Passage of thyroid hormone across the placenta in intact and hypophysectomized rats

The studies were designed to determine whether thyroid hormone traverses the placenta of the rat. 1. Pregnant rats were hypophysectomized on the twelfth day of the gestation period. The histological appearance and wet weights of the fetal thyroids at term were determined and found to be the same as that of fetuses of intact mothers. These results indicate that maternal TSH is not required for maturation of the fetal thyroid. 2. Intact and hypophysectomized pregnant rats were provided with a diet supplemented with 0.2% propylthiouracil (PTU) added in powdered form starting on the fifteenth day of gestation until term. This treatment led to enlargement of thyroids of fetuses of intact and hypophysectomized mothers. 3. Groups of intact and hypophysectomized pregnant rats given the same goitrogenic treatment as group 2 were injected daily with 100, 50, 25 and 12.5 μg of 1‐thyroxin in suspension. Twenty‐five μg of thyroxin were sufficient to inhibit goiter development in fetuses of hypophysectomized rats, while 50 μg was required to prevent the goiter formation in fetuses obtained from intact non‐hypophysectomized pregnant rats. These results suggest that maternal thyroid hormone passes the placenta from mother to fetus. The effectiveness with which thyroid hormone passes from the mother to the fetal circulation is probably influenced in some way by the maternal pituitary.