free thyroxine index

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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

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.

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.
References in periodicals archive ?
Serum free thyroxine in thyroidal and nonthyroidal illnesses: a comparison of measurements by radioimmunoassay, equilibrium dialysis and free thyroxine index.
Interestingly, although the authors give importance to the free thyroxine index, which has a tendency to disappear from the diagnostic tests menu, they do not discuss the presence of heterophilic antibodies, which are increasingly recognized as interfering with thyroid function tests, nor do they mention the availability of molecular diagnostic tools.