THYROXINE AND 3,5,3'-TRIIODOTHYRONINE PRODUCTION, METABOLISM, AND DISTRIBUTION IN PREGNANT RAT NEAR-TERM

In the pregnant rat near term thyroxine (T-4) and 3,5,3'-triiodothyron ine (T-3) concentrations are lower in plasma and extrathyroidal tissue s, except T-3 in the brain. To study the changes in T-4 and T-3 kineti cs a bolus injection of [I-125]T-4 and [I-131]T-3 was administered to nonpregnant controls and rats 14 and 19 days pregnant. Physiological p arameters of the production, interpool transport, distribution, and me tabolism of T-4 and T-3 were estimated by means of a three-compartment model. The production and partition of T-4 remained unchanged during pregnancy. The total distribution volume of T-4 was enlarged. On day 1 9 the plasma clearance rate was doubled, and transport to the fast poo l was more than tripled. The rate of production of T-3 was slightly di minished. 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 T-4 is responsi ble for the distribution of the available T-4 between the maternal and the fetal compartment.