Fetal and Neonatal Thyroid Physiology

Abstract

Thyroid hormone is critical to fetal and neonatal neurodevelopment. The tight control of fetal exposure to thyroid hormone begins early in gestation, as the placental thyroid hormone transporters facilitate passage of maternal thyroxine into fetal blood. With the development of the fetal thyroid gland and gradual maturation of the hypothalamic pituitary thyroid axis, fetal production of TSH, T4, and T3 increases, culminating in a surge of these hormones soon after birth. However, a number of genetic, maternal, and environmental factors can disrupt this process and have serious impacts on neonatal health. In this chapter, we review thyroid embryology and discuss fetal and neonatal thyroid physiology as well as pathology. While much is understood, human thyroid function ontogenesis remains an area of active discovery. Further research will help define topics such as the exact role of T3 in cortical development and the genetics of congenital hypothyroidism.