Thyroid hormone has profound effects on the heart and cardiovascular system. The cellular and systemic changes that occur with hypothyroidism include blood pressure changes, alterations in lipid metabolism, decreased cardiac contractility, and increased systemic vascular resistance. The resulting increased hypercholesterolemia and diastolic hypertension predispose these patients to atherosclerotic cardiovascular and coronary artery disease. Heart failure is associated with low-T3 syndrome, resulting from alterations in thyroid hormone metabolism. This nonthyroidal illness may cause cardiovascular alterations similar to the changes described for hypothyroidism. New research suggests that T3 treatment of patients with heart failure and low-T3 syndrome improves some of the clinical manifestations of heart failure.