Thyroid Hormones in Fetal Development

Normal fetal growth and development depend on several endocrine, metabolic, and nutritional factors (1). Among them, an important role is played by thyroid hormones (TH) (T4 thyroxine and T3 triiodothyronine) both of maternal and fetal origin. The supply of maternal TH to the human fetus depends mainly on the mother’s thyroid function and on several placental transport mechanisms. Moreover, during pregnancy, double the normal iodine intake of the mother is required to preserve normal TH concentrations (2). In fetus hypothalamus-pituitary-thyroid–target tissue axis is activated shortly after the thyroid gland has reached his anatomical site in the first gestation weeks. A full maturation of the complex system including TH transport, receptor availability, and normal function of postreceptoral mechanism is however needed to ensure the specific biological action at target tissues.