Subclinical Hypothyroidism and Cardiac Function

Abstract

The cardiovascular system is sensitive to the action of thyroid hormone. However, although a wide spectrum of cardiac abnormalities has long been recognized in patients with overt thyroid dysfunction, the question of cardiac involvement in patients with subclinical thyroid dysfunction has been investigated only in the last two to three decades. Most clinical studies have shown that subclinical hypothyroidism or hyperthyroidism is associated with changes in several cardiac parameters. More specifically, the literature on cardiac involvement in subclinical hypothyroidism consistently shows that patients have resting left ventricular diastolic dysfunction evidenced by delayed relaxation, and impaired systolic dysfunction on effort that results in poor exercise capacity. Whether or not subclinical hypothyroidism also affects left ventricular systolic function at rest remains controversial. Studies of subclinical hypothyroid patients before and after euthyroidism was achieved with levothyroxine replacement provided evidence of impaired resting left ventricular systolic function. Indeed, at-rest left ventricular systolic function was substantially normal in most studies of subclinical hypothyroid patients compared to normal control subjects. Drawing on these data, it appears that subclinical hypothyroidism should be considered a mild form of thyroid failure, associated with initial signs of cardiovascular hypothyroidism. Therefore, it would seem appropriate to initiate timely treatment of patients with mild thyroid failure to prevent cardiac involvement.

MECHANISMS IN ENDOCRINOLOGY: Heart failure and thyroid dysfunction

“…The relationship between thyroid hormone and the cardiovascular system has been extensively demonstrated in numerous experimental and clinical studies (14,15,16,17,18,19). This association has been recently confirmed by significant changes in cardiac structure and function in patients with persistent subclinical thyroid dysfunction (11,12,13,14,15,16).…”

“…The availability of T 3 , which is the active form of thyroid hormone in the heart, is controlled by the deiodinases, which regulate cardiac levels of T 3 (19,20). The heart is particularly vulnerable to the reduction in local T 3 levels because T 3 is essential to preserve both cardiac morphology and performance in adult life (16,17,18,19). In fact, thyroid hormone influences cardiac performance by genomic and non-genomic effects and increases cardiac output by affecting stroke volume and heart rate (14,15).…”

“…Short-term hyperthyroidism is characterized by a high cardiac output state with a remarkable increase in heart rate and cardiac preload and a reduction in peripheral vascular resistance, resulting in a hyperdynamic circulation (15,16,34). Cardiac preload (left ventricular end diastolic volume) is increased as a consequence of the increase in blood volume and the improvement in diastolic function (15,34).…”

“…Important changes in cardiac structure and function have been reported in patients with overt and subclinical hypothyroidism (SHypo) with a severity depending on the degree and the duration of thyroid hormone deficiency (14,15,16,77,78,79). Hypothyroidism is characterized by a low cardiac output due to the decreased heart rate and stroke volume (15,16).…”

Evaluation of the Echocardiographic Findings in cases of Thyroid Disorders

“…26 Study done by Sureshbabu KP et al on 100 patients of hypothyroid patients LVH was seen in 5% of patients. 27 Concentric LVH can be manifestation of presence of hypertension in the study subjects.…”