Subclinical Thyroid Dysfunction and Cardiovascular Disease

Thyroid hormones are intricately related to the cardiovascular system. Therefore, it is expected that thyroid dysfunction, including subclinical thyroid disorders, significantly impacts the cardiovascular system, contributing to both cardiovascular and all-cause mortality. Subclinical hyperthyroidism, defined by low or undetectable levels of serum thyroid-stimulating hormone with free triiodothyronine and thyroxine concentrations within the reference range, has been linked to an increased risk of hypertension, atrial fibrillation, coronary artery disease, endothelial dysfunction, and thromboembolic events. Subclinical hypothyroidism, defined as increased serum thyroid-stimulating hormone in the presence of normal circulating free triiodothyronine and thyroxine levels, is associated with increased prevalence of cardiovascular-associated risk factors, such as hypertension and obesity, as well as low levels of high-density lipoprotein. It also increases the risk for atherosclerosis and myocardial infarction. The prognosis of heart failure is known to be adversely impacted by both subclinical thyroid dysfunctions. Subclinical hyperthyroidism appears to increase the risk of acute heart failure, possibly by incrementing the risk of arrhythmias, such as atrial fibrillation. In turn, subclinical hypothyroidism has been associated with the development of heart failure in patients with and without underlying heart disease. Although treatment of these subclinical thyroid dysfunctions could be beneficial and alter the course of several cardiovascular diseases, its benefit on cardiovascular risk and mortality remains unclear. There is still no clear evidence to support an undeniable benefit of treatment of subclinical hypothyroidism or hyperthyroidism.