Thyroid hormones have profound cardiovascular effects. Chronic hypothyroidism is associated with cardiovascular abnormalities that include diminished cardiac output and increased systemic vascular resistance. Acute hypothyroidism, frequently referred to as the "euthyroid sick syndrome," is present in diverse clinical situations such as brain death, sepsis, congestive heart failure, and cardiopulmonary bypass. Significant cardiovascular dysfunction often complicates each of these clinical situations. This article reviews the laboratory experiments and clinical trials that have evaluated triiodothyronine (T3) repletion in cardiac surgery. Animal experiments have shown that T3 repletion ameliorates postischemic cardiovascular dysfunction. While anecdotal clinical experience suggests that T3 repletion should be of clinical benefit, rigorous clinical trials have failed to support routine repletion of T3 in cardiac surgery. Based on the results of these clinical trials, we do not recommend routine administration of T3 to patients undergoing cardiac surgery. However, anecdotal experience suggests that T3 may help in weaning patients from cardiopulmonary bypass who are unable to be weaned from bypass despite maximal inotropic support. In use as a "rescue" agent, we administer the 0.8 microgram/kg dose that has been demonstrated to safely improve cardiac output and decrease systemic vascular resistance in the postischemic cardiopulmonary bypass patient.