The kit is used for quantitative determination of total triiodothyronine (TT3) in human serum in vitro. Total triiodothyronine (TT3) is a kind of small molecular hormone synthesized by thyroid. Under the action of TSH, TT3 is released from thyroid and enters into blood. One part of TT3 in blood comes from thyroid and the other part comes from T4 deiodination. In blood, most of TT3 binds to TBG and other proteins, only about 0.4% of FT3 acts on target cells. The determination of TT3 in serum is of great significance in the diagnosis and treatment of thyroid function and related diseases.