Synopsis
Interpret common and discordant thyroid profiles in clinical context, recognise assay and illness effects, and select a safe confirmatory pathway before attaching a diagnosis.
- Start with the patient, indication, pregnancy status, current illness, medicines and timing of the sample; numbers without context invite diagnostic error.
- For most adults without suspected pituitary disease, NICE recommends TSH first, with laboratory reflex free T4 when TSH is high and free T4 plus free T3 when TSH is low.
- When hypothalamic or pituitary disease is plausible, request TSH and free T4 together because a reassuring-looking TSH can coexist with central hypothyroidism.
Key red flags
Free T4 is low while TSH is low, normal or inappropriately unremarkable for the degree of hormone deficiency. Seek headache, visual change, amenorrhoea, reduced libido, other pituitary deficits and glucocorticoid deficiency before treatment.
Investigation priorities
Classify thyroid status efficiently when pituitary disease is not suspected.
Management branches
Symptoms or signs make thyroid dysfunction a reasonable clinical possibility.
- Check for acute illness, pregnancy, pituitary features, previous thyroid treatment, relevant medicines and high-dose biotin before selecting tests.
- Request TSH alone with the local reflex cascade when pituitary function is likely intact; request paired TSH and free T4 if central disease is suspected.