Absence of Placental Transfer of L-Triiodothyronine (T3) in the Rat

Abstract

Summary: To detect placental transfer of L-triiodothyronine (T₃) in pregnant rats, we injected 1 μCi [¹²⁵I]T₃ on the 16th, 18th, and 20th day of gestation. Three hours after the injection, which corresponds to the equilibrium time determined by a method of constant infusion, the pregnant rats and their fetuses were killed. The [¹²⁵I]T₃ was extracted from the serum or the homogenate by butanol extractions and alkaline washes. The transfer rate was calculated from the quantity of [¹²⁵I]T₃ in the serum of the fetuses after 3 hr and from the maternal metabolic clearance rate (8.19 ± 0.45 ml/hr/100 g body weight; mean ± SEM). At the 16th day of gestation, the placental transfer of T₃ was 0.82 ± 0.11% of the total maternal clearance rate/litter weight; it was 1.05 ± 0.25% at the 18th day of gestation and 0.58 ± 0.10% at the 20th day of gestation. There were no significant differences between these results. The maternal T₃ concentration was 68.27 ± 20.6 ng/100 ml and its production rate was 5.57 ± 0.31 ng/hr/100 g; with these data we calculated a maternal-fetal T₃ transfer of 46 ± 6 pg/hr. Furthermore, there was no T₃ transfer observed when the mother received 1.9 μg unlabeled T₃, which led to a significant rise in maternal T₃ concentration (68.27 ± 20.6 ng to 102.23 ± 7.41 ng/100 ml; P < 0.01); there was no detectable T₃, in the fetal serum. From these results we conclude that there is minimal or no placental transfer of T₃ in the rat and that the hypothalamic-pituitary-thyroid axis of the fetus develops autonomously. Speculation: The present data plus the fact that the rats are hypothyroid at birth support the concept of an autonomous development of the hypothalamic-pituitary-thyroid axis. It suggests that the rat can serve as a useful model for development of this axis in the human.