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A Simplified Method for Estimating Free-Thyroxine Fraction in Serum

Veterans Administration Hospital, San Francisco, California

Correspondence: For reprints contact: Ralph R. Cavalieri, Veterans Administration Hospital, 4150 Clement St., San Francisco, Calif. 94121.

ABSTRACT

A method has been developed for estimating relative changes in free T₄ fraction (F T₄ F) in serum. The principle involves the partition of added labeled T₄ between binding sites on specific proteins and sites on Dextran gel (Sephadex). By using dilute serum and rapidly sedimenting particles of Sephadex, the method avoids the necessity for prolonged incubation and centrifugation. The Sephadex "uptake" of labeled T₄ from patient's serum is expressed as a percent of the uptake from a control serum. The results correlate well with those obtained by equilibrium dialysis over a wide range of thyroid states. The normalized F T₄ F, obtained by this method, is used together with the total T₄ value to calculate an index of free T₄ concentration. The latter is considered to be the parameter which best reflects the level of thyroid function.