Synopsis
Confirm a persistent raised-TSH normal-free-T4 pattern, estimate likelihood of progression and benefit, and use treatment trials or surveillance without medicalising transient results.
- Subclinical hypothyroidism is a persistently raised TSH with free T4 inside the assay reference range after transient and analytical causes are excluded.
- Confirm the abnormality on two samples separated by about three months before treatment in a stable non-pregnant adult, unless a different urgent context applies.
- NICE advises considering levothyroxine when TSH is 10 mIU/L or higher on two measurements three months apart.
Key red flags
A free T4 below range means overt hypothyroidism, not subclinical disease. Marked clinical deterioration, hypothermia, bradycardia or reduced consciousness requires urgent evaluation regardless of an earlier mild profile.
Investigation priorities
Confirm persistence and exclude evolution to overt deficiency.
Management branches
A stable adult has raised TSH with free T4 within range.
- Review acute illness, pregnancy possibility, medicines, supplements, prior thyroid treatment and the magnitude and chronology of TSH elevation.
- Repeat TSH and free T4 after approximately three months in the routine pathway, adding TPO antibodies once when autoimmune disease status is informative.
Treatment is not indicated, not beneficial or not preferred.
Key medicines
Levothyroxine for persistent subclinical hypothyroidismSelect an individual once-daily starting dose using age, body size, TSH elevation and cardiovascular disease; titrate in small steps against TSH and the current BNF rather than using a universal regimen.