Thyroxine (T4) is a thyroid hormone which circulates bound to various proteins. Only 0.03 % of total serum thyroxine is not bound. This free thyroxine (FT4) is responsible for biological activity. Its concentration is nearly independent of the concentration of the binding proteins and thus it reflects the true thyroid status. The determination of free thyroxine is intended to be used as an aid in diagnosis of various thyroid disorders.
Assay format prevents from potential interference with biotin.