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In the past, investigators have used thyroxine labeled either with JIP" in the 3',5'-position or with C14 in the carboxyl position to study the metabolism of thyroxine in vivo. Although these studies have yielded much valuable information, our knowledge about the metabolic fate of the ring structure in thyroxine is incomplete because both the 3'5'-IJ13 and the carboxyl-C14 are rapidly removed from thyroxine in vivo by deiodination and .decarboxylation. To investigate the metabolic fate of the thyroxine nucleus, L-thyroxine-C14 labeled uniformly in the phenolic ring has been synthesized in our laboratory. Studies on the metabolism of ring-labeled L-thyroxine-C14 following iv administration to rats, dogs, and human subjects are reported in this communication. Synthesis of ring-labeled L-thyroxine-C"4.1 The L-thy-roxine-C1' labeled in the phenolic ring was prepared from uniformly labeled phenol-C14 in 2.6% overall yield in the following manner: phenol-C1' was converted to 4-methoxyphenol-C1' by a series of reactions that included the formation of phenylbenzene sulfonate, nitration to the 4-nitro ester, saponification to 4-nitrophenol, methyla-tion to the ether, reduction of the nitro group to 4-methoxyaniline, diazotization of the amino group, and finally hydrolysis of the diazonium salt. The conversion of 4-methoxyphenol-C1' to L-thyroxine-C1' was accomplished by modification of a route developed by Chalmers, Dickson, Elks, and Hems (1) via 3,5-dinitro-N-acetyl-L-tyrosine ethyl ester.2 The L-thyroxine-C1' possessed the following physical constants: melting range 233 to 234° (with decomposition), [a] D'-5.6, SA 1.55 ,uc per mg. It was compared with nonradioactive and P`3-labeled L-thyroxine by de-cending paper chromatography in three different solvent systems: 2-methyl-2-butanol saturated with 2 N ammo-nium hydroxide (TAA), n-butanol: p-dioxane: 2 N am

THE METABOLISM OF RING-LABELED L-THYROXINE-C14IN VIVO