Free Triiodothyronine (fT3)

Triiodothyronine (T3) is a thyroid hormone that circulates in blood almost completely bound (]99.5%) to carrier proteins. The main transport protein is thyroxine-binding globulin (TBG). Furthermore, the concentrations of the carrier proteins are altered in many clinical conditions, such as pregnancy. In normal thyroid function, as the concentrations of the carrier proteins changes, the total triiodothyronine level also changes, so that the free triiodothyronine concentration remains constant. Measurements of free triiodothyronine (Free T3) concentrations, therefore, correlate more reliably with your clinical status than total triiodothyronine (T3) levels. For example, the increase in total triiodothyronine levels associated with pregnancy, oral contraceptives and estrogen therapy result in higher total T3 levels while the free T3 concentration remains unchanged .