Free Thyroxine (fT4)

Hyroxine(T4), a form of thyroid hormones is the major hormone secreted by the follicular cells of the thyroid gland. Over 99% of thyroxine (T4) circulates in blood is bound to thyroxine-binding globulin (TBG). However, only the free (unbound) portion of Thyroxine is responsible for the biological action. Further, 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 alters, the total T4 level changes so that the free T4 concentration remains constant. Thus, measurements of free T4 concentrations correlate more reliably with clinical status than total T4 levels. Free thyroxine (T4) can be measured directly (FT4) or calculated as the free thyroxine index (FTI). The FTI tells how much free T4 is present compared to bound T4. The FTI can help tell if abnormal amounts of T4 are present because of abnormal amounts of thyroxine-binding globulin.