Cardiac Morphology and Function in Mild Hypothyroidism

Abstract

Subclinical hypothyroidism is an early and mild form of thyroid failure characterized by serum levels of thyroid hormones within their reference range but elevated serum TSH concentrations. The prevalence of subclinical hypothyroidism is 4–10% and this condition increases significantly with age. The management and treatment of subclinical hypothyroidism are controversial, and there is no consensus about the TSH concentration at which treatment should be started. The cardiovascular risk is increased in patients with overt hypothyroidism, with several potential cardiovascular risk factors similarly reported in patients with subclinical hypothyroidism. Identification of the potential cardiovascular risk factors could facilitate decision-making about the treatment of patients with subclinical hypothyroidism. Impaired left ventricular diastolic function, characterized by slowed myocardial relaxation and impaired early ventricular filling, is the most consistent cardiac abnormality identified in young and middle-aged patients with subclinical hypothyroidism. Moreover, vascular function is impaired in patients with mild and subclinical hypothyroidism, as documented by the increase in systemic vascular resistance and arterial stiffness and by the impaired endothelial function, increasing the risk of atherosclerosis and coronary artery disease. Epidemiological studies have investigated cardiovascular morbidity and mortality in patients with subclinical hypothyroidism; the results suggest that middle-aged individuals with mild thyroid hormone deficiency have a higher cardiovascular risk for coronary heart disease than the elderly with this condition. Moreover, cardiac death, particularly from ischemic heart disease, is significantly higher in cardiac patients with subclinical hypothyroidism. Replacement therapy with l-thyroxine (l-T4) may reverse the cardiovascular risk associated with subclinical hypothyroidism. For this reason, l-T4 replacement therapy should be considered in subclinically hypothyroid patients who have associated high cardiovascular risk factors.