Hashimoto's Thyroiditis

Hashimoto's thyroiditis is a chronic autoimmune disorder in which antibodies target the thyroid gland, leading to lymphocytic infiltration, progressive destruction of thyroid tissue, and eventual hypothyroidism.

Hashimoto's thyroiditis is a common cause of hypothyroidism — but NOT the commonest overall (iatrogenic hypothyroidism is more common worldwide) ^[2]. However, among non-iatrogenic causes, it is the most common cause in iodine-sufficient populations.

Parameter	Detail
Prevalence	3.5/1,000 (F) vs 0.8/1,000 (M) ^[2]; higher in Western/iodine-replete populations
Sex ratio	F:M ≈ 7:1 ^[2] — strong female predominance, like most autoimmune diseases (estrogen modulates immune tolerance)
Age	Increases with age, especially in older women ^[2]; peak incidence 30–50 years, but can occur at any age including children
Spectrum	Varies from anti-TPO positivity alone (5%), to subclinical hypothyroidism (5%), to overt hypothyroidism (0.1–2%) ^[2] — most patients with positive antibodies never develop overt disease
Hong Kong context	In HK, there is no mandatory iodination of salt → borderline iodine intake → low incidence of autoimmune thyroiditis (1% vs 10% in West) but increasingly common nowadays ^[2]^[3] — likely due to dietary westernisation and increased iodine intake from processed food, seaweed, and supplements

Why is Hashimoto's more common in iodine-replete areas?

Iodine excess increases thyroid antigenicity. When thyroglobulin becomes heavily iodinated, it becomes more immunogenic — the immune system "sees" it as foreign. This is why autoimmune thyroiditis incidence rises after iodine supplementation programmes are introduced in previously deficient populations. This is an important concept for HKUMed exams ^[2]^[3].

Risk factors for Hashimoto's thyroiditis ^[2]:

Risk Factor	Mechanism / Explanation
Older female sex	Estrogen modulates B-cell survival and autoantibody production; X-chromosome inactivation skewing may expose self-antigens
Family history	30–60% monozygotic twin concordance ^[2] — strong genetic component
HLA-DR3	4–5× increased risk ^[2]; HLA-DR3 and HLA-DR5 are associated with antigen presentation of thyroid self-peptides to T cells. Also HLA-DR4 in some populations
Increased iodine intake	Heavily iodinated thyroglobulin is more immunogenic; iodine may also be directly toxic to thyrocytes generating neoantigens
Smoking	Paradoxically, smoking is a risk factor for Hashimoto's ^[2] (note: in Graves', smoking is more specifically associated with ophthalmopathy rather than the disease itself ^[4])
Other autoimmune diseases	Type 1 DM, Addison's disease, pernicious anaemia, vitiligo, coeliac disease, SLE, RA — autoimmune diseases cluster due to shared HLA susceptibility and generalised immune dysregulation ^[3]^[5]
Female sex hormones	Pregnancy, postpartum state — immune rebound after relative immunosuppression of pregnancy can trigger or unmask autoimmune thyroiditis
Drugs	Lithium (concentrates in thyroid, inhibits thyroid hormone release and may enhance autoimmunity), Amiodarone (high iodine content), Interferon-alpha, Immune checkpoint inhibitors (anti-PD-1, anti-CTLA-4)
Radiation exposure	External neck irradiation (e.g., for childhood leukaemia/NPC) can trigger thyroid autoimmunity

Anatomy and Thyroid Function (Brief Review)

Understanding Hashimoto's requires knowing what the immune system is destroying:

Hypothalamus → TRH → Anterior pituitary → TSH → Thyroid → T4/T3
                                    ↑___________________________|
                                    (negative feedback)

When thyroid hormone levels fall (as in Hashimoto's), negative feedback is lost → TSH rises → drives residual thyroid tissue to hypertrophy → goitre formation.

Key Targets in Hashimoto's

The two main enzymatic/structural targets of autoimmune attack are:

Thyroid peroxidase (TPO) — the enzyme that makes thyroid hormone
Thyroglobulin (Tg) — the scaffold protein on which thyroid hormone is assembled

This is why anti-TPO and anti-Tg antibodies are the hallmark serological markers ^[2]^[6].

Etiology and Pathophysiology

Etiology — Why Does Hashimoto's Happen?

Hashimoto's thyroiditis is a multifactorial autoimmune disease resulting from a breakdown in self-tolerance to thyroid antigens in a genetically susceptible individual, triggered by environmental factors.

The pathology is fundamentally destructive lymphoid infiltration ^[2]:

Step-by-Step Pathophysiology:

Initiation: Generally believed to arise from T-cell mediated damage to thyroid tissues → immune activation → lymphocytic infiltration and cytokine secretion ^[2]
- Dendritic cells within the thyroid present thyroid antigens (TPO, Tg) via MHC class II to CD4+ T cells
- This breaks peripheral tolerance
Effector Phase:
- Cellular immunity (primary mechanism): CD8+ cytotoxic T lymphocytes directly kill thyrocytes via perforin/granzyme pathway and Fas-FasL interaction
- Humoral immunity: B cells produce autoantibodies
- Cytokines: TNF-α, IFN-γ, IL-1 from infiltrating lymphocytes create a hostile microenvironment that further damages thyrocytes and promotes apoptosis
Role of anti-thyroid antibodies ^[2]:
- Anti-thyroglobulin (anti-Tg) Ab: nearly all patients with Hashimoto's have ↑anti-Tg but may also occur in other thyroid diseases and even in apparently clinically euthyroid patients ^[2]
- Anti-TPO (microsomal) Ab: more specific to hypothyroidism and may inhibit TPO activity ^[2] — this directly impairs thyroid hormone synthesis
- Anti-TSHr Ab (blocking type): mainly found in atrophic variant and contributes to hypothyroidism ^[2] — blocks TSH from stimulating the gland → no goitre, just atrophy
- The antibodies are likely secondary phenomena to T-mediated injury but may also contribute to secondary thyroid damage ^[2]
Histology ^[2]: Profuse lymphocytic infiltration, lymphoid germinal centres and destruction of thyroid follicles ± fibrosis
- The germinal centres within the thyroid are essentially ectopic lymphoid tissue — the thyroid becomes its own battlefield
- Hürthle cells (oxyphilic cells): Damaged follicular cells with abundant eosinophilic, mitochondria-rich cytoplasm — a characteristic histological finding
- Varying degrees of fibrosis in late-stage disease
Consequence: destruction of thyroid tissues → ↓fT4 → ↑TSH → goitre ^[2]
- The goitre in Hashimoto's is driven by TWO processes:
  - Lymphocytic infiltration expanding the gland
  - TSH-driven hyperplasia of remaining follicular cells trying to compensate
Hashitoxicosis: A minority of patients may initially present with hyperthyroidism and ↑iodine uptake due to severe follicular disruption and thyroid hormone release → subsequently progresses to typical Hashimoto's trajectory ^[2]
- Think of it as "uncontrolled spillage" of pre-formed thyroid hormone from destroyed follicles
- This is transient — once the stored hormone is depleted and enough follicular cells are destroyed, the patient becomes hypothyroid

Hashitoxicosis vs Graves' Disease

A common exam pitfall: Hashitoxicosis can mimic Graves' disease clinically. The key differences:

Hashitoxicosis: Destructive thyrotoxicosis → low radioiodine uptake (follicles are damaged, not hyperactive), anti-TPO strongly positive, TRAb usually negative
Graves': Stimulatory thyrotoxicosis → high radioiodine uptake (follicles are being stimulated by TRAb), TRAb positive

This distinction is critical because you must NOT give antithyroid drugs for destructive thyrotoxicosis — there is no overproduction to suppress ^[2]^[4].

Classification

Feature	Hashimoto's Thyroiditis	Atrophic Thyroiditis	Graves' Disease
Goitre	Yes (firm, rubbery)	No (gland atrophies)	Yes (diffuse, vascular, bruit)
Predominant antibody	Anti-TPO, Anti-Tg	Anti-TSHr (blocking)	Anti-TSHr (stimulating/TRAb)
Thyroid function	Hypothyroid (may be euthyroid or transiently thyrotoxic)	Hypothyroid	Hyperthyroid
Histology	Lymphocytic infiltration, germinal centres, Hürthle cells	Fibrosis, atrophy	Lymphocytic infiltration, hyperplasia

Hashimoto's is classified as a cause of primary hypothyroidism ^[3]^[6]:

Classification	Causes
Primary hypothyroidism	Autoimmune: Hashimoto's thyroiditis, atrophic thyroiditis; Iatrogenic (RAI, thyroidectomy, external radiation); Iodine deficiency or excess; Drug-induced (lithium, amiodarone); Congenital; Infiltrative (sarcoidosis, amyloidosis, Riedel's thyroiditis)
Secondary hypothyroidism	Hypothalamic or pituitary disease (tumours, surgery, Sheehan's syndrome, infiltrative disorders)
Transient hypothyroidism	Subacute (de Quervain's) thyroiditis, silent/postpartum thyroiditis, post-RAI, post-thyroidectomy

From the lecture slides ^[1]:

Goitre Classification	Examples
Neoplastic goitre	Benign; Malignant
Thyroiditis	Bacterial (acute suppurative); Viral (subacute); Lymphocytic/Hashimoto/autoimmune (chronic)
Simple goitre (endemic or sporadic)	Diffuse; Nodular
Toxic goitre	Diffuse toxic (Graves'); Toxic nodular (Plummer's); Toxic/functioning adenoma

Type	Aetiology	Pain	Thyroid Ab	Course
Hashimoto's (chronic lymphocytic)	Autoimmune	Painless	Anti-TPO, anti-Tg strongly positive	Progressive → permanent hypothyroidism
Subacute granulomatous (de Quervain's)	Post-viral	Painful (radiates to jaw/ears)	Low titre (if any)	Self-limiting (thyrotoxic → hypothyroid → recovery)
Subacute lymphocytic / Silent	Autoimmune (variant of Hashimoto's)	Painless	Moderate anti-TPO	Self-limiting (but may recur; ↑risk of permanent hypothyroidism)
Postpartum thyroiditis	Autoimmune (variant of silent)	Painless	Anti-TPO positive	Self-limiting in most; ~20–30% develop permanent hypothyroidism
Riedel's thyroiditis	Fibrosing (IgG4-related)	Hard, "woody" gland	Variable	Progressive fibrosis; may mimic carcinoma
Drug-induced	Amiodarone, lithium, checkpoint inhibitors	Variable	Variable	Depends on drug and mechanism

Clinical Features

The clinical presentation of Hashimoto's thyroiditis depends on where the patient sits on the disease spectrum at the time of presentation. Many patients are detected incidentally (positive antibodies, mildly elevated TSH) and are entirely asymptomatic.

