A slightly underactive thyroid can present as much of a cardiovascular threat as diabetes or hypertension, according to findings presented at the American Thyroid Association meeting in New York City in October.

To date, only small studies have shown an association between cardiac dysfunction and subclinical hypothyroidism or hyperthyroidism. The new trial involved 3,065 persons aged 65 and older who were free of heart failure (HF) at baseline. Over the course of 12 years of follow-up, HF developed twice as frequently in those with thyroid-stimulating hormone (TSH) levels ≥10.0 mU/L, compared with those who had normal TSH levels.

This risk is comparable to that associated with diabetes and hypertension, according to one of the researchers, Doug Bauer, MD, professor of medicine and biostatistics at the University of California, San Francisco.

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Subclinical hypothyroidism—a mildly underactive thyroid condition that can be detected only with a blood test—is defined as a TSH level >4.5 mU/L and normal free thyroxine levels. “There is no clear consensus, but most would say 20 [mU/L] is the top end of ‘mild,’” says Dr. Bauer. It is well known that overt low thyroid disease impairs the ability of the heart to contract normally, “but it was not clear that mild hypothyroidism was associated with heart failure,” Dr. Bauer pointed out. “Our study suggests it is. If other studies confirm these findings, clinicians might want to consider treating mild thyroid problems to prevent potential cardiac problems or to avoid increasing the severity of an existing heart condition.”Dr. Bauer also said that screening people for abnormal thyroid conditions to prevent the development of HF may eventually be a possibility.