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Simultaneous Determination of Free Thyroxine and Capacity of Thyroxine-Binding Globulin

University of California, School of Medicine, Los Angeles, California

Correspondence: For reprints contact: W. N. Paul Lee, MD, Dept. of Pediatrics, Univ. of California, Los Angeles, CA 90024.

ABSTRACT

A simple method is described for the simultaneous determination of capacity thyroxine-binding of globulin (TBG) and free thyroxine concentration (FT₄). The ratio of bound to free T₄ (B/F) is first determined for two total-T₄ concentrations using a Sephadex G-25 competitive-binding technique. TBG capacity and FT₄ can both be calculated assuming a known value of affinity constant of TBG. The method is linear over a twenty-fold serum dilution. FT₄ calculated is identical to that calculated using the method of Irvine. TBG capacity is shown to be linearly correlated to TBG concentration as determined by radioimmunoassay and is consistent with a molecular weight of 69,000 Daltons and one T₄ binding site per molecule. FT₄ is found to correlate with the free thyroxine index in a complicated way, depending on the degree of TBG saturation.