Hyperthyroidism increases risk for atrial fibrillation, but neither hyperthyroidism nor hypothyroidism increases risk for other cardiovascular conditions or death, according to an article in the March 1 issue of the Journal of the American Medical Association.

Although even mildly altered thyroid status has been reported to affect serum cholesterol levels, heart rhythm and rate, ventricular function, risk of coronary artery disease, and cardiovascular death, the relationship between abnormal thyroid function and cardiovascular outcomes has been unclear.

Anne R. Cappola, MD, ScM, 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 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 hypothyroidism, 1.6 percent had symptomatic hypothyroidism, and 1.5 percent had subclinical hyperthyroidism. After exclusion of people with baseline atrial fibrillation, individuals with subclinical hyperthyroidism had nearly twice the incidence of new-onset 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 subclinical hyperthyroidism is detected, older individuals may benefit from treatment to prevent atrial fibrillation," the authors wrote. "Our analyses do not support screening older individuals for thyroid disease to prevent cardiovascular disease, and, although our data are observational, they do not support treatment of individuals with subclinical hypothyroidism to prevent cardiovascular events."