Free triiodothyronine (FT3): mainly useful when hyperthyroidism is suspected.
What it is
Free T3 in pmol/L. Less useful for routine hypothyroidism monitoring than TSH and FT4.
Free T3 measures unbound triiodothyronine, the more receptor-active thyroid hormone. Reported in pmol/L. Many UK pathways use TSH and free T4 first; FT3 is added when hyperthyroidism is suspected or results disagree.
Why it matters
FT3 is interpreted with TSH and free T4. High or low FT3 can have several explanations, including patterns of excess thyroid hormone activity and temporary changes during illness. Only a qualified clinician can diagnose thyroid conditions. Do not start T3 tablets from a consumer panel reading; that can be dangerous.
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
T3 is the more receptor-active thyroid hormone influencing heart rate, temperature, and metabolic rate.
T3 binds thyroid hormone receptors and increases transcription of genes for metabolic rate, heart rate, and thermogenesis. Most circulating T3 comes from peripheral conversion of T4.
For athletes
Low energy availability can be associated with low T3 in research settings, this is a clinical/sports-medicine topic, not a cue to self-medicate T3.
Low T3 in energy-deficient athletes is a research and clinical observation pointing toward restoring fueling, not toward self-starting liothyronine, which is prohibited in sport without a TUE and medically risky.
