Congenital hypothyroidism is a relatively common endocrine disorder, affecting one in 4000 newborn infants. Undiagnosed and untreated congenital hypothyroidism will result in un-toward consequences, including mental retardation and other significant neurologic sequelae. For these reasons, programs to screen newborns were developed to detect congenital hypothyroidism before clinical features become obvious enough to suggest the diagnosis. The most common clinical features include prolonged jaundice, skin mottling, hypotonia, umbilical hernia, constipation, and macroglossia. Congenital hypothyroidism may be caused by several different disorders; ectopic thyroid glands represent the most common cause. There is accumulating evidence that autoimmune thyroid disease as manifested by TBII may be the cause of thyroid dysgenesis in some cases. The diagnosis is easily confirmed by finding a low serum free T4 or total T4 and elevated serum TSH concentration. The treatment of choice is levothyroxine; these infants must be followed carefully to ensure normal growth and development and maintenance of serum T4 and TSH within the normal ranges. With appropriate treatment and follow-up, the large majority of these infants have an excellent prognosis, with an IQ no different from comparison populations. However, it appears that there is still a small percentage of infants who are the most severely affected, who manifest the lowest serum T4 levels, thyroid aplasia, and retarded bone ages, and who may run the highest risk for some degree of retardation and other neurologic sequelae. Acquired hypothyroidism is also a relatively common disorder, occurring in one in 500 to one in 1000 school-age children. These children most commonly have a slowdown in growth, short stature, a goiter, and a drop in school performance. Other clinical features may be subtle or absent except in more severe or long-standing cases. The most common cause is chronic lymphocytic thyroiditis. The diagnosis is easily established by finding low serum-free T4 or total T4 and elevated serum TSH concentrations. Again, levothyroxine is the treatment of choice. With appropriate treatment and follow-up, all clinical features that develop after age 3 should be reversible and the prognosis should therefore be very good.