Direct measurement of the contributions of type I and type II 5′-deiodinases to whole body steady state 3,5,3′-triiodothyronine production from thyroxine in the rat

Production of T-3 from T-4 in tissues is catalyzed by two 5'-deiodinases, type I (D1) and type II (D2), but the quantitative contribution of each pathway to whole body T-3 production is not well established. In the presence of propylthiouracil (PTU), D1, but not D2, can be effectively blocked, providing an experimental probe for addressing this problem. Decades ago, this approach provided indirect estimates ranging from 23-44% contribution by D2, based on plasma T-3 appearance rate comparisons (PAR(3) = PCR3 [T-3](p)) in periodically T-4- injected athyreotic rats vs. controls. Two, more recent studies, using constant infusions of T-4 for replacement, achieved 22% and 65% estimates, respectively, from PAR(3) comparisons. We have revisited this problem more directly and precisely, with two major differences in experiment design. We used direct whole body steady state measurements of T-3 production, instead of indirect plasma-only data (PAR(3)). We also used (euthyroid) physiological doses of both T-4 (0.9 mu g/day-100 g BW) and T-2 (0.15 mu g/day-100 g BW) for replacement in two Lhyroidectomized rat groups, instead of T-4 only, in a 7-day constant steady state, dual tracer infusion protocol. The first group also had chronically implanted 150-mg PTU pellets (TXR-PTU); the other had implanted 0.1 N NaOH placebo pellets (TXR-EU); each delivered their product at constant rates. A third euthyroid intact group was used as the controls. The completeness of D1 inhibition was ascertained in a fourth group, identically treated with 150-mg PTU pellets, in which negligible D1 activity was found in liver and kidney using labeled 5'-D as substrate for the 5'-D assays and minimal (1 mM) dithiothreitol as cofactor. In the TXR-PTU group, the percentage of T-4 converted to T-3 was 11.8%, compared with 23.4% (P < 0.0005) in the TXR-EU group, and 22.7% (P = NS) in controls. Thus, in euthyroid steady state, D2 contributes about half of the Ta produced from T-4.