Free T3

Free Triiodothyronine - Triiodothyronine (T3) is one of two major hormones produced by the thyroid gland, a small butterfly-shaped organ that lies flat across the windpipe at the base of the throat. The other major thyroid hormone is called thyroxine (T4) and together they help control the rate at which the body uses energy. Almost all of the T3 (and T4) found in the blood is bound to protein. The rest is free (unbound) and is the biologically active form of the hormone. Tests can measure the amount of free T3 or the total T3 (bound plus unbound) in the blood. Most of the hormone produced by the thyroid is T4. This hormone is relatively inactive, but it is converted into the much more active T3 in the liver and other tissues. T3 levels are regulated by the feedback system that the body uses to maintain stable amounts of thyroid hormones in the blood. T4 and TSH, along with its regulatory hormone thyrotropin releasing hormone (TRH), which comes from the hypothalamus, are also part of the feedback system.