Oct. 23 (UPI) -- A review of 11 studies from Australia, Europe and the United States found that patients with atrial fibrillation have higher levels of thyroid hormone.
Atrial fibrillation impacts 2.7 to 6.1 million people in the United States and is expected to impact up to 12.1 million by 2030.
According to researchers, previous studies have found that the risk for atrial fibrillation, or irregular heartbeat, is higher in people with an overproduction of thyroid hormone.
A 2013 study found that both hypothyroidism and hyperthyroidism increased atrial fibrillation in studies on rats.
In the new study, published today in Circulation, researchers analyzed data from two cohort databases, finding that participants with higher levels of the thyroid hormone free thyroxine, FT4, were more likely to develop an irregular heartbeat than those with normal levels.
People with hypothyroidism typically are treated with medications containing thyroxine to increase their hormone levels.
"Our findings suggest that levels of the thyroid hormone, free thyroxine, circulating in the blood might be an additional risk factor for atrial fibrillation," Dr. Christine Baumgartner, specialist in General Internal Medicine from the University Hospital of Bern, Switzerland, and currently a postdoctoral scholar at University of California San Francisco, said in a press release. "Free thyroxine hormone levels might help to identify individuals at higher risk."
Researchers examined the incidence of atrial fibrillation in people with normal thyroid hormone levels. Participants who had higher blood levels of FT4, but still within the normal range when the study began, were at an increased risk of developing atrial fibrillation compared to those with lower levels of the hormone.
Patients who had the highest levels of FT4 had a 45 percent increased risk of developing atrial fibrillation and even modest increases in FT4 had an increased risk.
"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."