OBJECTIVES To detect newborns with congenital hypothyroidism (CH) by TSH screening and to treat the affected infants as early as possible. To find the incidence of CH in neonates born at Rajavithi Hospital from 1995 to 2000. The effects of thyroid pathology, ages at the initiation of treatment, associated conditions, and severity of CH on the intellectual outcome of the patients were also analyzed. STUDY DESIGNS Thyrotropin (TSH) screening for CH in Queen Sirikit National Institute of Child Health was done in neonates born at Rajavithi Hospital from 1995 to 2000. Infants with elevated TSH level of greater than 25 mU/L were recalled for tests including serum T4, TSH and roentgenogram of bone age. Infants with CH were treated with levothyroxine 10-12 Ug/Kg/day: growth and development, serum T4, TSH were followed-up every 3 months for at least 2 years to keep serum T4 between 10-14 Ug/dL and serum TSH below 6 mU/L. Levothyroxine was discontinued for 4 weeks when the infants were over 2 years old. Serum T4, TSH, thyroid scan, and uptake were then performed on these infants. The patients must be euthyroid at the time of the intelligence testing. The Stanford Binet or Vineland Social Maturity scale was used to evaluate the patient's intelligence between the ages of 2 and 5 years. RESULTS From 1995 to 2000, 62,681 neonates were screened for CH. The incidence of CH was 1:4,178 live births. The sex ratio female: male 2:1. Fifteen infants with CH were started on levothyroxine 10-12 Ug/Kg/day orally once a day. The age of initiation of treatment was between 2-8 weeks. There were 4 patients with lingual thyroid, 4 patients with thyroid hypoplasia, 2 patients with thyroid aplasia and one patient with organification defect. The average intelligence score was 93.8. The factor that significantly affected the intellectual outcome of the patients was associated with adverse conditions such as prematurity with complications, epilepsy and family disruption (p<0.05).