Thyroid Function During the Fetal and Neonatal Periods.

Thyroid hormones are essential during infancy and childhood for growth and brain development. The formation and maturation of the newborn's hypothalamic-pituitary-thyroid axis begin in utero with fetal dependence on maternal thyroid hormones early in the pregnancy. As the fetal thyroid gland begins to produce thyroid hormones in the second trimester, the reliance decreases and remains at lower levels until birth. After birth, the detachment from the placenta and the change in thermal environment lead to a rapid increase in circulating thyroid-stimulating hormone in the neonate within hours, resulting in subsequent increases in thyroxine and triiodothyronine concentrations. Preterm infants may have lower thyroxine concentrations because of an immature hypothalamic-pituitary-thyroid axis at the time of birth and premature discontinuation of transference of maternal thyroid hormones. Similarly, infants with critical illness unrelated to the thyroid gland may have lower thyroxine levels. Infants born to mothers with Graves' disease are at risk for hypothyroidism and hyperthyroidism, which is related to the placental transfer of maternal autoantibodies, as well as antithyroid medications. An understanding of the normal embryology and physiology of the fetal and neonatal thyroid will help in evaluating a newborn for thyroid disorders.