Thyroid system ontogeny in the sheep: a model for precocial mammalian species

There is now extensive data in two animal models to characterize the events of thyroid system maturation in mammalian species. Data from the rat have contributed importantly to our understanding of thyroid development in the altricial species while studies in the sheep have characterized thyroid ontogenesis in precocial mammals. More limited data are available from human studies and this information, with the animal data, indicate that the major features of thyroid system development are qualitatively similar in all three species. The combined data has provided an important overview of mammalian thyroid ontogenesis. However the picture remains unfinished. We have limited insights into the maturation of the molecular events of hypothalamic pituitary control, the maturation of TSH and thyroid nuclear receptors and postreceptor events, and the mechanisms of the thyroid effects on brain maturation. We can look forward to continuing contributions in these areas.