Triiodothyronine (T3) is a thyroid hormone produced by the thyroid gland that plays a crucial role in regulating metabolism. The majority of T3 in the bloodstream is bound to protein. Free T3 is the portion of T3 that is not bound to protein and is considered the biologically active form of the hormone that can regulate metabolism. Measuring free T3 levels is important in diagnosing and managing thyroid disorders.