Thyroid hormone has profound effects on the heart and cardiovascular system. This article describes the cellular mechanisms by which thyroid hormone acts at the level of the cardiac myocyte and the vascular smooth muscle cell to alter phenotype and physiology. Because it is well established that thyroid hormone, specifically T₃, acts on almost every cell and organ in the body, studies on the regulation of thyroid hormone transport into cardiac and vascular tissue have added clinical significance. The characteristic changes in cardiovascular hemodynamics and metabolism that accompany thyroid disease states can then be best understood at the cellular level.