A common hormone abnormality in older adults—a mild form of overactive thyroid called subclinical hyperthyroidism—is linked to a much higher risk of dying, a new study finds. The results will be presented Sunday at The Endocrine Society's 93rd Annual Meeting in Boston.

Because this condition sometimes does not even cause symptoms, elderly people may be unaware they have altered thyroid function until serious complications occur, said the study's main author, Graziano Ceresini, MD, PhD, a clinical researcher at the University of Parma in Italy.

"Subclinical hyperthyroidism can be responsible for important medical problems, such as cardiac arrhythmias—irregular heartbeat—as well as altered bone structure and cognitive abnormalities, especially in elderly individuals," Ceresini said. "Now we know that it also may be accompanied by increased mortality in people ages 65 and older."

In the new study, the investigators used data from the Italian Aging in the Chianti Area study to evaluate the relationship between thyroid function and death from all causes in older people. Thyroid function test results were available for 950 subjects age 65 or older. At enrollment in the study, 819 subjects (86 percent) had normal thyroid function and 83 (nearly 9 percent) had subclinical hyperthyroidism.

Technically, subclinical hyperthyroidism is a below-normal or undetectable blood concentration of thyroid-stimulating hormone (TSH) with normal levels of the thyroid hormones called free T3 and T4.

In evaluating the death statistics, the researchers adjusted for age, sex and other factors that could bias the results, such as congestive heart failure, body mass index, cancer and stroke. They found that subjects who had subclinical hyperthyroidism at the beginning of the study had a 65 percent higher risk of dying during the six-year follow-up than did subjects with normal thyroid function.

"Although our results would suggest the need for thyroid function testing in elderly people, confirmation of our data by further studies is needed before a screening recommendation can be made. There are no current recommendations to test all elderly individuals for subclinical hyperthyroidism," Ceresini said.

He suggested, however, that elderly persons with signs of subclinical hyperthyroidism, especially arrhythmias or thyroid disease, should ask their doctor about getting a thyroid function test. Other symptoms may include weight loss, feeling too hot and nervousness.

Subclinical hypothyroidism, or a mildly underactive thyroid, also is common in elderly people but in this study was not linked to decreased survival. There were not enough subjects with overt underactive or overactive thyroid to analyze their death risk, the authors reported.