Free T3, or free triiodothyronine, is a blood test that measures the unbound or “free” portion of the hormone T3 (triiodothyronine) circulating in the blood. T3 is one of the two major hormones produced by the thyroid gland and is more potent than T4, though present in smaller amounts.

Free T3 is an important test to assess thyroid function and is especially useful in diagnosing and managing hyperthyroidism, as T3 levels are often the last to decline in hypothyroidism.