A simple synthesis of [3,5-125I]Diiodo-L-thyroxine of high specific activity.

The customary methods for labeling T4 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 metnod. This synthesis involves the coupling of [125I]diiodo-L-tyrosine with 4-hydroxy-3,5-diiodophenylpyruvic acid to form L-thyroxine labeled in the nonphenolic ring, followed by removal of the two 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 approximately 36% in the coupling reaction and approximately 86% in the deiodination reaction, amounting to an overall yield of greater than 30%. L-Thyroxine and 3,5-diiodo-L-thyronine were purified by ion-exchange chromatography. Their radiochemical purity was checked by radiochromatography.