Thyroid System Ontogeny in the Sheep: A Model for Precocial Mammalian Species

Human thyroid system ontogenesis can be divided into three phases which roughly parallel the three classic trimesters of pregnancy. These include the period of embryogenesis during the first trimester, a period of relative hypothalamic-pituitary quiescence during the mid-trimester of pregnancy, and a final phase of synchronized maturation of hypothalamic-pituitary control of TSH secretion, thyroid hormone secretion, and peripheral iodothyronine metabolism (1). These phases of thyroid ontogenesis occur, as well, in other mammals and have been investigated in considerable detail in two models, the developing rat and the fetal sheep (1). In both models thyroid system development generally resembles human thyroid ontogenesis. However, in the rat, an altricial species, the total period of thyroid development encompasses about 50 days (21 fetal days + 28 postnatal days) and delivery occurs at approximately the midpoint of development during the phase of relative system quiescence. In the sheep, a precocial species, the period of thyroid system maturation extends from conception to two weeks of postnatal age, about 165 days. Birth occurs at approximately 150 days gestation in this species.