[Thyroid and cardiovascular system].

The cardiocirculatory changes in hyperthyroidism seem to be an accommodation to the increased metabolic demands and lead to an increased perfusion of the peripheral tissues. Due to the influence of elevated thyroid hormone levels, contractility, stroke volume, resting heart rate, and contraction and relaxation velocity of the left ventricle increase. Caused by direct effect on the smooth vascular muscle, systemic vascular resistance is decreased with the consequence of a diminished afterload and an increased cardiac efficiency. The activation of the renin-angiotensin-aldosteron system and the increased production of erythropoietin additionally lead to an increased blood volume, which increases cardiac preload together with the increased venous backflow. Manifest hypothyroidism is characterized by bradycardia and diastolic dysfunction in rest and systolic dysfunction at stress. Despite a slight increase of diastolic blood pressure due to an increased systemic vascular resistance, blood pressure remains nearly stable because of diminished cardiac output. Hypercholesterinaemia and diastolic hypertension in hypothyroid patients can lead to the development of arteriosclerosis and coronary heart disease (CHD). Also subclinical hypothyroidism is associated with a significantly higher risk for arteriosclerosis and CHD, whereas subclinical hyperthyroidism seems to be associated with an increased mortality for all reasons, especially for cardiovascular diseases.