Radioiodinated Impurities in Commercial Preparations of131I-Thyroxine and Their Effect on the Measurement of Free Thyroxine in Human Serum by Equilibrium Dialysis

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Abstract
Chromatographic analysis of 9 lots of commercially prepared 131I-thyroxine indicated the presence of significant amounts of contaminating 131I-labeled 3,5,3′-triiodothyronine (average 3.7%) and 3,3′5′-triiodothyronine (average 5.4%). Since these iodothyronines are much less strongly bound to serum proteins than thyroxine, they would be expected to make a disproportionately high contribution to the radioactivity in the dialysate of dialysis systems used for measuring the free thyroxine concentration in serum. This expectation was experimentally confirmed. Direct chromatographic analysis of the dialysate demonstrated that a substantial proportion of the radioactivity was derived from the contaminating iodothyronines. Purification of 131I-thyroxine by paper chromatography, thinlayer chromatography, or preparative dialysis resulted in a 25% reduction in the dialyzable fraction as determined in the dialysis system employing diluted serum. Commercial preparations of 125I-labeled thyroxine were found to contain much smaller quantities of contaminating thyronines, and results of dialysis studies using this tracer approximated those obtained with purified 131I-thyroxine. Since the binding of the iodinated contaminating thyronines to serum proteins differs qualitatively from that of thyroxine, it is essential to use only the purest thyroxine preparation available in studies of thyroxine binding by serum proteins. The presence of these radioiodinated contaminants also may be the source of potential error in the interpretation of physiological studies which involve the use of commercial preparations of 131I-thyroxine.