Treatment of Subclinical Hyperthyroidism and Incident Atrial Fibrillation

CONTEXT: Treating overt hyperthyroidism prevents atrial fibrillation (AF). Though subclinical hyperthyroidism (SH) has been associated with AF, it is unknown whether treating SH prevents AF. OBJECTIVE: We aimed to identify the association between treating SH and incident AF. DESIGN: In a pharmacoepidemiologic retrospective cohort study, patients diagnosed with SH between 2000 and 2021 were followed. PATIENTS: Outpatients ≥ 18 years with biochemical SH and without prior AF, hypothyroidism, thyroid cancer, pituitary disease, or pregnancy were included. MAIN OUTCOMES: The primary outcome was incident AF. Secondary outcomes were ECG and echocardiographic features associated with AF. RESULTS: Of 2169 patients screened, 360 (131 treated and 229 untreated) were followed up for a mean of 4.27 years. In the treated and untreated groups, AF occurred in 4 (3.1%) and 15 (6.6%) patients (p = 0.15), and AF incidence was 0.8% and 1.4%/year (p = 0.31), respectively. The hazard ratio (HR) for treatment as a time-dependent variable was 0.60 (95% CI 0.19-1.92; p = 0.39). As some cases of AF were documented nearly simultaneously with SH treatment, a sensitivity analysis was performed reassigning two patients diagnosed with AF < 30 days after starting SH treatment to the untreated group. Here, in the treated and untreated groups, AF occurred in 1.6% and 7.4% (p = 0.02), and AF incidence was 0.4% and 1.8%/year (p = 0.02), respectively. The HR was 0.25 (0.06-1.13; p = 0.07). There were no differences in ECG or echocardiographic features. CONCLUSION: There was an overall trend towards lower incidence and prevalence of AF following treatment of SH, supporting the need for larger scale studies.