Thyroid health
Free triiodothyronine (free T3)
FT3
Included in Baseline and Complete.
Compare membershipsT3 is your most active thyroid hormone.
T3 is the most active thyroid hormone, helping set how your body uses energy. Free T3 is the small share that isn't bound to proteins in your blood and can enter your tissues.
Most T3 starts out as T4.
Your thyroid releases mostly T4, and your body converts it into T3 to put it to use. Free T3 is usually read alongside TSH and free T4, which give the clearest first picture of thyroid function.
Free T3 is most useful when an overactive thyroid is suspected, since T3 tends to rise in that setting. It's less helpful for checking an underactive thyroid, and free T3 is technically harder to measure reliably, so results are interpreted with care.
TSH and free T4 lead. Free T3 fills in the detail.
Questions
What does a high free T3 mean?
A high free T3, especially with a low TSH, may suggest an overactive thyroid. Thyroid medicines that contain T3 and biotin supplements can also affect the result. Your clinician will look at your TSH, free T4, symptoms, and medicines together.
What does a low free T3 mean?
A low free T3 can occur with an underactive thyroid, though TSH and free T4 are better guides there. Serious illness, very low-calorie eating, and some medicines can also lower T3. On its own, a low free T3 is read cautiously.