Ellervik C, et al. - In this mendelian randomization (MR) study, researchers assessed the potential direct involvement of thyroid traits on atrial fibrillation (AF) in over 55,000 AF cases and > 480,000 referents included in summary-level MR. Overall, 11 studies were included in study-level MR. For standardized free thyroxine (FT₄) and thyrotropin levels within the reference range, standardized triiodothyronine (FT₃):FT₄ ratio, hypothyroidism, standardized thyroid peroxidase antibody levels, and hyperthyroidism, they used genome-wide significant variants as tools. Findings revealed that increased AF was found in relation to genetically increased FT₃:FT₄ ratio and hyperthyroidism, but not FT₄ within the reference range. They also observed decreased AF in association with increased thyrotropin within the reference range and hypothyroidism. These findings supported the premise of a pathway involving the pituitary-thyroid-cardiac axis.