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Metabolism of phenolic- and tyrosyl-ring labeled L-thyroxine in human beings and rats.

ABSTRACT The metabolism of L-thyroxine (T4) labeled with radioiodine in the phenolic (T4-3′,5′-131I) and tyrosyl (T4-3,5-125I) rings was studied simultaneously both in human subjects and groups of rats. A similar study was carried out using T4 labeled uniformly with 14C in the tyrosyl ring (T4-tyr-14C) in one group of rats. No significant differences in the plasma fractional disappearance rate, total distribution volume, or in the metabolic clearance rate were observed between the 2 radioiodinated T4 preparations in either the human subjects or rats. The daily and cumulative urinary excretion of radioiodine derived from either ring was the same in the human subjects whereas the urinary excretion rate of phenolic ring iodine slightly exceeded the tyrosyl ring iodine in the rat. Since, in the same rats, no corresponding decrease in the fecal excretion of phenolic ring iodine or any systematic differences in plasma values between the 2 isotopes occurred, the small increase in urinary phenolic iodine was attr...