Thyroid function test patterns
What it means
Interpret TSH against T4. Primary hypothyroidism: ↑TSH, ↓T4 (Hashimoto, post-RAI/thyroidectomy). Subclinical hypothyroidism: ↑TSH, normal T4. Primary hyperthyroidism: ↓TSH, ↑T4/T3 (Graves = TSH-receptor Ab; toxic nodule; thyroiditis). Secondary (pituitary): TSH inappropriate for the T4 — e.g. ↓T4 with a low/normal TSH = central hypothyroidism. Sick euthyroid: ↓T3 (± ↓T4/TSH) in any acute illness — don't check TFTs when acutely unwell.
Worked example
↓TSH, ↑T4 four months postpartum with TPO antibodies positive but TSH-receptor antibodies negative → postpartum thyroiditis (self-limiting → propranolol, NOT carbimazole).
In the exam
A woman four months postpartum is tremulous with ↑T4, ↑T3, undetectable TSH, positive TPO but negative TSH-receptor antibodies.
What settles it
Primary disease moves TSH and T4 in OPPOSITE directions; a TSH that is inappropriate for the T4 = a pituitary (secondary) cause.
Source: NICE NG145 (thyroid disease)