Symptoms

Symptom	Pathophysiological Basis
Anterior neck swelling	Lymphocytic infiltration + TSH-driven compensatory hypertrophy → gland enlargement. Patients may notice a visible or palpable fullness in the anterior neck
Sensation of neck pressure/fullness	Goitre expanding within the pretracheal fascia; usually mild because Hashimoto's goitres are typically only small to moderate in size
Dysphagia	Large goitre compressing the oesophagus posteriorly (uncommon in Hashimoto's; more relevant in large MNG or malignancy)
Dyspnoea/stridor	Compression of the trachea (rare in typical Hashimoto's; consider retrosternal extension or coexisting pathology)
Dysphonia/hoarseness	Not typical of uncomplicated Hashimoto's. If present, raises suspicion for thyroid lymphoma (a feared complication of long-standing Hashimoto's) or coexisting malignancy invading the RLN. However, hoarseness can also occur from myxoedema of vocal cords in severe hypothyroidism ^[3]^[7]

Only ~25% of Hashimoto's patients present with overt hypothyroidism ^[2]. The symptoms reflect a global metabolic slowdown — thyroid hormone drives basal metabolic rate, so deficiency affects virtually every organ system.

Symptom	Pathophysiological Basis
Weight gain	↓BMR → ↓energy expenditure; also fluid retention (myxoedema — mucopolysaccharide and fluid accumulation in tissues)
Cold intolerance	↓thermogenesis due to ↓metabolic rate; thyroid hormone normally drives uncoupling protein expression and mitochondrial heat production
Fatigue / easy fatiguability	↓cellular energy metabolism; ↓cardiac output; ↓oxygen delivery
Constipation	↓GI smooth muscle motility due to ↓sympathetic tone and direct metabolic slowing
Menstrual irregularities	Hypothyroidism → ↑TRH → ↑prolactin (TRH stimulates prolactin release) → menorrhagia, oligomenorrhoea, or anovulation. Also ↑SHBG clearance → altered oestrogen/progesterone balance
Muscle cramps	↓energy metabolism in muscle; delayed muscle relaxation (↓Ca²⁺-ATPase activity in sarcoplasmic reticulum)
Chest pain (angina)	↑LDL cholesterol (↓LDL receptor expression due to hypothyroidism) → accelerated atherosclerosis; also ↑peripheral vascular resistance
Depression, cognitive slowing	↓CNS neurotransmitter synthesis and metabolism; ↓serotonin, noradrenaline
Dry skin, hair loss	↓sebaceous gland activity; ↓hair follicle cycling due to metabolic slowing
Hoarseness	Myxoedema of vocal cords ^[3]^[7] — mucopolysaccharide deposition thickens the vocal folds

A minority of patients may initially present with hyperthyroidism ^[2]:

Symptom	Pathophysiological Basis
Palpitations, tremor, anxiety	Excess T4/T3 → ↑β-adrenergic receptor sensitivity → sympathetic overactivity
Weight loss despite increased appetite	↑BMR → ↑energy expenditure
Heat intolerance, sweating	↑thermogenesis
Diarrhoea	↑GI motility
Irritability, insomnia	CNS stimulation

This phase is self-limiting (weeks to a few months) — once stored thyroid hormone is depleted from destroyed follicles, the patient transitions to euthyroidism and then hypothyroidism.

Signs

Sign	Description & Pathophysiological Basis
Goitre	Usually small or moderately sized, diffuse, painless ^[2]. Moves with swallowing (confirms thyroid origin)
Firm, rubbery consistency	Characteristically firm, rubbery ^[2] — due to dense lymphocytic infiltration and fibrosis. Compare: Graves' = soft and smooth; MNG = irregular and nodular; Carcinoma = hard and fixed; de Quervain's = tender
Non-tender	Unlike subacute (de Quervain's) thyroiditis, Hashimoto's is painless — the inflammation is chronic and low-grade
Diffuse enlargement	Both lobes uniformly enlarged (though mild asymmetry is possible). The isthmus may be prominent ("Delphian node" enlargement suggests reactive lymphadenopathy)
Pyramidal lobe may be palpable	TSH stimulation can cause the embryological remnant to enlarge
Atrophic variant: no goitre	Predominantly TSH-receptor blocking Ab → no goitre ^[2]

Sign	Pathophysiological Basis
Dry skin	↓sebaceous secretion + mucopolysaccharide deposition in dermis
Periorbital oedema and myxoedema	Accumulation of hyaluronic acid and other glycosaminoglycans (GAGs) in subcutaneous tissue → osmotically draws water → non-pitting oedema. "Myxoedema" = mucin + oedema — this is NOT dependent oedema from heart failure; it is non-pitting and distributed in face (periorbital), hands, pretibial area
Slow-relaxing reflexes	Delayed relaxation phase of deep tendon reflexes (classically Achilles) — ↓Ca²⁺-ATPase activity in muscle → delayed cross-bridge cycling
Bradycardia	↓chronotropic effect (thyroid hormone normally upregulates β1-adrenergic receptors and HCN channels in the SA node)
Pallor and hypothyroid facies	Pallor from anaemia (↓EPO production, possible coexisting pernicious anaemia); puffy face from myxoedema; thinning of lateral third of eyebrows ("Queen Anne sign")
Oedema	Much less in secondary hypothyroidism ^[3]^[7] — because in secondary hypothyroidism, ACTH deficiency usually occurs before TSH depletion, and cortisol deficiency leads to less fluid retention than primary hypothyroidism
Hyperlipidaemia	Both triglycerides and cholesterol elevated ^[3]^[7] — ↓LDL receptor expression → ↓clearance of LDL-C; ↓lipoprotein lipase activity → ↑triglycerides
Carpal tunnel syndrome	Mucopolysaccharide deposition in the carpal tunnel compresses the median nerve
Macroglossia	GAG deposition in the tongue
Delayed relaxation of reflexes	The "hung-up" reflex — pathognomonic for hypothyroidism

Presentation	Mechanism
Hypothermia, myxoedematous coma	Severe prolonged hypothyroidism → ↓thermoregulation, ↓consciousness; precipitated by infection, cold exposure, sedatives
Pericardial effusion	↑capillary permeability + GAG deposition → transudative pericardial effusion (often asymptomatic, rarely causes tamponade)
Hyponatraemia, SIADH	↓free water excretion (↓GFR, ↓cardiac output) + ↑ADH secretion → dilutional hyponatraemia
Hyperprolactinaemia, galactorrhoea	↑TRH (due to loss of T4 negative feedback) directly stimulates lactotrophs → ↑prolactin
Ataxia	Cerebellar dysfunction from severe prolonged hypothyroidism (mechanism not fully elucidated; may involve ↓myelination)

This is high-yield for exams. Understand which antibodies go with which disease:

Antibody	Normal Population	Graves' Disease	Hashimoto's Thyroiditis	MNG
Anti-TSH receptor (TRAb)	0%	80–90%	10–20% (blocking type)	10–20%
Anti-TPO	10–15%	50–80%	90–100%	10–20%
Anti-Tg	10–20%	50–70%	80–90%	30–40%

^[6]

High-Yield Antibody Facts

Anti-TPO is the most useful single antibody for diagnosing Hashimoto's — present in 90–100% of cases ^[2]^[6]
Anti-Tg is less specific but present in 80–90% ^[2]^[6]
Euthyroid patients with positive anti-TPO have autoimmune thyroiditis and are at higher risk of developing hypothyroidism ^[7] — this is the concept of "subclinical" or "serological" Hashimoto's
Anti-TSHr (blocking type) is mainly in the atrophic variant ^[2]

The clinical approach is mainly directed to differentiate ^[3]^[7]:

Those who require life-long T4 replacement:
- Autoimmune thyroiditis (Hashimoto's, atrophic)
- Thyroid ablation (post-RAI, post-thyroidectomy)
Those who may only have transient hypothyroidism:
- Transient thyroiditis as suggested by:
  - Neck pain (suggests de Quervain's)
  - < 12 months post-partum (suggests postpartum thyroiditis)
  - Recent symptoms of thyrotoxicosis (suggests destructive phase)
- < 6 months since ¹³¹I or thyroidectomy
- On lithium or amiodarone

Clinical Pearl

If a patient presents with hypothyroidism and you find a firm, painless, diffuse goitre with strongly positive anti-TPO antibodies — you have Hashimoto's thyroiditis. No further imaging (ultrasound, scintigraphy) is needed for diagnosis unless there are palpable nodules that need separate evaluation for malignancy risk.

High Yield Summary

Hashimoto's Thyroiditis — Key Points for Exams:

Definition: Chronic autoimmune (lymphocytic) thyroiditis → most common non-iatrogenic cause of hypothyroidism in iodine-sufficient areas
Epidemiology: F:M = 7:1, increases with age, prevalence lower in HK (1% vs 10% West) due to borderline iodine intake but rising
Pathophysiology: T-cell mediated destruction → lymphocytic infiltration of thyroid → follicular destruction → ↓T4 → ↑TSH → goitre. Antibodies (anti-TPO, anti-Tg) are secondary phenomena but contribute to damage
Histology: Lymphocytic infiltration, germinal centres, Hürthle cells, follicular destruction, fibrosis
Clinical: Firm, rubbery, painless, diffuse goitre + hypothyroid symptoms (weight gain, cold intolerance, fatigue, constipation, dry skin, slow reflexes, bradycardia)
Hashitoxicosis: Minority present with transient hyperthyroidism from follicular destruction (low radioiodine uptake — distinguishes from Graves')
Antibodies: Anti-TPO (90–100%), Anti-Tg (80–90%), Anti-TSHr blocking type (10–20% — mainly atrophic variant)
Associations: Other autoimmune diseases (T1DM, pernicious anaemia, Addison's, vitiligo, coeliac); Thyroid lymphoma (long-standing disease)
HK context: No mandatory salt iodination → borderline iodine → lower incidence but increasing
Natural history: Progressive loss of function over years; 2–5% per year progression from subclinical to overt hypothyroidism if anti-TPO positive

Active Recall - Hashimoto's Thyroiditis (Definition, Epidemiology, Pathophysiology, Clinical Features)

Differential Diagnosis of Hashimoto's Thyroiditis

When a patient walks in with a diffuse goitre, hypothyroid symptoms, or simply an incidental finding of elevated TSH with positive thyroid antibodies, you need a systematic framework to differentiate Hashimoto's from its mimics. The differential diagnosis operates across three clinical axes simultaneously:

What is causing the goitre? (if present)
What is causing the hypothyroidism? (if present)
What is causing the positive thyroid antibodies? (if found)

Let's work through each systematically, then synthesise into a clinical approach.

