The Free T3 (FT3) test measures the level of free triiodothyronine (T3), the active form of the thyroid hormone that helps regulate metabolism, energy production, and overall growth. Unlike total T3, which measures both bound and unbound T3, the FT3 test specifically measures the unbound, biologically active portion of the hormone in the bloodstream. The test is done by drawing a blood sample, which is then analyzed in a laboratory to determine the level of FT3. This test is typically administered to assess thyroid function and diagnose conditions like hyperthyroidism or hypothyroidism, especially when symptoms such as unexplained weight changes, fatigue, or rapid heartbeat are present. It is often used when thyroid issues are suspected and may be ordered alongside other thyroid tests (like TSH and T4) to provide a more comprehensive picture of thyroid health. FT3 testing is also useful for monitoring treatment in individuals with thyroid disorders, particularly in those with conditions affecting the free hormone levels.