This kit is used for quantitative determination of free thyroxine (FT4) in human serum in vitro. Free thyroxine (FT4) is the free thyroxine (T4) in serum. T4 is a small hormone with molecular weight of 777 synthesized by thyroid gland. Under the action of TSH, T4 is released from the thyroid gland and enters the blood. Most of T4 in the blood combines with TBG, and only about 0.5% of FT4 acts on the target cells. It is very important to measure the level of serum FT4 in the diagnosis and treatment of thyroid function and related diseases.