The monodeiodination of triiodothyronine and reverse triiodothyronine in man: a quantitative evaluation of the pathway by the use of turnover rate techniques

Abstract

These studies were performed to evaluate the quantitative role of monodeiodination in the peripheral metabolism of T3 and rT3 in man. As a prerequisite step, the serum concentrations of two diiodothyronines (T2s), 3,5-T2 and 3',5'-T2, were established by specific RIAs. In 20 normal subjects, mean (+/- SEM) serum concentrations of 3,5-T2 and 3',5'-T2 were 0.40 +/- 0.18 and 2.07 +/- 0.13 ng/dl, respectively. The mean concentrations of both T2s were significantly increased in hyperthyroidism. In primary hypothyroidism, the mean 3,5-T2 concentration was not significantly different from normal, but 3',5'-T2 concentrations were undetectable in the majority of subjects. In the first experiments, the MCRs of rT3 and all three T2s were derived by the constant infusion method. Seven normal subjects were infused simultaneously with the three 125I-labeled T2s for 12 h, and in four of the subjects, [131I]rT3 was also administered. The MCRs (liters/day . 70 kg; mean +/- SEM) were: rT3, 130 +/- 17; 3,5-T2, 168 +/- 15; 3,3'-T2, 621 +/- 84; and 3',5'-T2, 305 +/- 45. The daily production rates (PR; micrograms per day/70 kg; mean +/- SEM) were: rT3, 29.1 +/- 1.0; 3,5-T2, 0.6 +/- 0.1; 3,3'-T2, 20.8 +/- 4.1; and 3',5'-T2, 5.7 +/- 2.1. In the four subjects who received [131I]rT3, the serum T2 concentrations and PRs were also derived by turnover rate techniques. At equilibrium, 2.0 +/- 0.7% and 6.0 +/- 1.6% of [131I] rT3 were found as [131I]3,3'-T2 and [131I]3',5'-T2, respectively. The serum T2 concentrations were derived by the product of these percentages and the serum rT3 concentrations and compared with those obtained by T2 RIA. The serum 3',5'-T2 concentration was 1.3 +/- 0.4 ng/dl (tracer), and its PR was 3.4 +/- 1.1 micrograms/day (tracer); these values were closely correlated with those obtained by RIA. Serum 3,3'-T2 concentrations were 0.4 +/- 0.2 ng/dl (tracer) and 2.7 +/- 0.4 ng/dl (RIA), and the PRs were 3.2 +/- 1.6 micrograms/day (tracer) and 20.3 +/- 5.7 micrograms/day (RIA), indicating that rT3 5'-deiodination contributes only a small proportion of serum 3,3'-T2 and its PR. An analysis of the rT3 PR and the 3,3'-T2 and 3',5'-T2 PRs derived from the turnover of rT3 revealed that 28% of the rT3 produced was degraded by monodeiodination. Of this total, 49% of the deiodination occurred at the 5' position and 51% occurred at the 5 position.(ABSTRACT TRUNCATED AT 400 WORDS)