Women’s risk of fatal coronary heart disease (CHD) rises with levels of thyrotropin, a hormone that stimulates thyroid function— even when those levels are within the normal range, a recent study suggests.

Thyrotropin is produced in the pituitary gland. Levels between 0.5 and 3.5 mIU/L are considered normal.

The research was based on data from a cohort study of people living in central Norway. About two thirds of the 25,313 subjects were women. During 8.3 years of follow-up, 228 women and 182 men died of CHD. Most of them (192 women and 164 men) had thyrotropin levels within the reference range.

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Using a low-normal category of 0.5 to 1.4 mIU/L for comparison, the researchers calculated hazard ratios (HRs) for CHD mortality. The HR was 1.41 for women with intermediate-normal thyrotropin levels (1.5-2.4 mIU/L) and 1.69 for those with high-normal readings (2.5-3.5 mIU/L). The trend did not apply to men (Arch Intern Med. 2008;168:855-860.)

“Thyrotropin levels were positively and linearly associated with CHD mortality in women,” the researchers conclude. “The results indicate that relatively low but clinically normal thyroid function may increase the risk of fatal CHD.”

But lead author Bjorn O. Asvold, MD, of the Norwegian University of Science and Technology in Trondheim, says it’s too soon to either prescribe a therapy or redefine the normal range.

Clinical studies are needed to assess whether a therapy would actually improve cardiovascular health and avoid such unwanted side effects as atrial fibrillation, he notes. And until research can define “a cutoff that separates thyrotropin levels that are associated with increased coronary risk and those that are not,” changing the reference range would be premature.