FT4 (25T/Kit) refers to a testing kit used to measure levels of free thyroxine (FT4) in the blood. FT4 is the unbound, biologically active form of thyroxine (T4), which is a thyroid hormone involved in regulating metabolism, growth, and development in the body. The kit, labeled as “25T/Kit,” indicates that it contains 25 tests, meaning it can be used to measure FT4 levels in 25 blood samples.