Background/Aims Hypothyroidism is more common among preterm infants than full-term infants. It was also known that preterm babies with congenital hypothyroidism have a higher tendency to develop transient cases than full-term babies with the same condition. But there are few, if any, large-sized studies which evaluating the difference in clinical characteristics and prognosis of congenital hypothyroidism between preterm and term infants. We examined how many of the infants diagnosed with hypothyroidism have permanent hypothyroidism. We also tried to see what differences there are between transient and permanent hypothyroidism groups and between preterm and full term infants with congenital hypothyroidism. Methods By retrospective chart review we gathered information about demographic characteristics, thyroid function test and synthyroid medication. Discontinuation of synthyroid was tried when patients were 3 years of age. Transient hypothyroidism was defined when thyroid function was normal without medication for more than 6 months. We compared the results between preterm and term infants. Results Patients (under review) were 61 infants who were born between July 2008-July 2014 and treated for congenital hypothyroidism in Gyeongsang National University Hospital (38 of them preterm infants and 23 full-term). Thirty of them (49.2%) were males. Nine (14.8 %) were permanent hypothyroidism. There was no difference in proportion of permanent cases between preterm and full term groups (10.5 vs 21.7 %, p=0.203). Neither thyroid function test results nor synthyroid dose at discontinuation was different between the two groups. Compared with transient cases, those with permanent hypothyroidism had higher TSH level at discontinuation (6.1 vs 2.9 IU/mL, p=0.008). There were no other differences in clinical characteristics or synthyroid dose between transient and permanent cases. Conclusions: In our study, among children with congenital hypothyroidism, proportion of transient cases were not higher in preterm infants than term infants. TSH level was higher in permanent cases than in transient cases.