The thyroid and the heart.

Cardiovascular manifestations are a frequent finding in hyperthyroid and hypothyroid states. In this review, potential mechanisms by which thyroid hormones may exert their cardiovascular effects and pathophysiological consequences of such effects are briefly discussed. Two major concepts have emerged about how thyroid hormones exert their cardiovascular effects. First, there is increasing evidence that thyroid hormones exert direct effects on the myocardium, which are mediated by stimulation of specific nuclear receptors, which in turn leads to specific mRNAs production. Furthermore, there is some evidence that thyroid hormones may also activate extranuclear sites and may directly alter plasma membrane function. Second, thyroid hormones interact with the sympathetic nervous system by altering responsiveness to sympathetic stimulation presumably by modulating adrenergic receptor function and/or density. Pathophysiological consequences of such direct and indirect thyroid hormone effects include increased my...