Description
Triiodothyronine (T3) is a thyroid hormone that is important in body metabolism. About 80% of triiodothyronine result from conversion of thyroxine (T4 ) to T3 in peripheral tissues, while the remaining 20% are synthesized by the follicular cells of the thyroid gland. Free T3 levels generally correlate closely with total T3 levels; however, total T3 is affected by levels of thyroid hormone-binding proteins, while free T3 is not. Thus, elevations of these proteins due to pregnancy, oral contraceptive use, or estrogen therapy will increase total T3 without affecting the free T3 concentration making free T3 an excellent biomarker for following active thyroid hormone levels.