Hashimoto's classically presents with a diffuse goitre. The differential diagnosis for a diffuse thyroid enlargement is organised by thyroid functional status ^[2]^[3]^[11]:

Thyroid Status	Condition	Key Distinguishing Features
Hypothyroid	Hashimoto's thyroiditis	Firm, rubbery, painless; anti-TPO 90–100%; gradual onset
	Iodine deficiency (endemic goitre)	Geographic context (mountainous regions); low urinary iodine; no antibodies
	Drug-induced (lithium, amiodarone)	Clear temporal relationship with drug initiation
	Late-stage subacute thyroiditis	History of preceding painful thyrotoxic phase; ↑ESR
Euthyroid	Simple diffuse goitre (physiological)	Pregnancy, puberty, iodine deficiency, goitrogen ^[3]^[11]; no antibodies; soft, non-tender
	Early MNG	Initially may appear diffuse before nodularity becomes apparent ^[3]^[11]
	Infiltrative disease (e.g. lymphoma)	Rapid enlargement in setting of long-standing Hashimoto's; firm/"woody" ^[3]^[11]
	Treated Graves' disease	History of prior antithyroid drugs/RAI; may now be euthyroid or hypothyroid ^[3]^[11]
Hyperthyroid	Graves' disease	Diffuse, smooth, vascular with audible bruit; TRAb positive; ophthalmopathy; pretibial myxoedema ^[4]
Mixed/Fluctuating	Destructive thyroiditis (subacute, postpartum, silent)	Biphasic course (thyrotoxic → hypothyroid → recovery); pain in de Quervain's ^[8]

Exam Tip: The Goitre DDx Table

This is a commonly tested table. The senior notes organise it neatly: hypothyroid diffuse goitre = Hashimoto's until proven otherwise. But don't forget that an euthyroid or even transiently thyrotoxic patient can still have Hashimoto's — it depends on the stage of disease ^[2]^[3]^[11].

Axis 2: Differential Diagnosis of Hypothyroidism

If the patient's presenting problem is hypothyroidism (↑TSH ± ↓fT4), you need to work out the cause. Hashimoto's is the most common non-iatrogenic cause in iodine-sufficient regions ^[2]^[3]:

Category	Condition	How to Differentiate from Hashimoto's
Autoimmune	Hashimoto's thyroiditis	Anti-TPO 90–100%, anti-Tg 80–90%; firm diffuse goitre ^[2]^[6]
	Atrophic thyroiditis	Predominantly TSHr-blocking Ab → no goitre ^[2]; same antibody profile but gland is small/absent
Iatrogenic	RAI treatment	History of prior radioactive iodine for Graves'/toxic nodule; < 6mo since treatment = may be transient ^[3]^[7]
	Thyroidectomy	Surgical scar; history of surgery
	External neck irradiation	History of radiotherapy (NPC, lymphoma, childhood leukaemia) ^[3]
Thyroiditis	Subacute (de Quervain's) thyroiditis	Pain (radiates to jaw/ears), fever, ↑ESR, ↑WBC; self-limiting; low titres of thyroid autoAb ^[8]
	Silent/lymphocytic thyroiditis	Painless but fluctuating thyroid status; often postpartum; self-limiting ^[8]
	Postpartum thyroiditis	< 12 months postpartum; biphasic course; may recur; ~20–30% progress to permanent hypothyroidism ^[7]
Drug-induced	Lithium	Concentrates in thyroid; inhibits T4 release + enhances autoimmunity; check drug history
	Amiodarone	High iodine content (75mg iodine per 200mg tablet); can cause type 1 (iodine-induced) or type 2 (destructive) thyrotoxicosis OR hypothyroidism
	Immune checkpoint inhibitors	Anti-PD-1/anti-CTLA-4 (nivolumab, pembrolizumab, ipilimumab); increasingly common cause; destructive mechanism
Iodine-related	Iodine deficiency	Geographic context; low urinary iodine; goitre without antibodies
	Iodine excess (Wolff–Chaikoff effect)	Recent iodine load (CT contrast, amiodarone, kelp supplements); transient
Infiltrative	Riedel's thyroiditis	"Woody" hard fixed thyroid; IgG4-related disease; mimics carcinoma on palpation
	Sarcoidosis, amyloidosis	Rare; systemic features of the underlying disease
Congenital	Thyroid dysgenesis, dyshormonogenesis	Neonatal screening; not relevant to adult differential but important in paediatrics ^[7]

Condition	Key Distinguishing Features
Pituitary tumour (adenoma, craniopharyngioma)	Other pituitary hormone deficits; visual field defects (bitemporal hemianopia); imaging findings
Pituitary surgery/irradiation	History of prior intervention
Sheehan's syndrome	Postpartum pituitary necrosis after massive haemorrhage; failure to lactate
Hypothalamic disease	Rare; usually with other hypothalamic features (DI, temperature dysregulation)

Primary vs Secondary — The TSH Tells the Story

A common mistake: students see a low fT4 and jump to "hypothyroidism" without checking the TSH direction. In primary hypothyroidism (like Hashimoto's), TSH is elevated because the pituitary is screaming at a failing thyroid. In secondary hypothyroidism, TSH is low or inappropriately normal because the pituitary itself is the problem. Also, oedema is much less in secondary hypothyroidism because ACTH deficiency usually occurs before TSH depletion ^[3]^[7] — so the aldosterone-mediated fluid retention is less.

Axis 3: Conditions That Can Mimic or Coexist with Hashimoto's

These deserve special attention because they are commonly tested and clinically important:

This is the most important differentiation in autoimmune thyroid disease. Both are autoimmune thyroid diseases sharing HLA associations and often coexisting in the same families, but they sit at opposite ends of the functional spectrum:

Feature	Hashimoto's Thyroiditis	Graves' Disease
Thyroid function	Hypothyroid (may be euthyroid or transiently thyrotoxic)	Hyperthyroid
Goitre character	Firm, rubbery, diffuse, painless ^[2]	Diffuse, smooth, vascular with bruit ^[4]
Key antibody	Anti-TPO (90–100%) ^[2]^[6]	TRAb / Anti-TSHr stimulating (80–90%) ^[4]^[6]
Ophthalmopathy	No (unless coexisting Graves')	Yes (~20–25%) ^[12] — proptosis, lid retraction, ophthalmoplegia
Pretibial myxoedema	No	Yes (< 10%) ^[4]
Radioiodine uptake	Normal or low	Diffusely increased ^[13]
Histology	Lymphocytic infiltration, germinal centres, Hürthle cells, follicular destruction	Lymphocytic infiltration, follicular hyperplasia, scalloping of colloid

Why this matters: Occasional patients have "overlap" features — Hashimoto's patients can harbour both stimulating and blocking TSHr antibodies, and the clinical picture can flip between hypo- and hyperthyroidism over time. This is the concept of the autoimmune thyroid disease continuum.

Feature	Hashimoto's	De Quervain's
Pain	Painless	Painful (cardinal feature) — radiates to jaw/ears ^[8]
Onset	Insidious (months to years)	Acute/subacute (days to weeks); often follows URTI
Goitre	Firm, rubbery	Tender, may be asymmetric
ESR	Normal	Markedly elevated (often > 50 mm/hr) ^[8]
Thyroid antibodies	Anti-TPO 90–100%	Low titres (if any) ^[8]
Course	Progressive → permanent hypothyroidism	Self-limiting (thyrotoxic → hypothyroid → resolution) ^[8]
Histology	Lymphocytic infiltration, germinal centres	Granulomatous inflammation, multinucleated giant cells

The key teaching point: pain + ↑ESR + recent viral illness + low-titre antibodies = de Quervain's, NOT Hashimoto's.

Several drugs can cause hypothyroidism that looks like Hashimoto's but is reversible on drug withdrawal:

Drug	Mechanism	Clue to Differentiation
Lithium	Inhibits thyroid hormone release; enhances autoimmunity; ↑anti-TPO	History of bipolar disorder; may actually trigger true Hashimoto's
Amiodarone	Iodine excess → Wolff–Chaikoff effect; may also cause destructive thyroiditis	History of AF/arrhythmia; check drug history
Interferon-alpha	Immune activation → thyroid autoimmunity	History of HCV/HBV treatment or haematological malignancy
Immune checkpoint inhibitors	Anti-PD-1/CTLA-4 → unmasked autoimmune thyroiditis	History of cancer immunotherapy; can be irreversible

Hashimoto's patients can develop coexisting thyroid nodules, and this raises the question of malignancy. From the lecture slides ^[1]^[14]:

Thyroid Nodule Pathology ^[14]	Frequency
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%

Around 10–15% of nodules are malignant ^[10]. In a Hashimoto's patient with a nodule, you must consider:

Pseudo-nodule: Heterogeneous lymphocytic infiltration in Hashimoto's can create the ultrasound appearance of nodules that are not true neoplastic lesions
Papillary thyroid carcinoma: Hashimoto's patients may have a slightly increased risk (debated); any suspicious nodule still warrants FNAC
Thyroid lymphoma: The feared complication — Hashimoto's thyroiditis is a risk factor ^[10]

Feature	Hashimoto's	Graves'	De Quervain's	Postpartum	Riedel's	Drug-induced
Pain	No	No	Yes	No	No	No
Goitre	Firm, rubbery, diffuse	Diffuse, vascular, bruit	Tender, asymmetric	Small, diffuse	"Woody", fixed	Variable
Function	Hypothyroid	Hyperthyroid	Biphasic	Biphasic	Hypothyroid	Hypothyroid
Anti-TPO	90–100%	50–80%	Low titre	Moderate	~67%	Variable
TRAb	10–20% (blocking)	80–90% (stimulating)	Negative	Negative	Negative	Negative
ESR	Normal	Normal	↑↑↑	Normal	Normal	Normal
Radioiodine uptake	Normal/low	Diffusely high	Low (damaged follicles)	Low (thyrotoxic phase)	Low	Variable
Course	Progressive	Relapsing-remitting	Self-limiting	Self-limiting (80%)	Progressive fibrosis	Reversible
Key clue	Anti-TPO +++, firm goitre	TRAb +++, bruit, eye signs	Pain, ↑ESR, viral prodrome	< 12mo postpartum	Fixed, hard, IgG4+	Drug history

Clinical Pearl: The single most useful first-line investigation to differentiate the cause of hypothyroidism is anti-TPO antibodies. If strongly positive (especially > 100 IU/mL) in a patient with ↑TSH, you can confidently diagnose Hashimoto's without the need for imaging, scintigraphy, or biopsy — unless there are palpable nodules or features concerning for lymphoma ^[2]^[3].

