(Philadelphia, PA) – Researchers at the University of Pennsylvania School of Medicine have found that leaving a mildly underactive thyroid gland (subclinical hypothyroidism) untreated does not lead to increased cardiovascular risk. The study results, to be published in the March 1st issue of the Journal of the American Medical Association, may lead to changes in the clinical care of this commonly diagnosed condition.

The thyroid gland, located at the base of the neck, produces thyroid hormone, which acts throughout the body to regulate metabolism. Patients with substantial underactivity of the thyroid gland, or hypothyroidism, may suffer from a variety of symptoms including fatigue, constipation, or weight gain. These patients are typically treated with medication to increase the thyroid hormone in the body. Likewise, patients with overactivity of the thyroid gland, or hyperthyroidism, may also be symptomatic and require treatment to lower their thyroid hormone levels.

In contrast, mild thyroid abnormalities are common and are usually not associated with any symptoms. Whether or not these mild abnormalities pose a risk to patients has been a questionable area. Many leading endocrinologists have advocated treatment of patients with mild thyroid problems based on the belief that mild thyroid problems increase cardiovascular risk.

"Previous studies have suggested that abnormal levels of thyroid stimulating hormone may represent a cardiac risk factor, the way that increased cholesterol or smoking are risk factors for heart disease." said lead author Anne Cappola, MD, ScM, Assistant Professor of Medicine and Epidemiology. “We set out to answer the question of whether there are cardiovascular consequences resulting from mild thyroid problems, using data from a large population studied for a long period of follow-up.”

Cappola measured thyroid function through a blood test in 3200 men and women, aged 65 and older, who participated in the Cardiovascular Health Study, a large multi-center cohort study sponsored by the National Institutes of Health. Study participants had no reason to have an abnormal thyroid test at enrollment and were not taking thyroid hormone replacement. Cappola categorized these patients into different groups based on their thyroid blood tests and determined the risk of cardiovascular problems over 13 years of follow-up.

Patients with a mildly underactive thyroid gland (subclinical hypothyroidism) made up 15 percent of the study, and showed no elevation in their risk of suffering an arrhythmia, heart attack, stroke or death. The only group that experienced adverse cardiac events was a small fraction of patients (1.5%) who had a mildly overactive thyroid (subclincial hyperthyroidism). These patients had an increased risk of developing common abnormal heart rhythm, atrial fibrillation, but no other cardiovascular threats such as heart attacks or death.

Cappola adds, “It’s our wish to figure out what is a ‘normal’ versus an ‘abnormal’ thyroid function… which thyroid hormone levels are associated with the highest level of physical function and health in the general population. Many patients with mild thyroid problems are being treated now and it is not clear if this is actually helping them. We need to put together a better understanding of the risks and benefits of treatment for mild thyroid abnormalities.”

So should general practitioners be screening patients over a certain age every so often for thyroid dysfunction? Some organizations recommend it. “Based on our findings, we believe there is no convincing evidence to support screening for mild thyroid problems in the general population in patients who have no symptoms. But if you do find someone over the age of 65 with a mildly overactive thyroid, our study suggests that they should be treated,” explains Cappola. “There’s no evidence you should treat someone with a mildly underactive thyroid in order to help deter cardiovascular disease.”

The results of this study will be published in the March 1st, 2006 issue of JAMA. You can access the journal on-line at: http://jama.ama-assn.org/. The article is titled, “Thyroid Status, Cardiovascular Risk and Mortality, in Older Adults.”

This study was supported by an American Heart Association Grand-in-Aid; contracts from the National Heart, Lung and Blood Institute; and a grant from the National Institute on Aging.

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