Free thyroxine (FT4): active T4 hormone paired with TSH to classify thyroid status.
What it is
Free T4 in pmol/L measures unbound thyroxine available to tissues.
Free T4 is unbound thyroxine available to tissues. Reported in pmol/L. Binding-protein changes (pregnancy, illness) make free assays preferable to older total T4 in most settings.
Why it matters
FT4 is interpreted with TSH (and sometimes FT3) and your symptoms. Patterns of high or low FT4 with matching TSH changes can relate to reduced or increased thyroid hormone activity, but discordant results (for example central thyroid disease, assay interference, or non-thyroidal illness) need specialist input. Only a qualified clinician can diagnose thyroid conditions.
This guide is educational only and cannot diagnose you. Discuss results, especially anything outside your laboratory’s reference interval, or that does not match how you feel, with a GP or other appropriately qualified clinician.
How it affects the body
T4 is converted to more active T3 in tissues. Free hormone better reflects status than total T4 when binding proteins change.
T4 is a prohormone reservoir converted to T3. Together they regulate basal metabolic rate and cardiac sensitivity to catecholamines.
For athletes
Useful when athlete TSH is abnormal; isolated FT4 quirks after illness should be repeated rather than acted on mid-season without advice.
Discordant TSH/FT4 patterns after races or illness should be repeated when well before major treatment decisions.
