Congenital hypothyroidism (CH) results from the effects of insufficient thyroid hormone on the developing fetus and infant, and is characterized by severe intellectual disability and growth inhibition. CH can result from maternal iodine deficiency, which can be abolished by appropriate dietary iodine supplementation. Alternately, CH may be caused by congenital defects of the thyroid gland and thyroid hormone biosynthesis abnormalities (primary congenital hypothyroidism), or by “central hypothyroidism,” in which the brain produces insufficient thyroid stimulating hormone. Treatment of these latter etiologies requires administration of thyroid hormone.