AJP - Endocrinology and Metabolism, Vol 267, Issue 6 E860-E867, Copyright © 1994 by American Physiological Society
Thyroxine and 3,5,3'-triiodothyronine production, metabolism, and distribution in pregnant rat near term
P. M. Versloot, J. Gerritsen, L. Boogerd, J. P. Schr oder-van der Elst and D. Van Der Heide
Department of Human and Animal Physiology, Agricultural University, Wageningen, The Netherlands.
In the pregnant rat near term thyroxine (T4) and 3,5,3'-triiodothyronine
(T3) concentrations are lower in plasma and extrathyroidal tissues, except
T3 in the brain. To study the changes in T4 and T3 kinetics a bolus
injection of [125I]T4 and [131I]T3 was administered to nonpregnant controls
and rats 14 and 19 days pregnant. Physiological parameters of the
production, interpool transport, distribution, and metabolism of T4 and T3
were estimated by means of a three-compartment model. The production and
partition of T4 remained unchanged during pregnancy. The total distribution
volume of T4 was enlarged. On day 19 the plasma clearance rate was doubled,
and transport to the fast pool was more than tripled. The rate of
production of T3 was slightly diminished. The plasma clearance rate was
increased, but no changes were found in the interpool transport rates.
These results suggest that in the pregnant rat near term the increased
transport of T4 is responsible for the distribution of the available T4
between the maternal and the fetal compartment.
