Thyroid hormones, specifically triiodothyronine (T₃), have significant effects on the heart and cardiovascular system. Hypothyroidism, hyperthyroidism, subclinical thyroid disease, and low T₃ syndrome each cause cardiac and cardiovascular abnormalities through both genomic and nongenomic effects on cardiac myocytes and vascular smooth muscle cells. In compromised health, such as occurs in heart disease, alterations in thyroid hormone metabolism may further impair cardiac and cardiovascular function. Diagnosis and treatment of cardiac disease may benefit from including analysis of thyroid hormone status, including serum total T₃ levels.