High Yield Summary — Differential Diagnosis

Hashimoto's presents as: Diffuse firm rubbery painless goitre + hypothyroidism + strongly positive anti-TPO/anti-Tg
Main DDx for diffuse goitre: Graves' (hyperthyroid, bruit, TRAb+), simple goitre (euthyroid, no antibodies), de Quervain's (painful, ↑ESR), early MNG, infiltrative disease, treated Graves'
Main DDx for hypothyroidism: Iatrogenic (RAI, surgery, radiation — most common overall), atrophic thyroiditis (no goitre), subacute/postpartum thyroiditis (transient), drug-induced (lithium, amiodarone, ICI), iodine deficiency/excess, secondary (pituitary)
Key differentiators: Pain (de Quervain's), postpartum timing, drug history, TRAb (Graves'), ESR (de Quervain's), rapid enlargement (lymphoma)
Hashimoto's + nodule: Must rule out coexisting carcinoma or lymphoma — USG + FNAC for suspicious nodules; core biopsy if lymphoma suspected
Sick euthyroidism trap: Avoid TFTs in acutely unwell patients — TSH may be transiently abnormal

Active Recall - Differential Diagnosis of Hashimoto's Thyroiditis

References

^[1] Lecture slides: GC 177. A thyroid nodule benign thyroid nodules; thyroid cancer.pdf (p4 — Goitre Classification) ^[2] Senior notes: Ryan Ho Endocrine.pdf (p30 — Hashimoto's Thyroiditis) ^[3] Senior notes: Ryan Ho Fundamentals.pdf (p423–426 — Hypothyroidism, Goitre and Thyroid Nodules) ^[4] Senior notes: Ryan Ho Endocrine.pdf (p23 — Graves' Disease) ^[6] Senior notes: felixlai.md (Thyroid antibodies table) ^[7] Senior notes: Adrian Lui Pediatrics.pdf (p274–275 — Hypothyroidism) ^[8] Senior notes: Ryan Ho Endocrine.pdf (p31 — Subacute Thyroiditis) ^[10] Senior notes: maxim.md (Risk factors — Hashimoto's thyroiditis and thyroid lymphoma) ^[11] Senior notes: Ryan Ho Endocrine.pdf (p17 — Subclinical hypothyroidism, Goitre and Thyroid Nodules) ^[12] Senior notes: Ryan Ho Opthalmology.pdf (p128 — Dysthyroid Eye Disease) ^[13] Senior notes: Ryan Ho Diagnostic Radiology.pdf (p59 — Thyroid Scintigraphy) ^[14] Lecture slides: GC 177. A thyroid nodule benign thyroid nodules; thyroid cancer.pdf (p5 — Thyroid nodule pathology)

Diagnostic Criteria for Hashimoto's Thyroiditis

A confident diagnosis of Hashimoto's thyroiditis can be made when the following are present:

Criterion	Detail
1. Biochemical evidence of hypothyroidism (overt or subclinical)	↑TSH ± ↓fT4 (overt) OR ↑TSH with normal fT4 (subclinical) ^[2]^[7]^[15]
2. Positive thyroid autoantibodies	Anti-TPO Ab (90–100%) is the most diagnostically useful. Anti-Tg Ab (80–90%) adds sensitivity ^[2]^[6]
3. Compatible clinical picture	Diffuse, firm, rubbery, painless goitre ± hypothyroid symptoms ^[2]

When is Biopsy Needed?

Histological confirmation is NOT required for routine diagnosis of Hashimoto's. The combination of positive anti-TPO + appropriate TFT is sufficient. Biopsy is only needed when:

There is a discrete nodule within the Hashimoto's goitre that needs evaluation for malignancy
There is rapid enlargement raising suspicion for thyroid lymphoma (requires core biopsy, not just FNAC) ^[10]
The clinical picture is atypical and an alternative diagnosis needs to be excluded

Investigation Modalities — Detailed Breakdown

1. Thyroid Function Tests (TFTs)

This is the first-line investigation for any patient with suspected thyroid disease. Understanding what you are measuring and why is critical.

TSH level is the MOST sensitive indicator of thyroid function ^[15] due to the logarithmic relationship between TSH and fT4 — small changes in fT4 produce large changes in TSH
Why TSH first? Because the pituitary amplifies even subtle thyroid dysfunction. A patient can have a "normal" fT4 but an elevated TSH — the pituitary has detected the decline before the fT4 falls out of range
Normal range: approximately 0.4–4.0 mIU/L (varies by assay and laboratory)
In Hashimoto's: ↑TSH ^[2] — ranging from mildly elevated (subclinical: 4–10 mIU/L) to markedly elevated ( > 50 mIU/L in overt disease)
Ultrasensitive TSH assays ^[3]^[11] can detect values as low as 0.01 mIU/L — important for distinguishing subclinical hyperthyroidism

Interpretation of TSH in Hashimoto's ^[7]^[11]:

TSH Level	fT4 Level	Interpretation	Action
↑TSH	↓fT4	Overt primary hypothyroidism	Confirm with anti-TPO; start T4 replacement
↑TSH	Normal fT4	Subclinical hypothyroidism	Check anti-TPO; risk of progression 2–4%/yr esp if TSH > 10 or anti-TPO+ ^[11]
Normal TSH	Normal fT4	Euthyroid	If antibody-positive only: monitor annually
↓TSH	↑fT4	Thyrotoxicosis (consider hashitoxicosis)	Check TRAb + radioiodine uptake to differentiate

2. Thyroid Autoantibodies

The second pillar of diagnosis. Understanding what each antibody tells you and its sensitivity/specificity is essential.

Thyroid antibodies tested ^[6]:

Thyrotropin receptor antibodies (TRAb / Anti-TSH antibodies)
Anti-thyroid peroxidase (TPO) antibodies
Anti-thyroglobulin (TG) antibodies

Antibody	Normal Population	Graves' Disease	Hashimoto's Thyroiditis	MNG
Anti-TSH (TRAb)	0%	80–90%	10–20%	10–20%
Anti-TPO	10–15%	50–80%	90–100%	10–20%
Anti-TG	10–20%	50–70%	80–90%	30–40%

^[6]

3. Thyroid Ultrasound (USG)

Performed in all patients with a suspected thyroid nodule or nodular goitre ^[6]^[15]. In the context of Hashimoto's, USG serves two purposes:

Characterise the thyroid parenchyma (supportive of diagnosis)
Identify any nodules that require separate evaluation for malignancy

Routine for all patients with goitre/palpable nodules: TFT, thyroid USG ^[10]^[16]

Feature	Description	Why It Occurs
Diffuse hypoechogenicity	The entire gland appears darker than normal	Lymphocytic infiltration replaces normal echogenic colloid-filled follicles; reduced colloid content
Heterogeneous echotexture	"Moth-eaten" or "pseudo-nodular" pattern	Patchy lymphocytic destruction creates alternating areas of inflammation and residual tissue
Increased vascularity	↑colour Doppler flow	TSH-driven hyperplasia + inflammatory hyperaemia
Diffuse enlargement (or atrophy in late stage)	Depends on disease stage	Early: infiltration + TSH-driven hyperplasia; Late: fibrosis and atrophy
Pseudo-nodules	Focal hypoechoic areas mimicking true nodules	Focal lymphocytic aggregates; important NOT to mistake these for neoplastic nodules
Fibrous septae	Linear echogenic bands	Progressive fibrosis in advanced disease

USG in Hashimoto's — Not for Diagnosis, but for Nodule Assessment

You do NOT need an ultrasound to diagnose Hashimoto's if TFT + anti-TPO are diagnostic. USG is indicated when there is a palpable goitre or nodule to: (1) assess the parenchyma, (2) identify discrete nodules that may need FNAC, and (3) evaluate cervical lymph nodes ^[3]^[11]^[16].

If a discrete nodule is identified within a Hashimoto's goitre, it must be assessed independently for malignancy risk. The ATA 2015 Sonographic Pattern and TI-RADS classification guide the need for FNAC ^[6]^[3]:

Sonographic Pattern	USG Findings	Risk of Malignancy	Size Cutoff for FNAC
High suspicion	Solid hypoechoic ± microcalcifications, taller-than-wide, irregular margins, extrathyroidal extension	> 70–90%	* > 1 cm*
Intermediate suspicion	Hypoechoic solid WITHOUT high-suspicion features	10–20%	* > 1 cm*
Low suspicion	Isoechoic/hyperechoic solid OR partially cystic with eccentric solid area	5–10%	* > 1.5 cm*
Very low suspicion	Spongiform or partially cystic without suspicious features	* < 3%*	* > 2 cm*
Benign	Purely cystic	* < 1%*	No FNAC

^[6]

Sonographic features suspicious of malignancy mnemonic: "SHIT CME" — Solid, Hypoechoic, Irregular margins, Taller than wide, Calcifications (micro), Margin invasion, Extrathyroidal extension ^[16]

4. Fine Needle Aspiration Cytology (FNAC)

FNAC is the single most important investigation for thyroid nodules if TSH is not depressed ^[3]. However, for diagnosing Hashimoto's itself, FNAC is not routinely required — it is indicated for nodule assessment within a Hashimoto's goitre.

Bethesda Category	Risk of Malignancy	Usual Management
I. Non-diagnostic	1–4%	Repeat FNAC or operate if radiologically suspicious
II. Benign	0–3%	Clinical follow-up
III. AUS/FLUS	5–15%	Repeat FNAC, molecular testing, hemiT if AUS x2
IV. Follicular neoplasm	15–30%	Hemithyroidectomy, molecular testing
V. Suspicious for malignancy	60–75%	Hemithyroidectomy + frozen section + total thyroidectomy
VI. Malignant	97–99%	Total thyroidectomy

^[3]

FNAC Pitfall in Hashimoto's

Hashimoto's thyroiditis can produce abundant lymphocytes on FNAC, which can be confused with thyroid lymphoma. If lymphoma is suspected (rapid enlargement, B-symptoms), a core biopsy is needed — FNAC alone cannot reliably distinguish lymphoma from severe lymphocytic thyroiditis ^[10]. Additionally, Hürthle cells in Hashimoto's can be misinterpreted as a "Hürthle cell neoplasm" (Bethesda IV) — clinical correlation with antibody status and USG findings is essential.

5. Thyroid Scintigraphy (Radionuclide Scan)

NOT recommended for routine diagnostic use ^[6]^[16] in Hashimoto's. Its primary role is in the workup of thyrotoxicosis and toxic nodules.

From the lecture slides ^[17]: Investigations for thyroid nodules include blood tests (TSH + free T4), ultrasound, FNAC (+ molecular testing), and selectively: ESR, thyroid antibodies, calcitonin, genetic testing, radioisotope scan, CT/MRI, PET scan, endoscopy, and thyroidectomy (diagnostic + therapeutic).

