Effects of Atrial Fibrillation Screening According to Thyroid Function: Post Hoc Analysis of the Randomized LOOP Study

Abstract Context Subclinical thyroid dysfunction is a marker for atrial fibrillation (AF) and stroke risk. Objective This study explored the effects of AF screening according to thyroid-stimulating hormone (TSH) levels. Methods An AF screening trial (the LOOP study) was analyzed post hoc according to baseline TSH. The primary outcome was stroke or systemic embolism (SE). Secondary outcomes included major bleeding, all-cause death, and the combination of stroke, SE, and cardiovascular death. Results TSH measurements were available in 6003 of 6004 trial participants, 1500 randomized to implantable loop recorder (ILR) screening for AF and anticoagulation upon detection vs 4503 to usual care; mean age was 74.7 ± 4.1 years and 2836 (47%) were women. AF detection was approximately triple for ILR vs usual care across TSH tertiles (adjusted P interaction = 0.44). In the first tertile, screening was associated with decreased risk of the primary outcome (hazard ratio [HR] 0.52, 95% CI 0.30-0.90; P = .02) and stroke, SE, or cardiovascular death (HR 0.54, 95% CI 0.34-0.84; P = .006) compared with usual care, while no effect was observed among participants with higher TSH (adjusted P interaction .03 and .01, respectively). There was no effect on other outcomes. Analyses of continuous TSH or excluding those with abnormal TSH or thyroid medication showed similar results. Conclusion AF screening and subsequent treatment was associated with decreased stroke risk among participants with low TSH, though the yield of screening was similar across TSH levels. TSH may be useful as a marker to indicate benefit from AF screening vs overdiagnosis and overtreatment. These findings should be considered exploratory and warrant further study.