Rheumatoid arthritis and risk of thyroid cancer: A nationwide cohort study.

Objective: Despite extensive cancer-related research in patients with rheumatoid arthritis (RA), there remains a paucity of studies on thyroid cancer in RA patients. We investigated risk of thyroid cancer in RA patients using updated definitions to identify RA patients and incident cases of thyroid cancer with consideration of RA serologic status.

Methods: Using a nationwide database, we identified 40,895 patients with newly diagnosed RA between 2010 and 2017, and matched them by sex and age at a 1:5 ratio to a control population of 204,475 non-RA individuals. The association of thyroid cancer and RA with consideration of serostatus was investigated using Cox regression analyses. Stratified analysis by sex and age were conducted using the same Cox modeling.

Results: During 5.5 years of follow-up, compared to the matched control group, the adjusted hazard ratio (aHR) of thyroid cancer in overall RA patients was 0.98 (95% confidence interval [CI] 0.85-1.13). Compared to the seronegative RA patients, seropositive RA patients did not show a significantly different risk of thyroid cancer (aHR 1.03, 95% CI 0.78-1.36). Stratified analyses by age or sex showed no statistical interaction.

Conclusion: Patients with newly diagnosed RA did not show a different risk of thyroid cancer compared to a matched control group. The risk of thyroid cancer incidence was not affected by serologic status of RA, sex, or age.