BACKGROUND: The purpose of this prospective study is to define the effect of cardiopulmonary bypass (CPB) on the concentration of thyroid hormones and metabolites. METHODS: Blood samples were obtained from 15 patients undergoing open heart surgery at 1) pre-induction, 2) after heparinization, 3) during CPB, 4) 2 hours after CPB, 5) 24 hours after CPB and 6) 48 hours after CPB. Thyroid stimulating hormone, albumin, thyroxine (T4), free thyroxine (FT4), triiodothyronine (T3), free triiodothyronine (FT3) and reverse T3 (T3) were measured. RESULTS: Concentration of T3 significantly decreased after infusion of heparin and maintained at the decreased level until postbypass 24 hours. Concentration of FT3 significantly increased after heparin administration but maintained at a control level during CPB and decreased after postbypass 24 , 48 hours (p<0.05). Reverse T3 increased at 24 and 48 hours after CPB (p<0.05). Thyroxine decreased during CPB and return to control level after CPB. Free thyroxine did not change significantly. Thyroid stimulating hormone was significantly depressed at 24 hours after CPB (p<0.05). CONCLUSIONS: This result suggest that the thyroid function is depressed until 48 hours after CPB and it seems to be associated with abnormal metabolism of thyroid hormones.