cautious K⁺ replacement (rebound hyperkalemia can occur as K⁺ redistributes back out of cells) + treat the underlying hyperthyroidism

Rare but life-threatening complication (10% mortality, medical emergency) ^[3].

• Pathophysiology: A rapid surge in thyroid hormone effects — not necessarily a massive increase in circulating hormone levels, but rather a sudden amplification of the body's response. This occurs when a precipitant (acute infection, surgery, iodine load, ATD withdrawal) overwhelms the compensatory mechanisms in a patient with pre-existing untreated/undertreated thyrotoxicosis ^[3][6]

Setting/Precipitants ^[3]:

• Acute infection in previously untreated or undertreated thyrotoxicosis
• Withdrawal of antithyroid drugs
• Shortly after treatment procedures (subtotal thyroidectomy or RAI)
• Acute iodine load, trauma, childbirth ^[4]

Clinical features ^[3]:

• CVS: tachycardia > 140/min, AF, high-output failure
• Hyperpyrexia: may reach > 40°C
• CNS disturbance: agitation, anxiety, delirium, psychosis, stupor, coma

Diagnosis: Burch and Wartofsky scoring system (sensitive but not specific): ≥ 45 = highly suggestive; 25–44 = supports diagnosis; < 25 = unlikely ^[3]

(Management was covered in detail in the Management section.)

• Pathophysiology: Progressive enlargement of nodules, particularly bilateral or with retrosternal extension, physically narrows the tracheal lumen → dyspnoea, stridor (especially on exertion or when lying flat)
• Can be insidious (chronic exertional dyspnoea) or acute (if haemorrhage into a nodule suddenly increases goitre size)
• Spirometry shows a blunted flow-volume loop characteristic of upper airway obstruction ^[2]

• Large posterior nodules or retrosternal extension compress the oesophagus → dysphagia (initially for solids, then liquids as compression worsens)
• Less common than tracheal compression because the oesophagus is more posteriorly located and more compressible

• Dysphonia (hoarseness) from stretching or compression of the RLN in the tracheoesophageal groove
• Important caveat: Hoarseness in a patient with MNG must raise suspicion for malignancy — benign goitres rarely cause RLN palsy by simple compression; true RLN invasion is more characteristic of thyroid carcinoma

• Pemberton's sign: Retrosternal goitre obstructs the thoracic inlet → venous congestion. When arms are raised above the head → further narrowing of the thoracic inlet → facial plethora, distended neck veins, inspiratory stridor
• Severe cases can cause superior vena cava syndrome (facial/upper limb oedema, collateral venous distension)

• Haemorrhage into nodule/cyst → sudden painful swelling ^[2]
• Pathophysiology: Hypervascular nodules can spontaneously bleed internally → rapid expansion of the nodule → acute pain + increased compressive symptoms
• Can mimic malignancy clinically (rapid enlargement, pain) but is benign
• Usually self-limiting; may need aspiration if causing airway compromise

• Around 10–15% of thyroid nodules are malignant ^[5] — this applies to nodules within an MNG too
• Long-standing MNG can progress to lymphoma ^[3]
• MNG is a risk factor for follicular thyroid carcinoma ^[5] (though note: follicular adenoma is NOT a risk factor for follicular carcinoma ^[5])
• A dominant or rapidly growing nodule within a long-standing MNG must be investigated (USG ± FNAC ± scintigraphy) to exclude malignancy
• Cold nodules on scintigraphy have a 10–20% risk of malignancy and warrant FNAC ^[6]

Important reassurances ^[9]:

• NO effect on fertility
• NO effect on congenital malformations (in future pregnancies after adequate washout period)
• NO effect on increased cancer risk of offspring

(Covered in detail in the Management section but summarised here for completeness)