Measure Thyroid Activity (FT3) - Free Triiodothyronine (FT3) Test

Free triiodothyronine (FT3) testing measures the level of the free (unbound) and biologically active form of the thyroid hormone T3 in blood serum. T3 is the most metabolically active of the thyroid hormones, produced in small amounts directly by the thyroid gland, with the majority being produced by the conversion of T4 (the main thyroid hormone) to T3 in peripheral tissues. FT3 measures the amount available for direct use by cells, making it an accurate indicator of true hormonal activity.