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This is a clinician-written, evidence-based summary aligned to the 2026 MLA Content Map. It is intended for medical students and junior doctors preparing for the UKMLA. Always cross-reference with NICE guidance, local protocols, and clinical judgement.
The Bottom Line
- Primary hypothyroidism: raised TSH + low free T4. Secondary (pituitary): low TSH + low free T4
- Commonest cause in UK: Hashimoto thyroiditis (autoimmune — anti-TPO antibodies positive)
- Treatment: levothyroxine — start low (25–50 mcg), titrate every 6–8 weeks to normalise TSH
- In elderly/cardiac disease: start 25 mcg and increase slowly (risk of precipitating angina/arrhythmia)
- Subclinical hypothyroidism: raised TSH + normal free T4 — treat if TSH >10 or symptomatic
- Myxoedema coma: rare but life-threatening — IV levothyroxine + IV hydrocortisone + supportive care
Overview
Hypothyroidism results from insufficient production of thyroid hormones (T3 and T4). Primary hypothyroidism (thyroid gland failure) accounts for >95% of cases, with Hashimoto thyroiditis (chronic autoimmune thyroiditis) being the commonest cause in iodine-sufficient countries like the UK. Other causes include post-thyroidectomy, radioiodine treatment, drugs (amiodarone, lithium, carbimazole), iodine deficiency (worldwide), and postpartum thyroiditis. Secondary hypothyroidism results from pituitary TSH deficiency. The condition is readily treated with levothyroxine replacement.
Epidemiology
Hypothyroidism affects approximately 2% of the UK population (overt disease), with subclinical hypothyroidism affecting a further 5–10%. It is 5–10 times more common in women. Incidence increases with age. Hashimoto thyroiditis is associated with other autoimmune conditions (type 1 diabetes, Addison disease, vitiligo, pernicious anaemia, coeliac disease). Anti-TPO antibodies are present in >90% of Hashimoto cases.
Clinical Features
Symptoms
Fatigue and lethargy — often the predominant complaint
Weight gain (modest — typically 2–5 kg, mostly fluid)
Cold intolerance
Constipation
Dry skin and hair, hair thinning/loss (including lateral eyebrow)
Menorrhagia (heavy periods) or amenorrhoea
Low mood, depression, cognitive slowing ("brain fog")
Myalgia, muscle cramps
Hoarse voice
Signs
Goitre (in Hashimoto — diffuse, firm, non-tender)
Bradycardia
Dry, coarse skin; non-pitting oedema (myxoedema — face, hands, feet)
Delayed relaxation of deep tendon reflexes
Periorbital puffiness
Carpal tunnel syndrome
Hypothermia, reduced consciousness, hypoventilation (myxoedema coma)
Investigations
First-line
TSHRaised in primary hypothyroidism (the most sensitive test). Low/normal in secondary (pituitary)
Free T4Low in overt hypothyroidism. Normal in subclinical hypothyroidism (raised TSH + normal fT4)
Second-line
Thyroid peroxidase (TPO) antibodiesPositive in >90% Hashimoto — confirms autoimmune aetiology. Useful to predict progression of subclinical disease
Lipid profileHypercholesterolaemia (elevated LDL) is common in hypothyroidism and may improve with treatment
FBCMacrocytic anaemia (associated pernicious anaemia) or normocytic anaemia
Specialist
Thyroid USSIf goitre or palpable nodule — assess for nodular disease, malignancy
Pituitary function tests + MRI pituitaryIf secondary hypothyroidism suspected (low TSH + low fT4): check cortisol, gonadotrophins, prolactin, GH/IGF-1
Management
NICE NG145 (Thyroid disease), 20191
Levothyroxine replacement
- Starting dose: 50–100 mcg OD (in young, otherwise healthy patients)
- In elderly or known cardiac disease: start 25 mcg OD and titrate slowly every 4–6 weeks
- Take on empty stomach, 30 min before breakfast (or at bedtime)
- Separate from calcium, iron, and PPI by ≥4 hours (reduce absorption)
2
Monitoring and dose adjustment
- Check TSH 6–8 weeks after starting or dose change
- Target: TSH in the normal reference range (typically 0.4–4.0 mU/L)
- Once stable: annual TSH monitoring
- In pregnancy: increase dose by ~25–50% early in first trimester; monitor TSH every 4 weeks
3
Subclinical hypothyroidism
- TSH >10 mU/L: treat with levothyroxine (high progression rate to overt disease)
- TSH 4–10 mU/L: consider treatment if symptomatic, positive TPO antibodies, or planning pregnancy
- If not treating: monitor TSH every 6–12 months
4
Myxoedema coma (rare but life-threatening)
- Medical emergency — ITU admission
- IV levothyroxine (loading dose) — oral absorption unreliable in this state
- IV hydrocortisone (must give before/with thyroxine — risk of adrenal crisis if coexistent adrenal insufficiency)
- Passive rewarming, ventilatory support, treat precipitant (infection, MI, CVA, sedatives)
Complications
- Cardiovascular: Hypercholesterolaemia → accelerated atherosclerosis; bradycardia; pericardial effusion
- Myxoedema coma: Extreme hypothyroidism with hypothermia, hypoventilation, hyponatraemia, reduced GCS — mortality >20%
- Infertility: Anovulation, menstrual irregularity — thyroid function should be checked in all women with infertility
- Congenital hypothyroidism: If untreated in neonates → cretinism (intellectual disability, growth failure) — screened on Guthrie test (day 5)
- Depression: Common association — check TFTs in all patients presenting with depression
- Carpal tunnel syndrome: From mucopolysaccharide deposition
UKMLA Exam Tips
- 1Primary hypothyroid = HIGH TSH + LOW fT4. Secondary = LOW TSH + LOW fT4. Know the pattern
- 2Hashimoto = commonest cause in UK. Anti-TPO positive. May initially cause transient thyrotoxicosis (Hashitoxicosis)
- 3Levothyroxine in elderly/cardiac disease: START LOW (25 mcg) GO SLOW — risk of angina/MI/arrhythmia
- 4In myxoedema coma: give IV hydrocortisone BEFORE thyroxine — otherwise risk of precipitating adrenal crisis
- 5Check TFTs: in unexplained fatigue, depression, weight gain, hypercholesterolaemia, infertility, constipation
- 6Amiodarone can cause BOTH hypothyroidism (more common in iodine-sufficient areas) and hyperthyroidism
- 7Lithium causes hypothyroidism — monitor TFTs every 6 months in patients on lithium
practicetest your knowledge on hypothyroidismApply what you've learnt with UKMLA-style questions from the iatroX Q-Bank — endocrine and beyond.
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