Proper thyroid function is essential for maintaining cardiovascular integrity during normal and stressful situations. In this study, the effects of isoflurane-O2 anesthesia and surgical stress on serum TSH, T4, free T4, T3, rT3, and cortisol were investigated in nine patients before, during, and after valve surgery. Compared with preoperative control values, serum TSH decreased in the postoperative period. Both T4 and free T4 had similar decreases after cardiopulmonary bypass (CPB) and remained depressed postoperatively. Both T3 and rT3 decreased at the start of cardiopulmonary bypass; T3 remained low in the postoperative period, while rT3 increased. Cortisol decreased during anesthesia and surgery in the prebypass period, but increased during cardiopulmonary bypass and in the postoperative period. The results suggest that isoflurane-O2 anesthesia during valve surgery produces a rapid decrease in T3, resulting in the low T3 syndrome postoperatively. Isoflurane, in the dose studied, similar to fentanyl, can suppress the cortisol response to anesthesia and surgery in the prebypass period, but not during and after CPB.