free thyroxine index


free thy·rox·ine in·dex (FTI),

an arbitrary value obtained by multiplying the triiodothyronine uptake by the serum thyroxine concentration; it largely corrects for variations in thyroid-bound globulin concentration by providing a clinically valid estimate of the physiologically active free thyroxine; direct assay or laboratory measurement of free serum thyroxine yields a more accurate value.

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.

free thy·rox·ine in·dex

(frē thī-roksēn indeks) Arbitrary value obtained by multiplying the triiodothyronine uptake by the serum thyroxine concentration.