Background-Triiodothyronine (T 3 ) supplementation may be a useful adjunct in the management of patients after cardiopulmonary bypass. Limited data are available regarding the use and pharmacokinetics of T 3 in children. The present study was performed to evaluate T 3 pharmacokinetics in a cohort of children undergoing the modified Fontan procedure. Methods and Results-A total of 28 patients were enrolled in this randomized, prospective study. The patients were divided into 4 groups: 1 group

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... eived a placebo and 3 groups received intravenous T 3 at dosages of 0.4, 0.6, and 0.8 g/kg, respectively. All 28 patients survived their operative procedures. Two patients developed low cardiac output, and 3 patients had pleural effusions. The median length of hospital stay was 7 days. The mean free T 3 level was 316Ϯ67 pg/dL after then administration of a placebo. Patients who received T 3 had mean peak free T 3 levels of 972Ϯ88, 1351Ϯ299, and 1869Ϯ281 pg/dL for the dosages of 0.4, 0.6, and 0.8 g/kg, respectively. The calculated half-life of T 3 was 7 hours. Conclusions-The half-life of intravenous T 3 in children is approximately one-third of that reported for adults. These results provide a framework for studying the efficacy of T 3 supplementation in children undergoing open-heart surgery.