AKT · Endocrine/Thyroid

Hypothyroidism

Thyroid hormone deficiency (commonly autoimmune — Hashimoto)

Overview

Underactivity of the thyroid, most commonly autoimmune (Hashimoto/atrophic), also post-thyroidectomy/radioiodine, drugs (amiodarone, lithium) and iodine deficiency. Insidious tiredness, weight gain, cold intolerance and slowed everything. Treated with levothyroxine monotherapy titrated to TSH.

Recognise

Fatigue, weight gain, cold intolerance, constipation, dry skin, hair loss, hoarseness, low mood, menorrhagia
Bradycardia, slow-relaxing reflexes, non-pitting oedema (myxoedema), goitre (Hashimoto) or atrophic gland
Biochemistry: high TSH with low free T4 (primary); subclinical = high TSH, normal T4

Red flags

Myxoedema coma (hypothermia, bradycardia, reduced consciousness, hyponatraemia) → emergency
Suspected secondary (pituitary) cause [low/normal TSH with low T4] or coexisting Addison's → don't start levothyroxine before glucocorticoid (precipitates adrenal crisis)

Differentials & how to tell them apart

Subclinical hypothyroidismraised TSH with NORMAL free T4 — treat if symptomatic, TSH >10, or pregnant/planning

Secondary (central) hypothyroidismlow/inappropriately normal TSH with low T4 — pituitary disease, needs specialist + exclude cortisol deficiency

Sick euthyroid (non-thyroidal illness)abnormal TFTs during acute illness — recheck when recovered, don't treat

Depression/anaemiaoverlapping fatigue/low mood — TFTs and FBC discriminate

Investigations

TFTs: primary = ↑TSH, ↓free T4. Anti-TPO antibodies (Hashimoto). Check for the precipitant (drugs, post-ablation).

Management

Levothyroxine monotherapy, titrated to TSH

1Start levothyroxine (full replacement in young/fit; lower dose, titrate slowly in elderly or ischaemic heart disease). Recheck TSH at 6–8 weeks and adjust; aim TSH within reference range.Gate: In suspected ADRENAL insufficiency, give glucocorticoid FIRST — starting levothyroxine alone can precipitate an adrenal crisis; in pregnancy, increase the dose (~25–50%) early and target a low-normal TSH
2Annual TSH once stable. Refer if secondary cause, goitre/nodule (cancer pathway), subacute thyroiditis, or difficult-to-interpret TFTs.

Levothyroxine (LT4) monotherapy — first-line; take on an empty stomach; lower starting dose in elderly/cardiac (start low, go slow)

Avoid LT3/combination/natural extract in primary care — not recommended

Key points

High TSH + low T4 = primary hypothyroidism → levothyroxine to TSH. The two traps: treat the adrenal axis first if Addison's is possible, and raise the dose promptly in pregnancy. Amiodarone and lithium both cause it.

Monitor & prognosis

TSH at 6–8 weeks after changes, then annually; dose up in pregnancy.

Excellent on replacement.

Source: NICE NG145/CKS Hypothyroidism