A controversy persists as to whether cardiopulmonary bypass (CPB) decreases plasma levels of triiodothyronine (T3), thereby justifying peri-operative administration of T3 to improve haemodynamic recovery. To examine the effects of T3 therapy on post-CPB haemodynamics and to determine whether the potential inotropic effects of T3 are mediated by an increase in β-adrenergic responsiveness, a prospective, randomized, double-blind, placebo-controlled study was performed in 20 patients undergoing cardiac surgery with CPB. T3 or placebo solution (10 patients in each group) was given intravenously at the time of aortic unclamping and 4, 8, 12 and 20 h thereafter. End points included (1) thyroid hormone levels measured by radioimmunoassay (2) standard haemodynamic parameters (3) the density of lymphocyte β-adrenoceptors measured by a radioligand (125I-iodocyanopindolol) binding technique.Post-CPB values (cross clamp removal) of T3 (pg . ml-1) were not significantly decreased compared with pre-CPB values: 3.3±0.2 vs 3.1±0.2 in controls and 3.3±0.4 vs 3.7±0.6 in T3-treated patients, respectively. The haemodynamic parameters were no different between the two groups at any postoperative time point. Likewise, density and affinity of lymphocyte β-adrenoceptors were not significantly different from pre-operative values in either group. Thus, there seems to be no sound justification for a routine use of T3 in patients undergoing open-heart procedures. © 1993 The Europen Society of Cardiology.