Subclinical hypothyroidism (SH) refers to patients who have an elevated level of the hormone thyrotropin, also known as thyroid stimulating hormone (TSH), and a normal level of the hormone thyroxine (T4). The prevalence of SH increases with age, and is about ten percent in women over the age of 70, and somewhat lower in men, according to background information in the article. Subclinical hypothyroidism has been associated with higher levels of some cardiovascular risk factors, but data on cardiovascular outcomes and death are limited.
Nicolas Rodondi, M.D., MAS, of the University of California, San Francisco, and colleagues studied 2,730 men and women, aged 70 to 79, to determine whether subclinical hypothyroidism was associated with congestive heart failure (CHF, failure of the heart to pump blood with normal efficiency), coronary heart disease (CHD), stroke, peripheral arterial disease (PAD, partial or total blockage of an artery, usually an artery leading to a leg or arm), death, and cardiovascular-related death. Subclinical hypothyroidism was defined as TSH levels of 4.5-6.9 mIU/L (mild), 7.0-9.9 mIU/L (moderate), and 10 mIU/L or greater (severe).
The authors found that the incidence of CHF during a four-year follow-up period was significantly increased in patients with moderate and severe SH, but not in patients with mild SH, who comprised the highest percentage (68 percent) of all patients with SH in the study.
"In this population-based study of older adults, subclinical hypothyroidism was associated with a higher rate of incident and recurrent CHF among participants with a TSH level of 7.0 mIU/L or greater compared with euthyroid participants [participants with normal thyroid function]," they write. "This association persisted after adjustment for cardiovascular risk factors."
"We found no consistent evidence that subclinical hypothyroidism was associated with CHD events, stroke, PAD, cardiovascular-related mortality, or total mortality," they continue.
"Because no other prospective study has assessed the risk of CHF events in subjects with subclinical hypothyroidism, to our knowledge, our results should be confirmed in other large prospective studies, including those in younger populations," the authors conclude. "Further investigation is also warranted to assess whether subclinical hypothyroidism causes or worsens pre-existing heart failure."
(Arch Intern Med. 2005;165:2460-2466. Available pre-embargo to the media at www.jamamedia.org.)
Editor's Note: This study was supported by contracts from the National Institute on Aging, Bethesda, Md., and by a grant from the Swiss National Foundation, Bern, Switzerland, to Dr. Rodondi.
Editorial: Subclinical Hypothyroidism and Cardiovascular Disease
In an accompanying editorial, Lawrence M. Crapo, M.D., of Santa Clara Valley Medical Center, San Jose, Calif., discusses potential treatment for patients, based on findings concerning subclinical hypothyroidism and cardiovascular disease.
"The data in these studies would certainly support the idea that the treatment of severe SH with levothyroxine in patients younger than 80 years may be beneficial, but this remains to be proved in a randomized prospective therapeutic trial," he writes.
"...Rodondi et al found no increased incidence of CHF, CVD, or deaths from CVD over a four-year period in subjects with initial TSH levels in the range of 4.5 to 7.0 mIU/L," he continues. "These data suggest that treatment of subjects with mild SH or high-normal TSH levels would probably not be beneficial in the prevention of CVD."
(Arch Intern Med. 2005;165:2451-2452. Available pre-embargo to the media at www.jamamedia.org.)
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