Endocrinology, Vol 123, 2514-2525, Copyright © 1988 by Endocrine Society

ARTICLES

Comparative aspects of the distribution, metabolism, and excretion of six iodothyronines in the rat

JJ DiStefano 3d and D Feng
Department of Computer Science, University of California, Los Angeles 90024.

We have studied the kinetics of 3 iodothyronines, 3,3'-diiodothyronine (T2), 3',5'-T2, and 3'-monoiodothyronine (T1), in groups of young adult male rats maintained under normal steady state physiological conditions. We have also performed a comparative analysis of these results, combined with corresponding kinetic indices of T4, T3, and rT3, to obtain a more comprehensive understanding of normal thyroid hormone production, distribution, and metabolism. Tracer doses of 125I- labeled 3,3'-T2, 3',5'-T2, and 3'-T1 were separately injected iv, and blood samples were collected 6-12 times for each iodothyronine in optimized sequential kinetic studies designed to maximize the precision of kinetic parameters. Labeled iodothyronines were separated quantitatively from their metabolites in each plasma sample by Sephadex G-25 column chromatography. Conventional kinetic analysis of the resulting data generated distribution volume, clearance, turnover, and mean residence time indices for each iodothyronine, and concomitant compartmental analysis of the same data provided additional results useful for integration and comparative analysis of the 6 iodothyronines. Kinetic parameters for all but T4 and T3 were similar, suggesting that similar mechanisms are responsible for the transport, metabolism, and distribution of nonhormonal iodothyronines. All but T4 and T3 (and, to a much lesser extent, 3'-T1) were almost completely and irreversibly metabolized, whereas 24-30% of the hormones (and 6% of 3'- T1) were excreted as such in feces only. Three-pool models fitted individual plasma kinetic data sets best in all cases (for all 6 iodothyronines), each with a plasma, a slowly exchanging (slow), and a rapidly exchanging (fast) pool, and kinetic parameters of interest were quantified for each iodothyronine (Ti). Quantitative analysis of an integrated 18-pool model for all 6 Tis revealed several other features of physiological interest. The fractional transport rate of T3 into the fast pool (liver, at least) is about an order of magnitude larger than that for all other Tis, supporting the hypothesis that transport of T3 into fast tissues (e.g. liver cells) is selectively amplified relative to that of the 5 other iodothyronines studied. Simultaneous and direct comparison of the 6 plasma kinetic data sets also supports this result. In addition, composite slow tissue pools, which probably exclude liver and kidney, contained the largest whole body fractions of all Tis (greater than 50%), and these also appear to be major sites of whole body T4 monodeiodinations.(ABSTRACT TRUNCATED AT 400 WORDS)

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