Thyroxine, the principal thyroid hormone, circulates in blood almost completely bound to carrier proteins. The main carrier is 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.
The Biorex Diagnostics FT4 kit is intended for use in the quantitative determination of Free Thyroxine concentration in human serum by a Microplate Enzyme Immunoassay.