Free T3 is a form of triiodothyronine, which is a thyroid hormone that helps control many body functions, including growth, body temperature, and heart rate. Free T3 is the active form of T3 that is not bound to protein, while bound T3 attaches to proteins, which prevents it from entering the bodys tissues. The free T3 test measures only the amount of free T3 that is floating in the blood, while the total T3 test measures both bound and free T3 in the blood. The tests for free T3 are generally less accurate than for total T3. A T3 test is most often used to diagnose hyperthyroidism, a condition in which the body makes too much thyroid hormone. Doctors may order the T3 test when a persons symptoms or previous blood tests suggest a thyroid problem.