Scenario	Indication	Expected Finding
Hashitoxicosis vs Graves'	Differentiating cause of thyrotoxicosis	Hashitoxicosis: diffuse ↓uptake (damaged follicles cannot trap iodine); Graves': diffuse ↑uptake ^[3]^[4]^[13]
Nodule + ↓TSH	Determine if nodule is "hot" or "cold"	Hot nodules rarely malignant (< 1%); cold nodules 10–20% malignant → warrant FNAC ^[3]^[18]
NOT indicated if TSH is normal/elevated	Because the nodule will never be hyperfunctioning	Would lead to unnecessary biopsies ^[6]^[18]

Pattern	Diagnosis
Diffuse ↓uptake	Destructive thyroiditis (including hashitoxicosis) vs factitious thyrotoxicosis
Diffuse ↑uptake	Graves' disease vs secondary hyperthyroidism
Heterogeneous ↑uptake	Toxic MNG
Focal ↑uptake with ↓uptake elsewhere	Toxic adenoma

These are not primary diagnostic tests for Hashimoto's but are important for assessing complications and associated conditions:

Test	Rationale in Hashimoto's	Expected Finding
CBC	Screen for associated anaemia (pernicious anaemia from coexisting autoimmune gastritis; iron deficiency from menorrhagia)	May show macrocytic anaemia (B12 def) or normocytic anaemia
Lipid profile	Hypothyroidism causes dyslipidaemia	↑Total cholesterol, ↑LDL, ↑triglycerides ^[3]^[7] — due to ↓LDL receptor expression
Serum Na+	Severe hypothyroidism → SIADH → hyponatraemia	↓Na+ in severe disease
CK (creatine kinase)	Hypothyroid myopathy	↑CK (↓clearance + muscle damage)
Prolactin	If galactorrhoea or menstrual irregularity	↑Prolactin (↑TRH stimulates lactotrophs)
ESR	To differentiate from subacute (de Quervain's) thyroiditis	Normal in Hashimoto's; markedly elevated in de Quervain's ^[3]^[8]
Serum B12	If macrocytosis or neurological symptoms	↓B12 suggests coexisting pernicious anaemia
HbA1c / glucose	Screen for associated T1DM	May be elevated

Not routine ^[3]^[11]^[16]. Only indicated in specific circumstances:

Indication	Reason
Retrosternal goitre	Cannot be visualised by USG; needed for surgical planning ^[16]
Locally advanced disease	Better delineation of important structures within cervical fascia ^[16]
Suspected thyroid lymphoma with extrathyroidal extension	Staging purposes

CT Contrast Warning

The use of iodinated contrast may affect post-op radioactive iodine body scan ^[3]^[11]. If a patient may need RAI therapy (e.g., for coexisting thyroid cancer), iodinated contrast should be avoided or planned appropriately (wait 6–8 weeks for iodine washout).

Investigation	When Indicated	Purpose
Calcitonin	Hx or clinical suspicion of familial medullary carcinoma or MEN2 ^[3]^[11]	Not relevant to Hashimoto's per se, but if a nodule within a Hashimoto's goitre is being worked up
Genetic testing (RET)	If medullary thyroid cancer confirmed	Not directly relevant to Hashimoto's
Direct laryngoscopy	For RLN palsy ^[3]^[11] — if hoarseness is present in a Hashimoto's patient	Exclude malignant invasion vs myxoedema of cords
Flow-volume loop	For airway obstruction ^[3]^[11] — if large goitre with dyspnoea	UAO results in a blunted flow-volume loop ^[3]
Core biopsy	Suspected thyroid lymphoma	Provides tissue architecture needed for lymphoma diagnosis

From the lecture slides and senior notes ^[16]^[17]:

	Routine	Selective
History + Physical exam	✓
Thyroid function test	✓
Anti-TPO antibodies	✓ (for hypothyroidism workup)
USG thyroid ± FNAC	✓ (if goitre/nodule present)
Thyroid scintigraphy	✗	Only if thyrotoxicosis + nodules (↓TSH)
CT scan	✗	Only for retrosternal goitre or locally advanced disease
PET scan	✗	No diagnostic role ^[16]
ESR		If thyroiditis DDx (de Quervain's)
Calcitonin		If MTC suspected
Core biopsy		If lymphoma suspected

The Bottom Line: Diagnosing Hashimoto's thyroiditis is usually simple — TSH + fT4 + anti-TPO is all you need in the majority of cases. The complexity arises when you need to (1) differentiate from other causes of hypothyroidism using clinical context, (2) assess coexisting nodules for malignancy, or (3) evaluate for the rare complication of thyroid lymphoma.

High Yield Summary — Diagnosis

No formal diagnostic criteria exist — diagnosis is clinical + biochemical: ↑TSH + positive anti-TPO ± compatible goitre
TSH is the most sensitive first-line test due to logarithmic TSH–fT4 relationship
Always measure fT4 (not total T4) to avoid confounding by TBG changes
Anti-TPO is the single most useful antibody (90–100% sensitive, specific for hypothyroidism, inhibits TPO activity)
USG is for nodule assessment, not for diagnosing Hashimoto's — look for diffuse hypoechogenicity, heterogeneous echotexture, pseudo-nodules
FNAC is only for discrete nodules — follow ATA criteria/TI-RADS; Bethesda classification for reporting
Scintigraphy is NOT routine — only for thyrotoxicosis workup or nodule + ↓TSH
Subclinical hypothyroidism: ↑TSH + normal fT4 — progression to overt disease 2–4%/yr if anti-TPO+ or TSH > 10
Core biopsy (not FNAC) if thyroid lymphoma is suspected
Screen for associated conditions: lipid profile, CBC, B12, glucose

Active Recall - Diagnosis of Hashimoto's Thyroiditis

The management of Hashimoto's thyroiditis is, at its core, deceptively simple: replace what the thyroid can no longer make. But the nuances — when to start, how much to give, how to monitor, what to watch for, and when surgery enters the picture — are what separate good clinical care from cookbook medicine.

The management approach depends entirely on where the patient sits on the Hashimoto's disease spectrum:

Clinical Stage	TSH	fT4	Treatment Approach
Anti-TPO positive, euthyroid	Normal	Normal	Observation — monitor annually
Subclinical hypothyroidism	↑ (4–10 mIU/L)	Normal	Consider treatment in selected patients
Overt hypothyroidism	↑↑	↓	Levothyroxine (T4) replacement — mainstay ^[2]^[19]
Hashitoxicosis (transient)	↓	↑	Symptomatic only (β-blockers); do NOT give antithyroid drugs
Goitre causing compression	Variable	Variable	Surgery (thyroidectomy)
Myxoedema coma	↑↑↑	↓↓↓	Emergency IV T4/T3 + hydrocortisone

Treatment Modalities — Detailed Breakdown

1. Levothyroxine (T4) Replacement — The Mainstay

"Levo" = levorotatory (the biologically active L-isomer); "thyroxine" = T4. This is synthetic T4 that is identical to the endogenous hormone. It is the single most important treatment for Hashimoto's thyroiditis ^[2]^[3]^[7]^[19].

Thyroxine replacement is the mainstay of treatment (ATA 2014) ^[7]^[19]

Feature	Levothyroxine (T4)	Liothyronine (T3)
Indications	Routine replacement therapy for hypothyroidism of any cause, due to its longer half-life ^[20]	Acute severe hypothyroid state — myxoedema coma — due to its faster onset ^[20]
Half-life	~7 days	~1 day
Dosing	Once daily ^[20] — the long half-life provides stable serum levels	TDS dosing required — fluctuating levels
Physiological	T4 is the predominant secretory product of the thyroid (~90%); peripheral deiodination converts it to the active T3 as needed	Direct T3 bypasses the body's regulatory deiodination
Monitoring	TSH is a reliable indicator of adequacy	TSH fluctuates unpredictably with T3 dosing

Bottom line: T4 acts as a prodrug — the body converts it to T3 at the tissues that need it, when they need it. This is more physiological and easier to monitor than giving T3 directly.

Full replacement dose: levothyroxine 1.6 µg/kg/day (~100 µg/day for 60 kg) ^[19] — but this is just a starting estimate.

Patient Population	Starting Dose	Rationale
Young, otherwise healthy	Full anticipated dose (e.g., 100 µg/day)	Can tolerate rapid correction
Elderly or IHD patients	Start low (25–50 µg/day), increase by 25 µg Q4–6 weeks ^[3]	Starting T4 may ↑cardiac output → exacerbate IHD ^[3]; risk of precipitating angina, arrhythmias, heart failure
Subclinical hypothyroidism	Low dose (25–50 µg/day)	Often lower dose sufficient as residual thyroid function remains
Pregnancy	↑ dose by ~30–50% from pre-pregnancy dose	↑metabolism and ↑TBG in pregnancy → need more T4; prevent congenital hypothyroidism ^[3]^[7]^[19]

Monitoring: monitor TSH Q4–6 weeks and titrate accordingly, then monitor Q1–2 years if within target range ^[7]^[19]

Phase	Frequency	What to Check	Action
Initiation / dose change	Q4–6 weeks	TSH (± fT4)	Adjust dose by 12.5–25 µg increments; TSH levels may decline ≤ 1 month upon initiation but smaller changes may require > 8 weeks to achieve goal TSH ^[7]
Stable maintenance	Q1–2 years	TSH	Ensure no drift in thyroid function (ongoing autoimmune destruction may require dose increases over time)
Pregnancy	Q4 weeks during 1st half, then Q trimester	TSH	Dose ↑ needed in most; maintain TSH < 2.5 in 1st trimester
Intercurrent illness / new medication	As needed	TSH	Many drugs/conditions alter T4 requirements

	Overtreatment (TSH suppressed)	Undertreatment (TSH elevated)
Consequences	Risk of osteoporosis and AF (especially in elderly) ^[7]^[19]	Risk of abnormal lipid profile → ↑risk of cardiovascular disease ^[7]^[19]
Why?	Excess T4 → ↑bone turnover (↑osteoclastic activity), ↑cardiac β-receptor sensitivity → AF	Persistent hypothyroidism → ↓LDL receptor expression → ↑LDL; ↓lipoprotein lipase → ↑TG

Adverse effects ^[20]:

Effect	Mechanism	Management
Thyrotoxicosis symptoms (palpitations, tremor, weight loss, heat intolerance)	Overdose → iatrogenic hyperthyroidism	Reduce dose; monitor TSH
Acute adrenal crisis	↑ metabolic clearance of adrenocortical hormones → ↓cortisol and aldosterone ^[20]	Contraindicated in patients with untreated adrenal insufficiency — must give hydrocortisone BEFORE starting T4
Deterioration of CVS disease	↑ workload of heart and worsens ischaemic symptoms → angina / arrhythmias / cardiac failure ^[20]	Start low, go slow in elderly/IHD; treat coronary disease first
Osteoporosis	Long-term suppressed TSH → ↑bone resorption	Avoid over-replacement; monitor bone density in at-risk patients
AF	↑atrial β-receptor sensitivity	Avoid over-replacement, especially in elderly

The Adrenal Crisis Pitfall

This is a commonly tested concept: if a patient has both hypothyroidism and adrenal insufficiency (e.g., Schmidt syndrome / autoimmune polyendocrine syndrome type 2), you must treat the adrenal insufficiency FIRST with hydrocortisone before starting levothyroxine. Why? Because T4 increases cortisol metabolism — starting T4 in a cortisol-deficient patient can precipitate an acute adrenal crisis. This is why levothyroxine is contraindicated in patients with untreated adrenal insufficiency ^[20].

