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.
- Raised TSH with low free T4 supports primary hypothyroidism; raised TSH with normal free T4 is the biochemical pattern of subclinical hypothyroidism.
- Suppressed TSH with raised free T4 and/or free T3 indicates overt thyrotoxicosis, whereas normal peripheral hormones describe subclinical hyperthyroidism after confounders are excluded.
- Low free T4 with low, normal or only modestly raised TSH prompts central disease, non-thyroidal illness, drug effect or assay interference rather than automatic levothyroxine prescribing.
- Ask specifically about high-dose biotin in supplements, amiodarone, lithium, glucocorticoids and sample timing after levothyroxine; each can distort the apparent pattern.
- Use pregnancy- and assay-specific reference intervals; ordinary adult ranges and total hormone concentrations are not interchangeable with pregnancy interpretation.
- If results conflict with the clinical picture, contact the biochemistry laboratory, repeat when appropriate and consider an alternative assay before investigating rare endocrine syndromes.
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.
TSH is suppressed with free T4 and/or free T3 above range. Pulse, temperature, rhythm, heart failure, agitation and gastrointestinal or hepatic features determine whether this is uncomplicated disease or a threatened thyroid emergency.
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.
- Interpret the reported pattern with the examination and assay range, then decide whether urgent physiological assessment, cause-specific testing or planned confirmation is required.