Availability of sensitive radioimunoassays for T4 and TSH has simplified the detection and treatment of congenital hypothyroidism. Early diagnosis by newborn screening and prompt treatment should minimize the serious complications of mental retardation. Absence of clear cut signs and symptoms early in the life requires laboratory testing for early diagnosis. The high cost effectiveness of the screening programme has resulted in the implementation of mass screening in many countries. Several studies indicate a favourable IQ with initiation of treatment before the age of 3 months. Neonatal laboratory screening programs have made it possible to initiate treatment before one month of age. This may further improve the outcome of this common cause of preventable mental retardation.