Absence of placental transfer of l-triiodothyronine (T3) in the rat.
To detect placental transfer of L-triiodothyronine (T3) in pregnant rats, we injected 1 muCi [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 mug 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 less than 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.
D000818:Animals; D005260:Female; D005865:Gestational Age; D007031:Hypothalamus / Q000254:growth & development; D007700:Kinetics; D008431:Maternal-Fetal Exchange; D008657:Metabolic Clearance Rate; D010902:Pituitary Gland / Q000254:growth & development; D010920:Placenta / Q000378:metabolism*; D011247:Pregnancy; D011270:Pregnancy, Animal; D051381:Rats; D013961:Thyroid Gland / Q000254:growth & development; D013997:Time Factors; D014284:Triiodothyronine / Q000378:metabolism*
D014284:Triiodothyronine