Older people taking levothyroxine at a dose of more than 0.075mg daily might be at increased risk of developing atrial fibrillation (AF), a Canadian study shows.

Researchers drew on healthcare data for nearly 392,000 people in Ontario aged 66 or older (mean age 82, 77% female), without an AF history who were prescribed levothyroxine at least once between 2007 and 2016.

During the study period 30,560 people in the cohort developed AF. Their baseline characteristics were compared with those of 152,800 controls, matched for age, sex and length of time in the cohort, who were taking levothyroxine but had not developed AF.

“After adjustment for potential confounders, we found a modest, but significantly increased association between increasing levothyroxine exposure and risk of incident AF,” the researchers wrote in the American Heart Journal.