Congenital hypothyroidism is considered the most common neonatal endocrine disorder, with an incidence of 1/3000–1/4000 newborns. It is defined by insufficient synthesis of thyroid hormones from the newborn thyroid. The hormonal deficiency can vary from a slightly low level to a severe deficiency, also called myxedema. It is often a chronic condition caused mainly by thyroid dysgenesis or a defect in the thyroid hormones synthesis (dyshormonogenesis). Less often, it is secondary to abnormal pituitary or hypothalamic control of thyroid function. Considering the major role played by thyroid hormones in the early development of the central nervous system, congenital hypothyroidism is considered the most common condition involved in the etiology of mental retardation in children. Thus, early detection through neonatal screening programs and initiation the earliest possible of thyroid hormone replacement treatment prevent irreversible neurodevelopmental delay and optimize developmental outcome of affected newborns.