Summary
	Thyroid hormone has profound effects on the heart and circulation. Measures of ventricular contractility have demonstrated supranormal systolic and diastolic function. These changes arise from alterations in systemic hemodynamics and T3-mediated effects on cardiac myocyte-specific gene expression. T3 effects like the enhanced velocity of cardiac contraction and the increased speed of diastolic relaxation can be traced to T3-induced changes in the level of specific mRNAs and proteins like MHC a and b or the Ca++ATPase of the SR. These T3 effects present direct nuclear mediated actions of T3. Other T3-induced changes result from increased sensitivity of the sympathetic system and some T3 effects may be mediated by an increased demand on the periphery. The most prominent cardiovascular signs and symptoms of H include palpitations, tachycardia, widened pulse pressure, a hyperdynamic precordium, and marked increases in cardiac output. Impaired cardiopulmonary function and exercise capacity is observed in H. Thyrotoxicosis can aggravate pre-existing heart disease and can also lead to AF, congestive heart failure, or worsening of angina pectoris. In the elderly patient, often with mild but prolonged elevation of plasma thyroid hormones, symptoms and signs of heart failure and complicating AF may dominate the clinical picture and mask the more classical endocrine manifestations of the disease. Regarding the high incidence of AF in older patients with H, it is also important to detect subclinical H, thus warranting the measurement of the serum TSH concentration for an early recognition and treatment. Most cardiac abnormalities return to normal once a euthyroid state has been achieved, although AF may persist in a minority. Optimal treatment requires rapid and definitive antithyroid therapy. Furthermore, anticoagulation is recommended for H patients with AF older than 50 yr, those who have histories of previous emboli, hypertension, or with echocardiographic evidence of left atrial enlargement and/or myxomatous cardiac valves.