WASHINGTON — An as-yet-unpublished update to a 2006 meta-analysis found that the risk of heart disease in the presence of subclinical hypothyroidism is attenuated somewhat, to about a 30%-35% increased risk from a previously estimated 65%.
However, the increase is still significant and “clinically meaningful,” Dr. Douglas C. Bauer said at a meeting sponsored by the American Thyroid Association, and what to do about it remains an open question. “The issue of primary versus secondary prevention hasn't really been well looked at. There have been no randomized controlled trials looking at replacement with thyroid hormone in individuals to determine the effect on ischemic heart disease, [which] has limited [our] ability to make practice guidelines.”
Even a relatively small increase in risk for heart disease is important for two reasons, said Dr. Bauer, professor of medicine, epidemiology, and biostatistics at the University of California, San Francisco.
First, subclinical hypothyroidism is a common risk factor, affecting up to 10% of the general population, and second, ischemic heart disease is the most common cause of death in the U.S. These two points add up to a potential major public health problem, said Dr. Bauer. He and his colleagues have submitted the revised meta-analysis for publication in the Annals of Internal Medicine.
The original 2006 meta-analysis included 14 studies, 1,362 coronary heart disease outcomes, and 10,540 patients (Am. J. Med. 2006;119:541–51).
In that study, researchers found a 65% increased risk of heart disease in those who had subclinical hypothyroidism compared with those who were euthyroid.
The risk was lessened slightly in some subgroup analyses according to the study design: There was an increased risk of about 40% in the five prospective cohort studies included in the meta-analysis (OR 1.42, CI 0.91–2.21), closer to the newly revised risk estimate; and a 70% increased risk in the case-control and cross-sectional studies alone (OR 1.72, CI 1.25–2.38).
Additional sensitivity and subgroup analyses in that initial meta-analysis—for example, in studies that adjusted for cardiovascular risk factors, or in studies that used different definitions of subclinical hypothyroidism according to thyroid-stimulating hormone (TSH) values—yielded variable risk factors, but none fell below 40%, said Dr. Bauer.
However, almost all of the studies relied on a single measurement of TSH, suggesting they may have underestimated the true relationship between subclinical hypothyroidism and the incidence of ischemic heart disease because it's “possible that some of those who were initially classified as having subclinical hypothyroidism later reverted to normal.”
In general, Dr. Bauer advocates screening for subclinical hypothyroidism for men and women over age 50 who present at well visits with other cardiac risk factors. And although there is no clear agreement on whether subclinical hypothyroidism should be included in guidelines for risk factors for heart disease, Dr. Bauer said he thinks that they should. He added that general physicians and cardiologists are “not at all” fully cognizant of the heart risks associated with subclinical hypothyroidism.
Dr. Bauer said that he had no disclosures to make in relation to this presentation. The meeting was sponsored in part by Abbott Laboratories and the Genzyme Corp.