Free T3 Test

Free T3 measures the level of unbound, biologically active triiodothyronine circulating in your blood — one of the two primary hormones produced directly by the thyroid gland.

Most T3 in the bloodstream is bound to carrier proteins and isn't immediately available for your cells to use; the 'free' fraction is the small percentage that's actually active, which is why measuring it directly gives a more accurate picture of thyroid activity than measuring total T3.

T3 is actually the more potent of the two thyroid hormones on a molecule-for-molecule basis, even though the thyroid produces far more T4, since much of the body's T4 is converted into T3 in peripheral tissues as needed.

This test is usually ordered alongside TSH and Free T4 rather than in isolation, since interpreting thyroid function typically requires looking at the relationship between all three values together.