A solid-phase radioimmunoassay for free thyroxine in serum compared with equilibrium dialysis method.

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Abstract
Serum free thyroxine (T4) was measured by a solid-phase radioimmunoassay in various thyroid states and in those associated with changes in serum T4-binding proteins. And it was compared with the value obtained by product of total serum T4 and of per cent free T4 by equilibrium dialysis. Serum free T4 concentrations were found to be 1.34±0.33ng/100ml (mean±s. D. ) in 51 normal subjects, 5.02±1.41ng/100ml in 29 hyperthyroid patients, 0.16±0.15ng/100ml in 12 hypothyroid patients, and the value in each individual of the latter two groups was significantly different from normal. Serum free T4 was normal of 1.28±0.22ng/100ml in 7 pregnant women and all had free T4 levels within the normal range. Moreover, five out of six patients with decreased thyroxine-binding globulin had normal free T4 levels, though their mean value (0.85±0.20ng/100ml) was significantly lower than the normal value. An excellent correlation (r=0.93) was observed between values for serum free T4 obtained by the present method and those by the equilibrium dialysis method. The simplicity, accuracy, sensitivity, and specificity of this assay render it highly attractive for routine clinical determination.