Metabolic.Clearance and production rates of 3,3′,5′- triiodothyronine in hyperthyroid, euthyroid, and hypothyroid subjects

To further elucidate the peripheral metabolism of rT₃ and to determine if rT₃ production rates vary directly with thyroid function, we measured the disappearance of [¹²⁵I]rT₃ in thyrotoxic and hypothyroid subjects as well as in athyreotic patients maintained eumetabolic on exogenous T₄. Kinetic parameters were determined by noncompartmental analysis, and serum concentrations of rT₃ and T₄ were determined by specific RIAs. In six hyperthyroid, seven euthyroid, and six hypothyroid subjects, the MCRs were 190.7 ± 15.7, 111.7 ± 13.2, and 71.8 ± 7.0 liters/day/70 kg, respectively (mean ± SE). Production rates (PR) for these same groups were 271.3 ± 40.5, 51.7 ± 9.1, and 4.3 ± 0.6 μg/day/70 kg. The observed differences in MCR and PR among the three study groups were highly significant (P < 0.002). These data indicate that in comparison to euthyroid subjects, rT₃ PR and MCR are increased in thyrotoxic and decreased in hypothyroid individuals.