Free Triiodothyronine (Free T3) is the biologically active form of the thyroid hormone T3 that circulates unbound to carrier proteins in the blood. This test measures the free, unbound T3 levels to evaluate thyroid function independently of binding protein variations. Free T3 represents only about 0.25% of total T3 but is the form that directly affects tissues and metabolism. Elevated free T3 levels typically indicate hyperthyroidism, while low levels suggest hypothyroidism. This test is particularly valuable for confirming hyperthyroidism when total T4 levels are normal, diagnosing T3 toxicosis, and evaluating thyroid function in patients with abnormal binding protein levels. Free T3 measurements provide more accurate assessment of thyroid status than total T3 in conditions that affect protein binding, such as pregnancy, liver disease, or certain medications.