Simultaneous measurement of thyroxine and triiodothyronine in trout plasma using a solid-phase radioimmunoassay

Abstract
A solid-phase radioimmunoassay (RIA) employing miniature G-25 Sephadex columns and a single isotope was evaluated for simultaneous measurement of T3 (3,5,3''-triiodo-L-thyronine) and T4 (L-thyroxine) in trout plasma. The method was judged reliable based on the characteristics of the standard curves, the high and consistent recoveries of T3 (87.0 or 86.7%) and T4 (103 or 98%) added either singly or in combination, low inter- (< 8%) and intra-assay (< 7%) coefficients of variation, predictable values of T3 and T4 after plasma dilution, and acceptable correlations between hormone values obtained using either the simultaneous or single RIA methods (rT3 = 0.89; rT4 = 0.91). The simultaneous RIA with its savings in time, plasma volume, and reagents can be used to advantage to measure T3 and T4 in plasma of trout and presumably other vertebrates.

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