This is where the controversy lies, and it is a favourite exam topic.

Subclinical hypothyroidism: ↑TSH with normal fT4 ^[3]^[11]

Management is controversial as benefit is uncertain ^[11]. The decision to treat depends on the balance of:

Risk of progression to overt hypothyroidism
Cardiovascular risk from untreated subclinical hypothyroidism
Risk of overtreatment (osteoporosis, AF) especially in elderly

When to Treat (Current Consensus)

Scenario	Recommendation	Rationale
TSH ≥ 10 mIU/L	Treat ^[11]	↑risk of CVD (IHD 1.89×, HF 1.86×) and ↑risk of progression ^[11]
Pregnant or planning pregnancy	Treat ^[11]	Maternal hypothyroidism → adverse fetal neurodevelopment; maintain TSH < 2.5 in 1st trimester
TSH 4–10 + anti-TPO positive	Consider treatment	Anti-TPO positivity predicts progression (2–4%/yr)
TSH 4–10 + symptoms attributable to hypothyroidism	Consider trial of T4	Improvement of symptoms supports continued treatment
TSH 4–10, asymptomatic, elderly	Observe + monitor	Risks of overtreatment may outweigh benefits; TSH reference range shifts higher with age

Risk of progression to overt hypothyroidism: 2–4%/year, especially if TSH > 10 mU/L or anti-TPO positive ^[11]

4. Management of the Goitre

T4 replacement treats hypothyroidism + shrinks goitre ^[2] — this is the dual benefit of levothyroxine in Hashimoto's.

5. Thyroidectomy in Hashimoto's Thyroiditis

Surgery is not first-line for Hashimoto's — it is reserved for specific indications. But when it is needed, understanding the types and complications is essential.

Indication	Explanation
Compressive goitre	Large goitre causing dysphagia, dyspnoea, stridor, or tracheal deviation despite T4 therapy
Suspected malignancy	Suspicious nodule on USG/FNAC (Bethesda IV–VI) within the Hashimoto's goitre
Suspected thyroid lymphoma	Rapidly enlarging goitre — though lymphoma is treated with R-CHOP + EBRT, surgery may be needed for tissue diagnosis or debulking
Cosmetic	Large visible goitre unresponsive to T4 therapy
Failed T4 suppression	Goitre continues to grow despite adequate T4 replacement

From the lecture slides ^[21]:

Type	Description	Indication in Hashimoto's	Key Consequences
Hemithyroidectomy (unilateral lobectomy)	Resection of one lobe + isthmus	Uninodular goitre; safe, minimal morbidities, diagnosis and cure; future reoperation on contralateral lobe without difficulty ^[21]	Around 10–20% chance of hypothyroidism ^[21]; lower risk of hypoparathyroidism and RLN injury
Total/near-total thyroidectomy	Resection of both lobes + isthmus + pyramidal lobe	Symptomatic multinodular goitre; no recurrence/need of reoperation ^[21]	Needs long-term thyroxine replacement ^[21]; additional surgical risk: hypoparathyroidism ^[21]; thyroid failure (100%) ^[10]
Subtotal thyroidectomy	Partial resection of both lobes	Rarely indicated ^[21]	Risk of recurrence + complications

Terminologies ^[10]:

Total thyroidectomy: resection of both lobes + isthmus + pyramidal lobe
Subtotal thyroidectomy: resection of > 1/2 of both lobes + isthmus
Hemithyroidectomy: resection of one lobe + isthmus
Lobectomy: resection of one lobe (isthmus preserved)

Complication	Mechanism	Management
Hypoparathyroidism → Hypocalcaemia	Parathyroid glands damaged/devascularised during surgery (especially in total thyroidectomy)	Acute: IV 10–20 mL of 10% calcium gluconate over 10 mins ^[20]; Long-term: calcium carbonate + calcitriol
RLN injury → Hoarseness	Recurrent laryngeal nerve damaged during dissection; runs in tracheo-oesophageal groove posterior to thyroid	Unilateral: hoarseness (usually recovers); Bilateral: stridor → emergency (may need tracheostomy)
Hypothyroidism	Loss of thyroid tissue	T4 replacement (universal after total thyroidectomy)
Haemorrhage / Haematoma	Post-op bleeding → neck swelling → can compress airway	Emergency: open wound at bedside to decompress; return to OT
Wound infection	Standard surgical complication	Antibiotics; wound care

Post-Thyroidectomy T4 Replacement

After hemithyroidectomy in Hashimoto's: do NOT start T4 therapy immediately postoperatively — measure serum TSH 6 weeks after surgery and determine the need for T4 ^[20]. The remaining lobe may compensate adequately (though in Hashimoto's, the remaining lobe is also diseased, so hypothyroidism is more likely than in other conditions).

After total thyroidectomy: T4 replacement is mandatory and lifelong ^[20]^[21].

This is the most extreme presentation of untreated/undertreated Hashimoto's hypothyroidism — a medical emergency with ~20–40% mortality.

Myxoedematous coma: very rare, medical emergency ^[3]^[19]

S/S: confusion, coma, ↓↓body temperature, convulsion, respiratory failure, hypoxia; prone to superimposed infections ^[3]^[19]

Treatment Protocol [3][19][20]

Component	Detail	Rationale
Treat precipitating cause	Infection (most common trigger), cold exposure, sedatives, non-compliance with T4	Mortality is driven by the precipitant as much as the hypothyroidism
Supportive care	Fluid replacement + maintain body temperature + correct fluid, electrolytes, hypoglycaemia ^[3]^[19]	Hypothermia, hyponatraemia, hypoglycaemia are life-threatening
Urgent T4 replacement	T4 200–500 µg PO stat then 100–200 µg PO daily ^[3]^[19]	Rapid restoration of thyroid hormone; loading dose needed because T4 has a long half-life
Urgent T3 replacement	T3 20–40 µg stat then 20 µg Q8H PO ^[3]^[19]	Faster onset than T4; T3 is used because peripheral conversion of T4→T3 may be impaired in critical illness
IV Hydrocortisone	100 mg Q6H ^[3]^[19]	To provide steroid cover for possible coexisting secondary hypothyroidism (autoimmune polyendocrine syndrome) or relative adrenal insufficiency — must give BEFORE T4 to prevent adrenal crisis
Ventilatory support	Intubation if respiratory failure / ↓GCS	Hypothyroidism depresses respiratory drive
Rewarming	Passive rewarming (blankets); avoid active rewarming (can cause vasodilation → cardiovascular collapse)	Core temperature may be < 35°C

Why Hydrocortisone Before T4 in Myxoedema Coma?

Two reasons: (1) Coexisting adrenal insufficiency may be present (autoimmune polyendocrine syndrome — Hashimoto's + Addison's). Starting T4 increases cortisol metabolism, which can precipitate adrenal crisis. (2) Even without Addison's, critical illness causes relative adrenal insufficiency. The hydrocortisone provides a safety net ^[3]^[19].

Because Hashimoto's clusters with other autoimmune diseases, comprehensive management includes:

Associated Condition	Screening / Action
Pernicious anaemia	Check B12, anti-IF/anti-parietal cell Ab if macrocytosis or neurological symptoms
Type 1 DM	Check HbA1c/glucose; co-management with diabetology
Coeliac disease	Check anti-tTG IgA if GI symptoms or unexplained iron deficiency
Addison's disease	Check morning cortisol + ACTH if unexplained fatigue, hypotension, hyperpigmentation, hyponatraemia
Dyslipidaemia	Lipid profile; hypothyroidism-driven dyslipidaemia often improves with adequate T4 replacement — reassess before starting statins
Thyroid lymphoma	Surveillance for rapid goitre enlargement; low threshold for core biopsy

8. Special Populations

Clinical Scenario	Management	Key Points
Euthyroid + anti-TPO positive	Observe; TSH Q12 months	Do NOT treat; 2–4%/yr progression risk
Subclinical hypothyroidism	Treat if TSH ≥ 10, pregnant/planning, symptomatic + anti-TPO+	Controversial if mild; weigh risks vs benefits
Overt hypothyroidism	Levothyroxine 1.6 µg/kg/day; titrate to normalise TSH	Lifelong therapy; monitor Q4–6w then Q1–2y
Hashitoxicosis	β-blocker only; NO antithyroid drugs	Self-limiting; will transition to hypothyroid phase
Goitre (small, stable)	T4 replacement ± T4 suppressive therapy	T4 treats hypothyroidism AND shrinks goitre
Goitre (large, compressive)	Thyroidectomy (total or hemi depending on extent)	Indications: 4Cs
Suspected lymphoma	Core biopsy → R-CHOP + EBRT	FNAC insufficient; urgency required
Myxoedema coma	IV hydrocortisone first → IV/PO T4 + T3 + supportive	Medical emergency; 20–40% mortality
Pregnancy	↑ T4 dose 30–50%; target TSH < 2.5 (1st trim)	Prevent congenital hypothyroidism

Hashimoto's thyroiditis requires life-long T4 replacement ^[3]^[7] — this is the key management principle. The autoimmune destruction is irreversible in the vast majority of cases (though a minority may have spontaneous remission ^[2]).

High Yield Summary — Management

Levothyroxine (T4) replacement is the mainstay — treats hypothyroidism AND shrinks goitre
Dose: 1.6 µg/kg/day; start low in elderly/IHD; ↑ in pregnancy
Aims: clinically euthyroid, normalise TSH, avoid overtreatment
Monitoring: TSH Q4–6 weeks initially → Q1–2 years once stable
Overtreatment risks: osteoporosis, AF (especially elderly)
Undertreatment risks: dyslipidaemia, CVD, persistent symptoms
Subclinical hypothyroidism: treat if TSH ≥ 10, pregnant, or symptomatic with anti-TPO+
Hashitoxicosis: β-blocker only — NO antithyroid drugs (nothing to inhibit)
Myxoedema coma: hydrocortisone FIRST → then IV T4/T3 + supportive care
Surgery: reserved for compression (4Cs), suspected malignancy/lymphoma
Always rule out coexisting adrenal insufficiency before starting T4 — give hydrocortisone first if suspected
Pregnancy: increase dose immediately; target TSH < 2.5; monitor Q4 weeks
Associated conditions: screen for pernicious anaemia, T1DM, coeliac, Addison's, dyslipidaemia

Active Recall - Management of Hashimoto's Thyroiditis

Complications of Hashimoto's Thyroiditis

Hashimoto's thyroiditis is a slowly progressive autoimmune condition. Left untreated or inadequately treated, it leads to complications that span virtually every organ system — because thyroid hormone affects everything. The complications can be conceptually divided into:

Complications of the hypothyroid state itself (the consequence of inadequate thyroid hormone)
Complications of the goitre (local mass effects)
Complications of the autoimmune process (associated autoimmune diseases and malignancy)
Complications of treatment (levothyroxine and surgery)

Let's dissect each category from first principles.

