High-normal free thyroxine (fT4) levels are associated with an increased risk of atrial fibrillation, but TSH levels do not  appear to have the same association, a new study shows.

A meta-analysis of data from more than 30,000 individuals in 11 cohorts who had thyroid function tests found a 45% higher risk of incident AF in the euthyroid subjects who had  the highest quartile of fT4 compared to the lowest.

Published in Circulation, the study showed no association between risk of AF and different levels of TSH within the reference range or with subclinical hypothyroidism.

“Our findings suggest that levels of the thyroid hormone, free thyroxine, circulating in the blood might be an additional risk factor for atrial fibrillation,” says study lead author Dr Christine Baumgartner, of the Department of General Internal Medicine, Bern University Hospital, Switzerland.

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

An accompanying commentary notes the meta-analysis confirms findings of previous studies showing that subclinical hypothyroidism is not associated with an increased risk of AF.

But the findings raise the question of whether physicians treating individuals with thyroid dysfunction should be careful not to achieve a fT4 value at the high end of the normal reference range, especially in the presence of other risk factors for AF.

“Should we have a different goal for fT4 when treating subclinical hypothyroidism? Current guidelines recommend a target of normal/low-normal TSH that often results in fT4 in the high-normal range, which may have adverse effects,” it says.

Previous studies have shown that treatment with thyroxine leads to small changes in fT4 but relatively large changes in TSH, and most clinicians therefore focus primarily on TSH and less on fT4, the authors note.

“This study challenges this notion to some degree, but does not show that we should abolish TSH as a target when treating individuals with thyroid dysfunction; rather, we probably should give more attention to the fT4 levels in individuals at high risk of AF,” they conclude.