Congenital hypothyroidism.

Congenital hypothyroidism (CH) is the one of the most common preventable cause of mental retardation. In the majority of patients, CH is caused by an abnormal development of the thyroid gland (thyroid dysgenesis) that is a sporadic disorder and accounts for 85% of cases and the remaining 15% of cases are caused by dyshormonogenesis. The clinical features of congenital hypothyroidism are so subtle that many newborn infants remain undiagnosed at birth and delayed diagnosis leads to the most severe outcome of CH, mental retardation, emphasizing the importance of neonatal screening.