Higher levels of thyroid hormone in the blood were associated with an increased risk of atrial fibrillation (AFib), even when the levels were within normal range, according to an analysis of 11 studies containing more than 30,000 people.
Lead researcher Christine Baumgartner, MD, and colleagues studied only individuals with thyroid hormone (free thyroxine, FT4) levels in the normal range. However, those people were grouped into quartiles based on where their levels were within that range at baseline. Study participants were an average of 69 years old and slightly more than half were women.
Baumgartner et al. found patients with the highest FT4 levels at baseline had a 45 percent increased risk of AFib when compared to the group with the lowest levels. Versus the lowest quartile, the group with the second-highest FT4 levels had a 17 percent greater risk of AFib and the third-highest group had a 25 percent increased risk.
Blood levels of thyroid-stimulating hormone were not associated with increased risk of AFib.
“Our findings suggest that levels of the thyroid hormone, free thyroxine, circulating in the blood might be an additional risk factor for atrial fibrillation,” Baumgartner said in a press release. “Free thyroxine hormone levels might help to identify individuals at higher risk.”
In their study published online Oct. 23 in Circulation, the authors noted these findings highlight the need for further investigation into thyroxine medication, which is given to individuals with hypothyroidism to increase their hormonal levels. It could be dangerous to increase the levels too much, the researchers said.
Previous studies have shown the risk of irregular heartbeat is greater among individuals with abnormally high thyroid hormone levels, but Baumgartner and colleagues said this is the first to show the association remains intact for patients with higher levels but within the normal range.
“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.”
AFib affects between 2.7 and 6.1 million people in the U.S., according to the American Heart Association, and up to 12.1 million Americans could have the condition by 2030.
The researchers’ analysis encompassed studies from Europe, Australia and the United States. Average follow-up for the studies ranged from 1.3 to 17 years—contributing to 278,955 person-years of follow-up. A total of 8.6 percent of the study population developed AFib.