Metabolism of L-Thyroxine (3:5-131I) in Man

Publisher Website
Google Scholar
Abstract
Thyroxine (3:5-131I) was synthesized by coupling diiodohyroxyphenylpyruvic acid with diiodotyrosine-131I. The labeled thyroxine was given orally to 1 hypothyroid and 4 euthyroid patients and the serum radioactivity was followed for up to 16 days by means of paper chromatography. Three solvent systems were used as well as radioautography of 2-dimensional chromatograms. Only thyroxine and small amounts of iodide could be identified in the serum samples. No labeled diiodotyrosine or monoiodotyrosine was found as a product of thyroxine degradation. However, stable iodotyrosines were identified in other aliquots of the same sera by the identical procedure using nbutanol extraction, and paper chromatography, followed by elution, and estimation by the eerie sulfate-arsenious acid reaction. The half-life in serum of ingested radioactivity appeared longer than that reported for thyroxine labeled in the 3′:5′-position, but the significance of this finding is uncertain. It is concluded that the α-phenyl ring of thyroxine is deiodinated to iodide with no intermediates that are detectable in the serum.