The effects of thyroid hormone on the cardiovascular system include the following three aspects: First, the effects on the arterial vascular system, thyroid hormone can directly reduce vascular resistance, regulate blood pressure and cardiac output. 50% of patients with thyrotoxicosis have a decrease in systemic vascular resistance, and vascular smooth muscle cells relax rapidly under the effect of thyroid hormone. Second, it is the effect on the venous system. Thyroid hormones have an effect on venous compliance and blood volume, mainly in the sense that thyroid hormones can increase the mean filling pressure of patients and decrease venous compliance, but blood volume does not change. In mucinous edema, the heart output per beat is reduced, blood volume is decreased, glomerular filtration rate is also reduced, and systemic vascular resistance is increased. Third, it is the effect on the heart. Thyroid hormone can increase cardiac output, leading to an increase in systolic blood pressure, but the increase in cardiac output can also increase myocardial oxygen consumption, leaving cardiac muscle cells in a state of relative hypoxia. This long-term hypoxia can cause a decrease in the function of the heart, so patients with hyperthyroidism can have more serious changes in heart function, with cardiac ultrasound indicating heart enlargement and valve closure insufficiency. For example, patients with severe hyperthyroidism can develop atrial fibrillation, congestive heart failure, and even stroke. Severe hypothyroidism can also cause decreased myocardial contractility, enlargement of the heart, increased vascular resistance, and heart failure.