PURPOSE: Alteration in the serum level of thyroid hormone can occur following open heart surgery due to major stress and hemodilution after extracorporeal circulation but these changes have been ignored. The purpose of this study is to evaluate the changes of thyroid hormone level after open heart surgery and if these changes influence the prognosis. METHODS: We evaluated 26 children who had undergone open heart surgery from Sept. 1994 to Jun. 1996 at Korea University Hospital. Five ml of blood were collected before surgery, immediately after surgery and 24 hours after surgery. The blood was centrifused and the serum stored at -40degrees C until examinations were made. Serum T3, T4, reverse T3 were measured by sensitive and specific radioimmunoassay. RESULTS: The mean T3 level was 1.49 +/- 0.50ng/ml (normal 0.08-2.0ng/ml) at pre op, 0.75 +/- 0.40ng/ ml immediately after surgery and 0.69 +/- 0.50ng/ml 24 hours after surgery. The mean T4 level was 10.73 +/- 4.40ug/dl (normal 6.1-11.8 microgram/dl) at pre op, 5.80 +/- 1.90ug/dl immediately after surgery and 5.60 +/- 2.10ug/dl 24 hours after surgery. The mean TSH level was 1.69 +/- 1.13uIU/ml (normal 0.25-4.0 uIU/ml) at pre op, 1.37 +/- 0.80uIU/ml immediately after surgery and 1.61 +/- 1.00uIU/ml 24 hours after surgery. Serum T3 and T4 levels in cardiac surgical patients were significantly reduced (P<0.01), and serum reverse T3 levels were significantly increased (P<0.05), and serum TSH levels were not significantly different in comparison with the preoperative state. CONCLUSION: We conclued that patients with congenital heart disease who underwent open heart surgery show euthyroid sick states like other severely ill patients. It is likely that reduction in T3 and T4 without increased TSH represents an adaptive response by the body to minimize catabolism when undergoing major stress.