PURPOSE:The project of the neonatal mass screening test for inborn errors of metabolism are just at the beginning in Korea and there was a few reports about the overall incidence and subtypes of congenital hypothyroidism. METHODS:In this study, we analysed the 97 cases of newborns with hyperthyrotropinemia who were detected by neonatal screening test to identify the incidence and early clinical manifestations of each subtypes of congenital hypothyroidism. Thyroid function were measured by thyrotropin(TSH) level, T4, T3, Free T4, Free T3, thyroglobulin, T3 resin uptake, TBII, TBG in serum, thyroid ultrasonography and 99mTc thyroid scan. We reanalysed the thyroid functions 1 week after discontinuance of L-thyroxine treatment for 1 year. RESULTS: 1) The time of neonatal screening test were between 3 and 7 days after birth in 46 cases(47.4%) and 8 and 14 days after birth in 35 cases(36.1%). Two cases (2.1%) were done neonatal screening test at the age of 2 days old. 2) The major cause of thyroid disorders were primary hypothyroidism in 45 cases of the total due to thyroid aplasia(7 cases), thyroid hypoplasia(17 cases), ectopic thyroid gland(12 cases) and dyshormonogenesis(9 cases). Other causes of thyroid disorders were TBG deficiency(11 cases), TBG dysfuction(1 case), transient hyperthyrotropinemia(28 cases) and transient hypothyroidism(12 cases). 3) Serum level of thyrotropin(TSH) at diagnosis were 223.5+/-229.6microU/ml in thyroid aplasia, 41.6+/-42.9microU/ml in thyroid hypoplasia, 52.4+/-55.6microU/ml in ectopic thyroid gland. TSH levels were significantly high in thyroid aplasia. T4 levels in thyroid aplasia are 1.7+/-2.0microg/dl and this is significantly lower than other types of thyroid disorders. T3 levels were within normal range except in thyroid aplasia and TBG deficiency. 4) Prolongation of physiologic jaundice was the most common clinical manifestation(33.3%) in patients with primary hypothyroidism and macroglossia, hypothermia, umbilical hernia and cold skin were the next commom clinical manifestations in order to present. 5) Osseous development was normal in 57 cases(82.6%) out of 69 cases who accomplished roentgenographic examination of knees. Only 12 cases(17.4%) showed retardation of osseous development, but there was no significant differences between types of thyroid disorders. 6) Most of the newborn(93.3%) with primary hypothyroidism started to treatment within 8 weeks of age. 7) Initial dosage of L-thyroxine was 10microg/kg/day and decreased 6 to 12 months after treatment. 8) There was significantly decreased thyroid uptake of 99mTc after 1 year follow-up in 5 cases of dyshormonogenesis. 9) The serum TSH levels returned to normal ranges within 6 month after treatment in transient hypothyroidism and transient hyperthyrotropinemia. CONCLUSIONS:Special attention should be paid to transient hyperthyrotropinemia and transient hypothyroidism because many of the congenital thyroid disorders showed transient type and it is necessary to establish the diagnostic guideline to early detect these transient types of congenital thyroid disorders.