Metabolic Clearance Rate of L-Triiodothyronine in Man: A Comparison of Results by Single-Injection and Constant Infusion Methods

Publisher Website
Google Scholar
Abstract
In order to test the validity of the single-compartment kinetics model upon which most studies of triiodothyronine (T3) are based, we have made estimates of metabolic clearance rate (MCR) of T3 by both single tracer injection and constant infusion methods in euthyroid and hyperthyroid patients. The conventional mode of analysis (final exponential component of plasma curve) was applied to the single injection data, which method assumes a single-compartment system. The results (MCRsi) were compared to those obtained by the infusion technique (MCRinf), which is independent of assumptions regarding compartmental model. In five euthyroid (control) subjects, MCRinf averaged 26.0 1/day ± 4.0 (sd), and mean MCRsi was 27.2 ± 7.1 1/day. Seven thyrotoxic Graves disease patients exhibited a mean MCRinf of 52.3 ± 6.7 and MCRsi of 79.0 ± 13.5 1/day. Two subjects with euthyroid Graves' disease gave values for MCRinf of 30.0 and 33.6 1/day (all corrected to 70 Kg body weight.) The ratio MCRinf/MCRsi, calculated from individual values, averaged 1.00 ± 0.16, 0.68 ± 0.14, and 0.88, in the controls, toxic, and euthyroid Graves' disease groups, respectively Thus, although the conventional, single-compartment model seems to yield a reasonable approximation of MCR in normal individuals, it tends to give an overestimation in Graves' disease, especially in thyrotoxic patients. The deviation from single-compartment kinetics in the latter group may be related to the alterations in distributional kinetics of T3 reported previously. In any case, it is apparent that the single-compartment kinetics model of T3 metabolism is inadequate. Such studies must take into account variations of tracer distribution in various thyroid states.

This publication has 0 references indexed in Scilit: