INFLUENCE OF CORONARY-ARTERY BYPASS-SURGERY ON THYROID-HORMONE PARAMETERS

The postoperative period after cardiac surgery with cardiopulmonary by pass (CPB) is associated with a low T3 syndrome, i.e. low T3 and fT3 c oncentrations 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 thyr oid function in 28 patients before, during and up to 3 days after coro nary artery bypass surgery. We could demonstrate the most significant changes in thyroid hormone concentrations on day 1 after CPB (low T3 a nd 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 fr om 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 con centrations on day 1 after CPB. Cortisol usually has a suppressive eff ect on TSH secretion. However, the effect of cortisol on TSH in patien ts 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 con centrations. Also, the application of high doses of catecholamines see ms to have only minor effects on TSH secretion, because those patients requiring high doses of dopamine over a prolonged time period had ess entially the same TSH values after CPB. Patients who had been exposed preoperatively to high doses of iodine did not demonstrate significant ly different thyroid hormone concentrations. In conclusion: We could d emonstrate that CPB induces a low T3 syndrome up to 3 days after surge ry. Those patients with low T3 concentrations prior to surgery demonst rate 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 contaminat ion is negligible.