The postoperative period after cardiac surgery with cardiopulmonary bypass (CPB) is associated with a low T3 syndrome, i.e. low T3 and fT3 concentrations in the presence of normal T4 and TSH concentrations. So far, results from studies evaluating thyroid function during and after CPB are rather conflicting. We therefore evaluated prospectively thyroid function in 28 patients before, during and up to 3 days after coronary artery bypass surgery. We could demonstrate the most significant changes in thyroid hormone concentrations on day 1 after CPB (low T3 and fT3 concentrations, elevated rT3 concentrations in the presence of a significant fall of TSH). T3 fell from 1.93 to 0.6 nmol/l and fT3 from 5.5 to 1.42 pmol/l. Those patients with low cardiac output syndrome after surgery had significantly lower T3 concentrations than patients without this complication. Moreover, those patients, who already had significant lower T3 values prior to CPB, also demonstrated low T3 concentrations on day 1 after CPB. Cortisol usually has a suppressive effect on TSH secretion. However, the effect of cortisol on TSH in patients undergoing CPB seems to be not that important: those patients with high endogenous cortisol concentrations on day 1 after CPB had similar TSH values to those patients with only slightly elevated cortisol concentrations. Also, the application of high doses of catecholamines seems to have only minor effects on TSH secretion, because those patients requiring high doses of dopamine over a prolonged time period had essentially the same TSH values after CPB. Patients who had been exposed preoperatively to high doses of iodine did not demonstrate significantly different thyroid hormone concentrations. In conclusion: We could demonstrate that CPB induces a low T3 syndrome up to 3 days after surgery. Those patients with low T3 concentrations prior to surgery demonstrate postoperatively a more severe degree of nonthyroidal illness (NTI).Catecholamines and cortisol seem to have only minor effects on the TSH secretion after CPB. The influence of a previous iodine contamination is negligible.