Patients undergoing the modified Fontan procedure may develop low cardiac output postoperatively. Since thyroid hormone has important effects on cardiovascular function, the present study was undertaken to evaluate the influence of triiodothyronine supplementation. Ten consecutive patients under-going the Fontan procedure were administered intravenous triiodothyronine (0.4 mcg per kg) following surgery. Clinical outcome and thyroid hormone profiles were assessed and then compared to a previous series of patients undergoing the Fontan procedure who had not received triiodothyronine supplementation. Both groups initially demonstrated marked decreases in serum free triiodothyronine levels. The group which received triiodothyronine supplementation demonstrated a more rapid return of serum triiodothyronine levels to baseline [259±17 vs 121±15 pg/dl (p<0.05) on the fifth postoperative day and 336±18 vs 178±12 pg/dl (p<0.05) on the eighth day]. In addition, patients receiving supplemental triiodothyronine demonstrated more rapid recovery of total triiodothyronine, free thyroxine, total thyroxine and thyroglobulin levels. The group which received triiodothyronine supplementation had a shorter length of hospital stay [9±2 vs 14±3 (p<0.05)] as compared to patients who did not receive exogenous triiodothyronine. The results of this study demonstrate that triiodothyronine supplementation aids in the recovery of thyroid hormone levels following Fontan procedure. This endocrinologic finding correlated with improved clinical outcome.