Alterations in serum concentration of thyroid hormones occur even in euthyroid patients undergoing cardiopulmonary bypass. The purpose of our prospective study was to define the effects of cardiopulmonary bypass on thyroid hormones. Twenty euthyroid patients for mitral valve replacement were included in this study. Heparinised arterial sample for thyroid function tests were obtained before cardiopulmonary bypass (CPB), 30 and 60 minutes after initiation of CPB, immediate post-CPB and 24 hours after termination of CPB. T3, T4 and thyroid stimulating hormone (TSH) were estimated using radioimmunoassay method. There was more than 50% decrease in T3 levels after initiation of CPB and it remained persistently below the physiological range until 24 hours after termination of CPB. T4 and TSH remained within normal limits throughout the study period. These results indicate that CPB simulates the euthyroid sick syndrome as seen in critically ill and burn patients. Whether routine administration of intravenous tri-iodo thyronine is beneficial in the prevention of low cardiac output syndrome seen after CPB remains to be elucidated in future.