Analysis of the influence of thyroid hormone on prenatal and postnatal maturation of the rat

The effects of hyper‐ and hypothyroidism on fetal development and on the postnatal maturation of the central nervous system was studied in the rat. Our observations indicated that the development of the rat fetus, as measured by birthweight and skeletal maturation of the newborn animal is not markedly influenced by presence or absence of thyroid hormone during the prenatal stage. Availability of thyroid hormone is a more critical factor with respect to maturation of the nervous system taking place in the postnatal animal. Delay in cerebellar maturation and delay in increase in dry weight of cerebrum and cerebellum were noted in hypothyroid rats. A transitory acceleration of oxygen consumption of these structures during their maturation was also noted in hyperthyroid litters. A more permanent suppression of learning behavior and of the thermoregulating mechanism was observed in young hypothyroid rats deprived of thyroid hormone since birth. It is concluded that requirements for thyroid hormone during development of the rat are limited to a critical period coinciding with the first two to three weeks of postnatal age.