Six hypothyroid patients with ischemic heart disease underwent coronary artery bypass grafting (CABG). Three of them were in euthyroid state by preoperative thyroid supplementation. Although remaining three who had not received preoperative supplementation were in overt hypothyroid state at CABG, their intra- and postoperative courses were satisfactory without any complications. We, therefore, found no benefit from preoperative thyroid supplementation. Serum TSH levels in hypothyroid patients were suppressed for more than three days postoperatively despite concomitant low FT3 and FT4 concentrations. The inhibition of TSH secretion could have been produced by therapeutic dose of dopamine which had been infused from intraoperative through postoperative period. Postoperative thyroid replacement should be initiated as soon as possible in hypothyroid patients to counteract the low FT4 concentration that could be prolonged and aggravated by the infusion of dopamine.