The integrity of the ether linkage during thyroxine metabolism in man

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Abstract
The structures of thyroxine metabolites after total deiodination bear on the mode of thyroxine (T4) action in vivo. The present study was undertaken to determine the integrity of the ether linkage during thyroxine metabolism in man. Normal volunteers were given simultaneous intravenous injections of two thyroxines labeled with either 14C or 3H on the opposite sides of the ether linkage, D,L-[α,β-3H]T4 and D,L-[phenolic ring-14C]T4. The ratio of alanine side chain to phenolic ring which was measured as 3H:14C ratio was found to remain constant in the serum, urine, and feces during the subsequent 3 wk. The disappearance rates of the 3H and 14C radioactivity from blood were similar. The values of half-life were in the ranges of 4.2-6.7 days. 51-63% of the 3H and 50-57% of the 14C doses were recovered from urine and 13-20% of the 3H and 15-20% of the 14C doses were recovered from feces. Chromatography of the urinary metabolites confirmed that the phenolic ring and the nonphenolic ring including at least part of the side chain remained linked together.