FREE T3 / T4 - T3 (triiodothyronine) is a hormone produced by the thyroid gland. It also makes the hormone thyroxine, also known as T4. These hormones work together to control your body's temperature, metabolism, and heart rate. The majority of T3 in your body is bound to protein. Free T3 does not bind to protein and circulates in your bloodstream unbound. The T3 total test, which is the most common type of T3 test, evaluates both types free and bound of T3 in your blood.