Congenital Hypothyroidism (CH) is one of the most common preventable causes of mental retardation with a worldwide incidence of 1:4000 live births. Ideally universal screening at 3-4 days of age should be done for detecting CH. Abnormal values on screening (T4 < 6.5 ug/dL, TSH > 20 micro/L) should be confirmed by a venous sample (using age appropriate cutoffs) before initiating treatment. Term as well as preterm infants with low T4 and elevated TSH should be started on L-thyroxine at a dose of 10-15 microg/ kg/ day as soon as the diagnosis is made. Regular monitoring should be done to ensure that T4 is in the upper half of normal range. The outcome of CH depends on the time of initiation of therapy and the dose of L-thyroxine used with the best outcome in infants started on treatment before 2 weeks of age with a dose > 9.5 microg/ kg/ day.