Initially, patients are randomized to receive either 1 of 3 different dosages of liothyronine or placebo after undergoing the Fontan procedure. If no unacceptable toxicity is observed in this group, a third dose level of liothyronine is added to the randomization. A total of 7 patients are enrolled at each dose level. All randomized study drugs are administered by continuous infusion over 1 hour after surgery.