free thyroxine index, the amount of unbound, physiologically active thyroxine (T4) in serum. This amount is determined by direct assay or, more frequently, calculated on the basis of an in vitro uptake test. In this test the uptake (by resin or charcoal) of labeled triiodothyronine (T3) is measured; because T3 is less strongly bound by serum, it is used instead of T4. The free T4 index is then obtained by multiplying the T3 uptake by the total concentration of T4 in serum.
