A Simple Synthesis of [3,5-125I]Diiodo-L-Thyronine and of [3,5-125I]L-Thyroxine of High Specific Activity

Abstract

The customary methods for labeling T4 [thyroxine] and its analogs, iodination or exchange-labeling, are not applicable to those iodothyronines whose iodine atoms are exclusively in the nonphenolic ring. Therefore, [3,5-125I]diiodo-L-thyronine had to be synthesized by a different method. This synthesis involves the coupling of [125I]diiodo-L-tyrosine with 4-hydroxy-3,5-diiodophenylpyruvic acid to form L-T4 labeled in the nonphenolic ring, followed by removal of the 2 unlabeled iodine atoms in the phenolic ring. High specific activity, limited only by that of the [125I]diiodo-L-tyrosine used as starting material, can be achieved. Radioactivity yields are .apprx. 36% in the coupling reaction and .apprx. 86% in the deiodination reaction, amounting to an overall yield of > 30%. L-T4 and 3,5-diiodo-L-thyronine were purified by ion-exchange chromatography. Their radiochemical purity was checked by radiochromatography. [Application to radioimmunoassay in humans with thyroid disorders in discussed.].