Sept. 23 -- MONDAY, Sept. 22 (HealthDay News) -- A new study can help doctors decide when to treat people who have an underactive thyroid gland that does not cause symptoms strong enough to arouse worry, researchers report.
As many as 27 million Americans have some type of thyroid disease, according to the American Association of Clinical Endocrinologists; about half of these people are undiagnosed.
"At this point, how to manage older people who have subclinical thyroid dysfunction isn't clear," said Dr. Anne R. Cappola, an assistant professor of medicine and epidemiology at the University of Pennsylvania and a member of the group reporting the finding in the Sept. 30 issue of the Journal of the American College of Cardiology.
Cappola and her colleagues looked at one possible result of abnormal thyroid activity, the development of heart failure. They followed more than 3,000 people for an average of 12 years, monitoring thyroid activity by measuring blood levels of thyroid-stimulating hormone (TSH), looking for a relationship between thyroid activity and heart failure, the progressive loss of the ability of the heart to pump blood.
TSH is a hormone produced by the pituitary gland that stimulates thyroid activity. A high TSH reading means that the pituitary is working hard to get the thyroid into action.
A clear indicator of danger emerged from the study, Cappola said: a TSH level of 10 mU/liter. A normal TSH level is 4.5 mU/l or lower.
Study participants with a TSH reading of 10 or higher had about twice the incidence of heart failure as those with lower readings. "Between 4.5 and 10, there was no effect," Cappola said.
The term "subclinical" can mean simply that thyroid problems are being ignored, said Dr. Anne B. Newman, a professor of epidemiology and medicine at the University of Pittsburgh and another member of the research team.
"It is subclinical in the sense that no one has checked it before, and if you do more screening, the chances are that people will show up in the office with hypothyroidism [low thyroid function]," Newman said.
People with low thyroid function can feel very tired, gain excess weight, have a puffy look and slow down in general, she said, phenomena that are not uncommon among older people.
"The reason we looked at this is that there has been a question about whether you really need to start treatment to replace the thyroid hormone with more modest levels of TSH," Newman said. "We didn't see an increased risk of heart disease with readings under 10."
Treatment of low thyroid function is thyroid replacement hormone, "a single small pill once a day, adjusted to body size and metabolism," she said.
Hypothyroidism can be difficult for a physician to detect unless a screening blood test is done, and that test is not done by many physicians who deal with older people, Newman said.
"I think the rate of hypothyroidism is high enough to make it worth while to screen for it," she said.
This study alone is not enough to support treatment for anyone with a TSH reading of 10 or higher, Cappola said. "We're reluctant to recommend that with an observational study," she said. "But this is, clinically, more evidence that someone with a TSH level 10 or above should be treated."
Learn about thyroid problems and their treatment from the U.S. National Library of Medicine.
SOURCES: Anne R. Cappola, assistant professor, medicine and epidemiology, University of Pennsylvania, Philadelphia; Anne B. Newman, M.D., professor, epidemiology and medicine, University of Pittsburgh; Sept. 23, 2008, Journal of the American College of Cardiology