Impact of Thyroid Function on Atrial Fibrillation Screening Outcomes
Overview
This post hoc analysis of the LOOP randomized trial evaluated the influence of baseline thyroid-stimulating hormone (TSH) levels on atrial fibrillation (AF) screening efficacy. AF screening with implantable loop recorders (ILR) significantly increased AF detection across all TSH levels, but stroke risk reduction was observed only in participants with low TSH levels.
Background
Atrial fibrillation screening aims to prevent strokes in high-risk populations, yet optimal strategies remain uncertain. The LOOP study demonstrated increased AF detection and a nonsignificant stroke risk reduction with ILR screening. Thyroid function, particularly TSH levels, is linked to AF risk, with low or subclinical hyperthyroid states associated with higher AF and stroke risk. This analysis investigates whether TSH levels modify the benefit of AF screening on stroke prevention.
Data Highlights
TSH Tertile
Participants (n)
AF Detection Increase (ILR vs Usual Care)
Stroke or SE HR (95% CI)
Stroke, SE, or CV Death HR (95% CI)
Lowest Tertile
~2000
~3-fold increase
0.52 (0.30-0.90), P=0.02
0.54 (0.34-0.84), P=0.006
Middle Tertile
~2000
~3-fold increase
No significant effect
No significant effect
Highest Tertile
~2000
~3-fold increase
No significant effect
No significant effect
Key Findings
ILR screening tripled AF detection rates compared to usual care across all TSH tertiles (adjusted P interaction = 0.44).
Significant reduction in stroke or systemic embolism (HR 0.52) was observed only in participants with the lowest TSH tertile.
Composite outcome of stroke, systemic embolism, or cardiovascular death was also significantly reduced in the lowest TSH group (HR 0.54).
No significant stroke risk reduction was found in participants with middle or highest TSH tertiles despite similar AF detection rates.
Results remained consistent when excluding participants with abnormal TSH or thyroid medication and when analyzing TSH as a continuous variable.
There was no significant effect of screening on major bleeding, all-cause death, or other secondary outcomes.
Clinical Implications
TSH measurement may help identify patients who derive the greatest stroke prevention benefit from AF screening and anticoagulation. Screening in individuals with low TSH could optimize resource use and reduce overdiagnosis or overtreatment in those with higher TSH levels. These findings support a more personalized approach to AF screening based on thyroid function.
Conclusion
AF screening with ILR increases AF detection regardless of thyroid function, but stroke risk reduction is confined to patients with low baseline TSH. TSH may serve as a useful biomarker to guide targeted AF screening strategies, warranting further prospective validation.
References
Svendsen et al. 2021 -- The LOOP Study: AF Screening Using Implantable Loop Recorders
Engdahl et al. 2013 -- STROKESTOP Study: Systematic ECG Screening for AF
Díez et al. 2019 -- Thyroid Function and Atrial Fibrillation Risk