1. Complications of Untreated/Undertreated Hypothyroidism

These complications arise because thyroid hormone drives basal metabolic rate, protein synthesis, and catecholamine sensitivity. When it is deficient, every system decelerates. The severity is proportional to the degree and duration of hypothyroidism.

Complication	Pathophysiological Mechanism	Clinical Relevance
Dyslipidaemia	Hyperlipidaemia — both TG and cholesterol ^[3]^[7]: ↓T4 → ↓hepatic LDL receptor expression → ↓LDL clearance → ↑LDL-C; ↓lipoprotein lipase activity → ↑triglycerides	Leads to accelerated atherosclerosis → ↑risk of IHD ^[3]^[7]^[11]; TSH > 10 associated with ↑risk of IHD (1.89×) and HF (1.86×) ^[11]
Coronary artery disease	Dyslipidaemia + ↑peripheral vascular resistance + diastolic hypertension → accelerated coronary atherosclerosis	Hypothyroidism can lead to hyperlipidaemia → coronary atherosclerosis; starting T4 may ↑CO → exacerbate IHD ^[3] — a therapeutic double-bind
Pericardial effusion	↑capillary permeability + GAG deposition in pericardium → slow accumulation of transudative fluid ^[3]^[7]^[22]	Usually asymptomatic and detected incidentally on echocardiography; rarely causes tamponade because accumulation is gradual and the pericardium stretches
Bradycardia and conduction abnormalities	↓T4 → ↓β1-adrenergic receptor expression in myocardium → ↓chronotropy and ↓dromotropy	Sinus bradycardia is almost universal; heart block is rare but reported
Diastolic hypertension	↑peripheral vascular resistance (↓T4 → ↓vasodilatory effect of T3 on smooth muscle)	Contributing factor to coronary disease
Heart failure	↓contractility (↓myosin heavy-chain α expression) + diastolic dysfunction + pericardial effusion	Hypothyroidism may mask underlying HF; therefore T4 should be started slowly in susceptible individuals ^[22]

The Cardiovascular Paradox

Hypothyroidism causes atherosclerosis through dyslipidaemia, but at the same time, the low metabolic rate is somewhat "protective" because the heart is not being pushed hard. The danger comes when you start treating with T4 — suddenly increasing metabolic demand on a heart with already-compromised coronary arteries can precipitate angina, MI, or arrhythmias. This is why we start low and go slow in elderly/IHD patients ^[3]^[7].

Complication	Mechanism	Details
Depression and cognitive slowing	↓CNS neurotransmitter synthesis (↓serotonin, ↓noradrenaline) + ↓cerebral blood flow	Very common; often the presenting complaint; may be misdiagnosed as primary depression
Dementia	Prolonged ↓T4 → ↓myelination, ↓synaptic plasticity	Reversible in early stages with T4 replacement; irreversible if prolonged
Hashimoto encephalopathy	An uncommon syndrome due to autoimmune vasculitis thought to be associated with Hashimoto's thyroiditis, presenting with subacute onset of confusion, altered mentation, seizures, myoclonus ^[22]	Also called "steroid-responsive encephalopathy associated with autoimmune thyroiditis" (SREAT); responds to corticosteroids; anti-TPO strongly positive; diagnosis of exclusion
Ataxia	Cerebellar dysfunction from severe prolonged hypothyroidism ^[3]^[7]	Mechanism not fully elucidated; may involve ↓myelination of cerebellar white matter tracts
Carpal tunnel syndrome	Sensory loss as carpal tunnel is thickened in myxoedema ^[22] — mucopolysaccharide deposition compresses the median nerve	Predisposing factor for entrapment neuropathy ^[23]; may improve with T4 replacement
Peripheral neuropathy	↓T4 is a cause of polyneuropathy ^[23] — ↓Schwann cell function and ↓nerve conduction velocity	Usually a length-dependent sensorimotor polyneuropathy

Complication	Mechanism	Management
Hyponatraemia / SIADH	↓free water excretion (↓GFR + ↓cardiac output) + ↑ADH secretion → dilutional hyponatraemia ^[3]^[7]	Correct with T4 replacement + fluid restriction; severe cases may need hypertonic saline
Weight gain	↓BMR + fluid retention (myxoedema) — not primarily fat gain but mixed fluid/fat	Usually modest (5–10 kg); largely reverses with T4 replacement
Hypoglycaemia	↓gluconeogenesis + ↓cortisol (if coexisting Addison's)	Important in myxoedema coma — correct with D10

Complication	Mechanism
Anaemia	Multiple mechanisms: (1) Anaemia of chronic disease — ↓EPO production due to ↓metabolic demand; (2) Iron deficiency from menorrhagia (a consequence of hypothyroid menstrual irregularity); (3) Folate deficiency from bacterial overgrowth due to ↓GI motility; (4) Pernicious anaemia from coexisting autoimmune gastritis ^[22]
Macrocytosis	↓DNA synthesis from B12/folate deficiency (pernicious anaemia association) or direct effect of hypothyroidism on erythropoiesis

Complication	Mechanism
Menstrual irregularities	↑TRH → ↑prolactin → disrupts GnRH pulsatility → menorrhagia, oligomenorrhoea, anovulation ^[3]^[7]^[22]
Infertility	Anovulation + ↓progesterone → difficulty conceiving; ↑risk of early miscarriage ^[11]^[22]
Adverse pregnancy outcomes	Untreated maternal hypothyroidism → preeclampsia, placental abruption, preterm birth, low birth weight, impaired fetal neurodevelopment
Hyperprolactinaemia and galactorrhoea	↑TRH directly stimulates lactotrophs → ↑prolactin ^[3]^[7]
↓Libido, ED, delayed ejaculation in males ^[22]	↓testosterone from ↓GnRH + ↓SHBG alterations

The goitre in Hashimoto's is usually small to moderate and rarely causes significant compression. However, in a minority of patients, the goitre can be large enough to produce local effects, or a coexisting pathology (lymphoma) may cause rapid enlargement.

Complication	Mechanism	When to Suspect
Tracheal compression → dyspnoea, stridor	Large goitre compresses the trachea (especially with retrosternal extension)	Progressive exertional dyspnoea; inspiratory stridor; flow-volume loop shows blunted pattern
Oesophageal compression → dysphagia	Goitre pushes posteriorly against the oesophagus	Difficulty swallowing solids more than liquids
Recurrent laryngeal nerve compression → dysphonia	Very rare in uncomplicated Hashimoto's; if present, suspect malignancy (lymphoma or coexisting carcinoma) invading the nerve ^[3]	New-onset hoarseness in a Hashimoto's patient is a red flag
Venous congestion	Large goitre (especially retrosternal) compresses jugular veins	Facial plethora, distended neck veins, Pemberton's sign (facial congestion + arm tingling on raising arms above head)

3. Complications of the Autoimmune Process

The autoimmune process in Hashimoto's has consequences that extend beyond the thyroid gland itself.

Hashimoto's thyroiditis frequently coexists with other organ-specific autoimmune diseases due to shared HLA susceptibility and generalised immune dysregulation:

Associated Condition	Approximate Prevalence in Hashimoto's	Pathological Basis	Clinical Impact
Pernicious anaemia	~5–10%	Anti-parietal cell / anti-IF antibodies → ↓B12 absorption	Macrocytic anaemia, subacute combined degeneration of cord; screen with B12 levels
Type 1 DM	~2–5% (and vice versa)	Shared HLA susceptibility; islet cell autoimmunity	T1DM is associated with autoimmune thyroid disease (2–5%) ^[5]; co-manage
Addison's disease	~1–2%	Anti-adrenal autoantibodies → adrenal cortex destruction	Must treat adrenal insufficiency BEFORE starting T4 to prevent adrenal crisis
Vitiligo	~5%	Anti-melanocyte autoantibodies	Cosmetic; marker of autoimmune diathesis
Coeliac disease	~2–5%	Anti-tTG IgA; shared HLA-DQ2/DQ8	Can impair T4 absorption — suspect if patient needs unusually high T4 doses
Primary biliary cholangitis	10–15% of PBC patients have Hashimoto's ^[24]	Shared autoimmune pathogenesis	Screen for thyroid function in PBC patients
Autoimmune hepatitis	8–23% of AIH patients have autoimmune thyroiditis ^[9]	Shared autoimmune diathesis
Sjögren's syndrome	~5–10%	Lymphocytic infiltration of exocrine glands	Dry eyes, dry mouth
Alopecia areata	Variable	Anti-hair follicle autoimmunity	Patchy hair loss

The clustering of Hashimoto's with Addison's disease (± Type 1 DM) is called Schmidt syndrome (autoimmune polyendocrine syndrome type 2, APS-2). This is clinically important because the adrenal insufficiency can be fatal if unrecognised.

Teaching point: When you diagnose Hashimoto's, always think about what else may be lurking. Screen for associated autoimmune conditions based on clinical suspicion — particularly pernicious anaemia (if macrocytosis), T1DM (if hyperglycaemia), and Addison's (if unexplained fatigue, hypotension, or hyponatraemia out of proportion to hypothyroidism).

