Treatment of non-thyroidal illness by intravenous triiodothyronine (T 3 ) after cardiac surgery causes a disproportional elevation of hormone levels. The administration of oral T 3 , which has never been studied in this context, could cause physiological hormone levels. The aim of this study was to test oral T 3 for the prevention of T 3 reduction during the postoperative period of valvular cardiac surgery in adults. Eighteen patients who underwent cardiac surgery for valvular disease with invasive hemodynamic monitoring were randomly assigned to 2 groups: the T group received oral T 3 (N = 8), 25 µg three times/day, initiated 24 h before surgery and maintained for 48 h and the NT group (N = 10) received placebo. Serum T 3 , thyroxine and thyrotropin were determined at baseline, 1 h before surgery, within 30 min of cardiopulmonary bypass and 6, 12, 24, and 48 h after removal of the aortic cross-clamp. Baseline T 3 was similar in both groups (T: 119 ± 13; NT: 131 ± 9 ng/dL). Serum T 3 increased during the first 24 h in the T group compared to the NT group (232 ± 18 vs 151 ± 13 ng/dL; P < 0.001). In the NT group, T 3 was reduced by 24% (P = 0.007) 6 h after removal of the aortic cross-clamp, confirming the non-thyroidal illness syndrome. There were no differences in clinical or hemodynamic parameters between groups. Administration of oral T 3 prevented its serum reduction after valvular cardiac surgery in adults, with normal serum levels for 48 h without disproportional elevations.