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A simple synthesis of various forms of radioactive L-thyroxine, carrying the label (I/sup 131/ or C/sup 14/) either in the phenolic ring or in the nonphenolic ring and in the side chain, is described, It is based on the coupling of 4hydroxy-3,5-diiodophenylpyruvic acid and 3,5-diiodotyrosine. Labeled 4- hydroxy-3,5-diiodophenylpyruvic acid was prepared by iodination of p- hydroxybenzaidehyde foilowed by condensation with acetylglycine and hydrolysis of the azlactone formed, or by enzymic oxidative deamination of diiodotyrosine. (auth)