4. Complications of Treatment

Complication	Mechanism	Prevention
Overtreatment → osteoporosis	Excess T4 → suppressed TSH → ↑bone turnover → ↓bone density (especially postmenopausal women)	Avoid overtreatment; monitor TSH; do not suppress TSH below normal ^[7]
Overtreatment → atrial fibrillation	Excess T4 → ↑atrial β-receptor sensitivity → ↑automaticity	Especially in elderly; maintain TSH in normal range ^[7]
Acute adrenal crisis	Starting T4 in a patient with unrecognised adrenal insufficiency → ↑cortisol metabolism → crisis	Always consider coexisting Addison's; give hydrocortisone first if suspected
Exacerbation of IHD	↑workload of heart → angina, arrhythmias, cardiac failure ^[20]	Start low (25 µg), go slow; treat coronary disease first ^[3]

Complications of thyroidectomy ^[20]^[22]^[25]:

Classification	Complications	Mechanism and Details
Immediate (intraoperative)	Intraoperative bleeding	Direct vascular injury
	Oesophageal / tracheal injury	Dissection injury
	Tracheomalacia	Degeneration of tracheal cartilage after removal of compressive goitre
	Superior laryngeal nerve injury	SLN supplies cricothyroid → vocal fatigue, cannot produce high-pitched sound ^[20]
	Recurrent laryngeal nerve injury	Ipsilateral: unilateral vocal cord palsy → hoarseness, ineffective cough; Bilateral: stridor, dyspnoea → emergency re-intubation ± tracheostomy ^[20]
Early (1 day – 1 month)	Haematoma formation	Potentially fatal if compression on airways; management: remove stitches and allow drainage ^[20]
	Wound infection	Standard surgical complication
	Hypocalcaemia / hypoparathyroidism	MOST common complication; risk 1–4% permanent, 10–20% transient ^[20]^[25]; parathyroid glands damaged/devascularised; presents with perioral and acral paraesthesia, carpopedal spasm, Trousseau's and Chvostek's signs ^[20]
Late	Recurrence	If subtotal thyroidectomy performed (rare in Hashimoto's)
	Hypothyroidism	100% after total thyroidectomy; 10–20% after hemithyroidectomy ^[21]
	Hypertrophic scar / keloid	Individual wound healing tendency
	Hungry bone syndrome	Relevant if pre-existing hyperparathyroidism (rare in Hashimoto's); sudden ↓PTH → ↑bone ossification → severe hypocalcaemia ^[20]

5. Complications in Special Populations

Complication	Mechanism
Miscarriage	Inadequate T4 → impaired trophoblast function and implantation
Preeclampsia	↑peripheral vascular resistance + endothelial dysfunction
Preterm birth	Hypothyroidism disrupts prostaglandin and progesterone balance
Impaired fetal neurodevelopment	Fetal brain depends on maternal T4 in first trimester (before fetal thyroid is functional at ~12 weeks); ↑dosage in pregnancy to prevent congenital hypothyroidism (associated with devastating consequences) ^[7]
Placental abruption	Mechanism not fully elucidated; ↑risk in hypothyroid pregnancies

Complication	Mechanism
Growth retardation	T4 is essential for GH action at the growth plate; hypothyroidism → ↓linear growth
Delayed puberty	↓GnRH pulsatility from ↑prolactin; ↓sex steroid synthesis
Intellectual disability (juvenile myxoedema)	↓T4 during critical periods of brain development → ↓myelination, ↓synaptogenesis

High Yield Summary — Complications

Cardiovascular: Dyslipidaemia (↑LDL, ↑TG) → accelerated atherosclerosis → IHD; pericardial effusion; bradycardia; diastolic hypertension
Myxoedema coma: Most severe complication; hypothermia, coma, respiratory failure; 20–40% mortality; treat with hydrocortisone → T4/T3 → supportive
Thyroid lymphoma: 60× risk in Hashimoto's; suspect rapid painless goitre enlargement; core biopsy required; R-CHOP + EBRT
Neuropsychiatric: Depression, cognitive decline, Hashimoto encephalopathy (steroid-responsive), ataxia, carpal tunnel syndrome
Hyponatraemia/SIADH: From ↓free water excretion + ↑ADH
Reproductive: Menorrhagia, infertility, miscarriage, preeclampsia, impaired fetal neurodevelopment
Anaemia: Multifactorial — chronic disease, iron deficiency (menorrhagia), B12 deficiency (pernicious anaemia)
Associated autoimmune diseases: T1DM, Addison's (Schmidt syndrome), pernicious anaemia, coeliac, PBC, vitiligo
Treatment complications: Overtreatment → osteoporosis + AF; adrenal crisis if untreated Addison's; thyroidectomy → hypoparathyroidism, RLN injury
Pregnancy: Must increase T4 dose; untreated → miscarriage, preeclampsia, fetal neurodevelopmental impairment

Active Recall - Complications of Hashimoto's Thyroiditis

References

^[2] Senior notes: Ryan Ho Endocrine.pdf (p30 — Hashimoto's Thyroiditis; p38 — Thyroid lymphoma) ^[3] Senior notes: Ryan Ho Fundamentals.pdf (p423–429 — Hypothyroidism complications, Goitre, Myxoedema coma) ^[5] Senior notes: Adrian Lui Pediatrics.pdf (p290 — Type 1 DM associations with autoimmune thyroid disease) ^[7] Senior notes: Adrian Lui Pediatrics.pdf (p274–275 — Hypothyroidism clinical features, complications, unusual presentations) ^[9] Senior notes: Ryan Ho GI.pdf (p280 — Autoimmune Hepatitis associations with autoimmune thyroiditis) ^[10] Senior notes: maxim.md (Risk factors for thyroid cancer — Hashimoto's and thyroid lymphoma; core biopsy requirement) ^[11] Senior notes: Ryan Ho Endocrine.pdf (p17 — Subclinical hypothyroidism: cardiovascular consequences, progression risk) ^[20] Senior notes: felixlai.md (Complications of thyroidectomy; Treatment of hypothyroidism adverse effects; Myxoedema coma management) ^[21] Lecture slides: GC 177. A thyroid nodule benign thyroid nodules; thyroid cancer.pdf (p15 — Surgical treatment types and consequences) ^[22] Senior notes: felixlai.md (Classical features of hypothyroidism table; Adrian Lui Pediatrics.pdf p270 — Hashimoto encephalopathy footnote; hypothyroidism masking HF) ^[23] Senior notes: Ryan Ho Neurology.pdf (p180 — Entrapment neuropathy predisposing factors including myxoedema; polyneuropathy causes including hypothyroidism) ^[24] Senior notes: felixlai.md (Primary biliary cholangitis — associated with Hashimoto's thyroiditis 10–15%) ^[25] Senior notes: Ryan Ho Endocrine.pdf (p22 — Thyroidectomy complications: immediate, intermediate, late)

Hashimoto's Thyroiditis

Anatomy and Thyroid Function (Brief Review)

Etiology and Pathophysiology

Etiology — Why Does Hashimoto's Happen?

Step-by-Step Pathophysiology:

Classification

Clinical Features

Symptoms

Signs

Active Recall - Hashimoto's Thyroiditis (Definition, Epidemiology, Pathophysiology, Clinical Features)

Differential Diagnosis of Hashimoto's Thyroiditis

Axis 2: Differential Diagnosis of Hypothyroidism

Axis 3: Conditions That Can Mimic or Coexist with Hashimoto's

Active Recall - Differential Diagnosis of Hashimoto's Thyroiditis

References

Diagnostic Criteria for Hashimoto's Thyroiditis

Step-by-Step Approach

Investigation Modalities — Detailed Breakdown

1. Thyroid Function Tests (TFTs)

2. Thyroid Autoantibodies

3. Thyroid Ultrasound (USG)

4. Fine Needle Aspiration Cytology (FNAC)

5. Thyroid Scintigraphy (Radionuclide Scan)

Active Recall - Diagnosis of Hashimoto's Thyroiditis

Treatment Modalities — Detailed Breakdown

1. Levothyroxine (T4) Replacement — The Mainstay

When to Treat (Current Consensus)

4. Management of the Goitre

5. Thyroidectomy in Hashimoto's Thyroiditis

Treatment Protocol [3][19][20]

8. Special Populations

Active Recall - Management of Hashimoto's Thyroiditis

Complications of Hashimoto's Thyroiditis

1. Complications of Untreated/Undertreated Hypothyroidism

3. Complications of the Autoimmune Process

4. Complications of Treatment

5. Complications in Special Populations

Active Recall - Complications of Hashimoto's Thyroiditis

References

On this page

Hashimoto's Thyroiditis

Definition

Epidemiology

Risk Factors

Anatomy and Thyroid Function (Brief Review)

Thyroid Gland Anatomy

Functional Unit — The Thyroid Follicle

HPT Axis

Etiology and Pathophysiology

Etiology — Why Does Hashimoto's Happen?

1. Genetic Susceptibility

2. Environmental Triggers

Pathophysiology — The Autoimmune Cascade

Classification

Within the Spectrum of Autoimmune Thyroid Disease

Within the Classification of Hypothyroidism

Within the Classification of Goitre

Within the Classification of Thyroiditis

Clinical Features

Symptoms

A. Symptoms Related to the Goitre (Local Effects)

B. Symptoms of Hypothyroidism (Systemic Effects)

C. Symptoms of Hashitoxicosis (Transient Hyperthyroid Phase)

Signs

A. Signs on Thyroid Examination

B. Signs of Hypothyroidism (Systemic)

C. Unusual / Severe Presentations

D. Associated Autoimmune Conditions (Look for These on Examination)

Thyroid Antibody Profile — The Serological Fingerprint

Natural History

Approach to Hashimoto's Thyroiditis — Differentiation from Other Causes of Hypothyroidism

Active Recall - Hashimoto's Thyroiditis (Definition, Epidemiology, Pathophysiology, Clinical Features)

References

Axis 1: Differential Diagnosis of a Diffuse Goitre

Axis 2: Differential Diagnosis of Hypothyroidism

Primary Hypothyroidism (↑TSH, ↓fT4)

Secondary Hypothyroidism (↓/normal TSH, ↓fT4)

Axis 3: Conditions That Can Mimic or Coexist with Hashimoto's

1. Graves' Disease vs Hashimoto's

2. Subacute (de Quervain's) Thyroiditis

3. Postpartum Thyroiditis

4. Riedel's Thyroiditis (Invasive Fibrous Thyroiditis)

5. Primary Thyroid Lymphoma

6. Subclinical Hypothyroidism (Early/Mild Hashimoto's vs Other Causes)

7. Drug-Induced Hypothyroidism Mimicking Hashimoto's

Differential Diagnosis of Thyroid Nodule(s) in the Context of Hashimoto's

Clinical Decision-Making Flowchart

Summary Table: Key Differentiating Features

Active Recall - Differential Diagnosis of Hashimoto's Thyroiditis

Diagnostic Criteria for Hashimoto's Thyroiditis

The Fundamental Principle

Practical Diagnostic Criteria (Working Diagnosis)

Important Nuances

Diagnostic Algorithm

Investigation Modalities — Detailed Breakdown

1. Thyroid Function Tests (TFTs)

a. Serum TSH (Thyroid-Stimulating Hormone)

b. Free T4 (fT4)

c. Free T3 (fT3)

2. Thyroid Autoantibodies

Anti-TPO Antibodies — The Workhorse

Anti-Thyroglobulin (Anti-Tg) Antibodies

Anti-TSH Receptor Antibodies (TRAb)

3. Thyroid Ultrasound (USG)

Technical Details

Characteristic USG Findings in Hashimoto's

Assessment of Coexisting Nodules — Suspicion for Malignancy

Assessment of Cervical Lymph Nodes

4. Fine Needle Aspiration Cytology (FNAC)

Indications for FNAC in the Setting of Hashimoto's

FNAC Findings in Hashimoto's (If Sampled)

Bethesda Classification — Standard Reporting for Thyroid FNAC [3]

5. Thyroid Scintigraphy (Radionuclide Scan)

When Is Scintigraphy Indicated in the Context of Hashimoto's?

Radiopharmaceuticals [13]

Interpretation Summary [3][4]

6. Additional Blood Tests

7. CT/MRI

8. Other Selective Investigations

Summary: Routine vs Selective Investigations