• Maria Kravchenko, Whitney Forbes, and Sky Graybill.
• Department of Medicine, Division of Endocrinology, Brooke Army Medical Center and Wilford Hall Ambulatory Surgical Center, San Antonio, TX 78234, USA.
• Mil Med. 2024 Jun 24.

IntroductionExcess thyroid hormone is a well-documented risk factor for the development of atrial fibrillation (AF). The purpose of the study is to assess incidence of AF in patients taking levothyroxine for hypothyroidism and correlate it with biochemical thyroid function.Materials And MethodsThis was a retrospective cohort study of patients aged 18 years and older who were treated with levothyroxine. Exclusion criteria were pre-existing diagnosis of AF and use of amiodarone in the prior year. Patients were followed 2012 through 2019 and stratified into 4 groups based on mean thyroid-stimulating hormone (TSH) value or mean fT4 value in 2012. Primary outcome was incidence of AF. Rates of AF between groups were assessed via Poisson regression with control of underlying confounders.ResultsOf 21,035 patients, 1091 (5.2%) developed AF during follow-up. Thyroid-stimulating hormone at baseline was not significantly associated with incident AF. Higher fT4 levels at baseline were associated with increased AF risk in age- and sex-adjusted analyses (hazard ratio 1.22; 95% CI, 1.03-1.44) for the highest quartile versus the lowest quartile of fT4.ConclusionsIn hypothyroid patients treated with levothyroxine, higher circulating fT4 levels are associated with increased risk of incident AF. There is no association of serum TSH with risk of AF. In patients at risk for AF, consideration should be given to avoiding fT4 levels in the highest quartile.Published by Oxford University Press on behalf of the Association of Military Surgeons of the United States 2024. This work is written by (a) US Government employee(s) and is in the public domain in the US.

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