Development of Fetal Thyroid System Control

The hypothalamic-pituitary-thyroid system in the fetus is comprised of a complex of hypothalamic centers, anterior pituitary thyrotroph cells, thyroid follicular cells, and peripheral tissues which metabolize and respond to thyroid hormones. Embryogenesis of the hypothalamus and of the pituitary and thyroid glands is largely completed by 12 weeks of gestation in the human fetus. Hypothalamic histogenesis and differentiation and continued growth and functional maturation of the pituitary and thyroid glands proceed into the neonatal period (1). Studies of thyroid system maturation have been conducted in many species, but most detailed data have been developed in the sheep and rat models. The period of thyroid system development in man, a precocial species, extends to one month of postnatal life (some 44 weeks). In the sheep (also a precocial species), comparable thyroid system maturation encompasses 150 days of intrauterine gestation plus two weeks of postnatal life (165 days total). In the rat (an altricial species) thyroid system development requires some 50 days (21 fetal days + 28 postnatal days). Relative thyroid system maturation in these species is quite comparable and thyroid control matures during the latter half of the period of ontogenesis. Thus, the third trimester fetal sheep and neonatal lamb and the neonatal rat have served as useful models for the study of thyroid control maturation. The following discussion of the development of various aspects of control of thyroid hormone production will review data in the three species normalized as relative development time to facilitate species comparisons.