Hypothyroidism

Types: primary (most common), central (pituitary/hypothalamic), drug-induced, and congenital. Most common in women over 55.

Murtagh DxT — tiredness + husky voice + cold intolerance ➜ hypothyroidism.

Clinical features

Symptoms: fatigue, weight gain, cold intolerance, constipation, dry skin, physical and mental slowing, depression, huskiness of voice, facial/periorbital puffiness, pallor, hair loss.

Signs: sinus bradycardia; delayed reflexes (slow relaxation phase); coarse dry brittle hair; thinning of the outer third of the eyebrows; dry cool skin; skin pallor/yellowing; obesity; goitre.

Hypothyroidism raises cardiovascular risk factors (hypercholesterolaemia, hypertension, LV systolic dysfunction) — treatment may improve these.

Associations / causes: Graves’ disease and other autoimmune disorders (type 1 diabetes, coeliac disease), Down syndrome, Turner syndrome; drugs (iodine excess, lithium, amiodarone, interferon alfa, interleukin-2, tyrosine kinase inhibitors, immune checkpoint inhibitors).

Investigations

• TSH and free T4 are the core tests. Do not order a thyroid ultrasound unless there are palpable nodules.
• Most spontaneous primary hypothyroidism is autoimmune (Hashimoto) thyroiditis — thyroid peroxidase (anti-TPO) antibodies are positive.

Refer suspected secondary/tertiary (central) hypothyroidism to an endocrinologist.

When to treat

Pattern	TSH / T4	Action
Overt, symptomatic	↑TSH, ↓T4	Clear indication to start levothyroxine
Overt, asymptomatic (rare)	↑TSH, ↓T4	Confirm persistence — repeat in 4–8 weeks before treating (may follow critical illness)
Subclinical, symptomatic	↑TSH, normal T4	Reasonable to trial therapy for symptoms
Subclinical, asymptomatic, TSH >10	↑↑TSH, normal T4	High progression risk — repeat in 4–8 weeks, treat if persistent
Subclinical, asymptomatic, TSH <10	mildly ↑TSH, normal T4	May not need treatment — retest with anti-TPO in 4–8 weeks; if antibody-positive, monitor at 3 then 6–12 months and treat if TSH rises or symptoms appear; if antibody-negative, review ~12-monthly

In younger patients (<60) with significant cardiovascular risk factors, consider a 3–6 month levothyroxine trial and stop if there’s no clear improvement (e.g. reduced LDL).

Levothyroxine replacement (primary hypothyroidism)

• Initial full replacement (preferred in most overt hypothyroidism): levothyroxine ~1.6 microgram/kg daily (to the nearest 25 microgram); use lean body weight at extremes of weight. Adjust every 4–8 weeks.
• Initial partial replacement (mild/subclinical disease, cardiovascular disease, older patients — a high starting dose can precipitate cardiac ischaemia): levothyroxine 25–50 microgram daily, titrated every 4–8 weeks.

Dose targets (titrate TSH every 4–8 weeks to an age-specific range):

• <60 years — TSH 0.5–2.5 mIU/L
• 60+ years — TSH 1–5 mIU/L
• >80 years — consider TSH 4–6 mIU/L

Targets are less aggressive with age because thyroid requirements fall, the normal TSH rises, and older patients are more susceptible to over-treatment (atrial fibrillation, accelerated bone loss, cardiac ischaemia). Once stable, measure TSH at 3–6 months, then annually.