Researchers may have found a new risk factor for an irregular heartbeat: high thyroid hormone levels.

Older people with higher levels of thyroid hormone were more likely to develop atrial fibrillation than people with lower levels, even when levels were within the normal range, according to a new study in the American Heart Association’s journal Circulation.

“Our findings suggest that levels of the thyroid hormone, free thyroxine, circulating in the blood might be an additional risk factor for atrial fibrillation,” said the study’s lead author Christine Baumgartner, M.D., an internal medicine specialist at the University Hospital of Bern in Switzerland and currently a postdoctoral scholar at the University of California, San Francisco.

“Free thyroxine hormone levels might help to identify individuals at higher risk,” she said.

AFib affects 2.7 to 6.1 million Americans, with an estimated 12.1 million affected by 2030. It occurs when the two upper chambers of the heart beat irregularly and faster than normal, causing heart palpitations, dizziness, sweating or fainting. The condition can cause chronic fatigue and increases the risk of stroke and heart failure.

The thyroid gland is a small gland in the neck that secretes thyroid hormones required to regulate energy metabolism. Patients with low levels of thyroid hormone, or hypothyroidism, may require medications containing the thyroid hormone thyroxine. But thyroxine can sometimes increase levels too much.

Previous studies have shown AFib risk is higher among people who produce too much thyroid hormone than among those with normal hormone levels. But unclear was whether levels on the high end of the normal range could also increase AFib risk.

In the new study, researchers found that people with higher blood levels of thyroid hormone, or FT4, that was within the normal range at the start of the study were much more likely to develop AFib than those with lower levels.

The group with the highest FT4 levels had a 45 percent increased risk of AFib compared to the group with the lowest levels. Even among those with the second-highest levels, the risk was 17 percent greater compared to those with the lowest levels, and those with the third-highest levels had a 25 percent increased risk.

High levels of thyroid-stimulating hormone within the normal range, however, were not associated with an increased risk of AFib.

“Patients who are treated with thyroxine, one of the most frequently prescribed drugs in the United States, generally have higher circulating free thyroxine levels compared to untreated individuals,” Baumgartner said. “So, an important next step is to see whether our results also apply to these patients, in order to assess whether target free thyroxine thyroid hormone concentrations for thyroid replacement therapy need to be modified.”

The investigators analyzed data from more than 30,000 older people taking part in 11 studies from Europe, Australia, and the United States.

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