Thyroid hormone is essential for normal fetal brain development in utero and for the first 2 years of life. The developing fetus is initially reliant upon maternal thyroid hormones that cross the placenta, until the fetal thyroid begins to supply thyroid hormone for the fetus. Maternal thyroid status affects fetal thyroid function and maternal thyroid dysfunction can have a significant impact on the fetus and neonate. There are also several neonatal factors that can influence thyroid function. Here, we describe thyroid function in the fetus and neonate and discuss the most common thyroid disorders seen in neonates.