The Tri-Iodothyronine Free (FT3) test is a blood test that measures the amount of free, unbound triiodothyronine hormone in the bloodstream. Triiodothyronine is one of the thyroid hormones that helps regulate metabolism, body temperature, and other vital functions. The free, unbound form of the hormone is the biologically active form and is a more accurate reflection of thyroid function than total triiodothyronine levels. The FT3 test is often ordered along with other thyroid function tests to help diagnose thyroid disorders such as hyperthyroidism or hypothyroidism. Results from the FT3 test are typically interpreted in conjunction with other clinical and laboratory findings to determine the appropriate course of treatment.