free thyroxine index

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.

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free thyroxine index

FT₄I, T7 assay, T12 assay Endocrinology A lab value for T₃ uptake combined with total T₄; FTI is a clinical parameter measured by RIA, used to evaluate thyroid function, calculated by T₄ x %T₃RU–resin uptake; the FTI is ↑ in hyperthyroidism and factitious hyperthyroidism and ↓ in hypothyroidism; it is falsely ↑ in heparin therapy and falsely ↓ in phenytoin and valproic acid therapy, and in the euthyroid sick syndrome. See Thyroxine.