Abstract
The coupling reaction be-tween thyroxine and diazonium salts has been investigated with thyroxine labeled in either ring. The results indicate that coupling occurs readily in the 3,5'' positions, with the elimination of I. Coupling in the 3,5 positions occurs only slightly or not at all. This reaction has been utilized as the basis of a method for the localization of I131 in the thyroxine molecule. The method is applicable to biosynthesized I131 thyroxine.