Last update:
14-Jan-2006

Subclinical hyperthyroidism and hypothyroidism are characterized by respectively low or high serum thyrotropin hormone, and normal serum free thyroid hormone levels. Although apparently symptom-free, both conditions have been associated with changes in cardiac structure and function, increased cardiovascular morbidity and possibly mortality. Patients with subclinical hyperthyroidism have been reported to have increased left ventricular (LV) mass, LV diastolic dysfunction at rest and LV systolic dysfunction with exercise. Subclinical hyperthyroidism has been associated with increased mean resting heart rate and a higher incidence of atrial fibrillation in the elderly, similar to clinical hyperthyroidism. Increased cardiovascular mortality in the elderly with subclinical hyperthyroidism has also been reported. Patients with subclinical hypothyroidism have been reported to have LV diastolic dysfunction at rest, systolic dysfunction with exercise, and increased risk for coronary artery disease. Subclinical hypothyroidism is also associated with hyperlipidemia, diastolic hypertension, hyperhomocysteinemia, increased systemic C-reactive protein levels and endothelial dysfunction, and these associations may in part account for the observed association of subclinical hypothyroidism with coronary artery disease. Thyroid hormones exert both direct and indirect actions on the heart and blood vessels, but the precise pathophysiological mechanisms leading to changes in heart function and increased cardiovascular morbidity in patients with subclinical thyroid disease have not been fully elucidated. Restoration of euthyroidism in patients with subclinical abnormalities of thyroid function may improve or reverse some of the cardiovascular abnormalities. Further studies are necessary to fully elucidate the effects of subclinical thyroid disease on the cardiovascular system and establish the efficacy of available treatments in preventing excess cardiovascular morbidity and mortality.

Key words: Atrial fibrillation, Heart, Subclinical hyperthyroidism, Subclinical hypothyroidism, Thyroid.