Overactive thyroid gland linked to increased risk for atrial fibrillation

Having an overactive thyroid gland is linked with an increased risk for atrial fibrillation (a type of abnormal heart rhythm), but neither an over- or under active thyroid gland is associated with a higher risk for other cardiovascular problems or increased risk of death, according to a study in the March 1 issue of JAMA: The Journal of the American Medical Association.

Thyroid hormone excess and deficiency are common, and can be readily diagnosed and treated. Previous studies have suggested that abnormal levels of thyroid stimulating hormone (TSH) may represent a cardiac risk factor. Cardiovascular diseases (CVD) are the most common cause of death in the U.S. Even mildly altered thyroid status reportedly affects serum cholesterol levels, heart rhythm and rate, ventricular function, risk of coronary artery disease, and cardiovascular death. However, the relationship between abnormal thyroid function and cardiovascular outcomes remains unclear, according to background information in the article.

Anne R. Cappola, M.D., Sc.M., of the University of Pennsylvania School of Medicine, Philadelphia, and colleagues tested the hypothesis that abnormal thyroid status is associated with increased cardiovascular risk and death in individuals with unrecognized thyroid dysfunction. The study included 3,233 U.S. individuals aged 65 years or older who had their serum thyroid-stimulating hormone levels measured when enrolled in 1989-1990. The cardiovascular health of the patients, who were not taking thyroid medication, was assessed through June 2002.

The researchers found that 82 percent of participants had normal thyroid function, 15 percent had subclinical (before symptoms) hypothyroidism (an underactive thyroid gland), 1.6 percent had symptomatic hypothyroidism, and 1.5 percent had subclinical hyperthyroidism (an overactive thyroid gland). After exclusion of those who had atrial fibrillation at the start of the study, individuals with subclinical hyperthyroidism had nearly twice the incidence of developing atrial fibrillation compared with those with normal thyroid function. No differences were seen between the subclinical hyperthyroidism group and the normal thyroid function group for the occurrence of coronary heart disease, cerebrovascular disease, cardiovascular death, or all-cause death. Likewise, there were no differences between the subclinical hypothyroidism or symptomatic hypothyroidism groups and the normal thyroid function group for cardiovascular outcomes or death.

"Our analyses do not support screening older individuals solely to prevent atrial fibrillation, with an estimated number needed to screen of 2,500 older individuals to find 1 case of atrial fibrillation associated with subclinical hyperthyroidism. Our findings suggest that if endogenous [originating from within the thyroid] subclinical hyperthyroidism is detected, older individuals may benefit from treatment to prevent atrial fibrillation," the authors write. "Our analyses do not support screening older individuals for thyroid disease to prevent CVD, and, although our data are observational, they do not support treatment of individuals with subclinical hypothyroidism to prevent cardiovascular events."