This kit is used for quantitative determination of free triiodothyronine (FT3) in human serum in vitro. 3,5,3 '- triiodothyronine (T3) is an important thyroid hormone secreted by the thyroid gland. Under the action of TSH, thyroglobulin (TG) releases T3 into the blood. About 60% of T3 in the serum is made by deiodination of T4. The total amount of T3 is about 1 / 60 of T4, but its biological activity is far more than T4. Most of the T3 in the blood is combined with TBG and so on. The combined T3 and free T3 (FT3) are in a dynamic balance. A small amount of free T3 acts on the target cells, and cooperates with T4 to maintain normal thyroid function.