The non-thyroidal illness syndrome (NTIS) is considered a transient and completely reversible phenomenon, but it has been shown that it may last for several days postoperatively after coronary artery bypass grafting (CABG) surgery. This study was undertaken to assess thyroid function 6 months after uncomplicated CABG. The thyroid profile was evaluated in 40 consecutive patients undergoing CABG preoperatively, at 0, 12, 48, and 120h postoperatively, and at 6-month follow-up. Triiodothyronine (T ₃), free T ₃ (FT ₃), free thyroxine (FT ₄) and thyroid stimulating hormone (TSH) were assayed using a microparticle enzyme immunoassay. T ₄ and total serum thyroid hormone-binding capacity (T-uptake) were measured on the same samples using a fluorescence polarization immunoassay. Patients with severe systemic illness and patients treated with amiodarone were excluded. All patients were euthyroid at admission. Mean age was 67.4±9.0years. There were 31 (77.5%) men. Typical NTIS was observed in all patients, and the FT ₃ concentration was still reduced by postoperative day 5 (p<0.0001). At 6-month follow-up, all patients were free from cardiac symptoms, and no new cardiac events were recorded. The thyroid profile was normal in 35 patients (87.5%). One patient (4.5%) had developed overt hypothyroidism. Two patients had isolated low T ₃ and FT ₃ levels with normal TSH. Two patients had moderately increased FT ₃ levels with suppressed TSH. In most uncomplicated patients, thyroid function returns to normal 6months after CABG. However, we observed significant alterations of the thyroid profile in 5 out of 40 patients. Further studies are needed to define the long-term consequences of postoperative NTIS.