OBJECTIVE: To investigate if autoimmune thyroid disease (AITD) impacts 
rheumatoid arthritis (RA) disease activity or response to methotrexate.
METHODS: A nationwide register-based cohort study of 9 004 patients with 
new-onset RA from the Swedish Rheumatology Quality Register year 2006-2016, with 
linkage to other nationwide registers to identify comorbidity with AITD defined 
as thyroxine prescription before RA diagnosis, excluding non-autoimmune causes. 
We compared RA disease activity using 28-joint Disease Activity Score (DAS28) 
and its components, and EULAR response, between patients with and without AITD, 
using logistic regression.
RESULTS: At diagnosis, patient reported outcome measures (PROMs; patient global, 
Health Assessment Questionnaire Disability Index and pain) but not objective 
disease activity measures (erythrocyte sedimentation rate and swollen joint 
count) were significantly higher (p<0.05 for all PROMs) among RA patients with 
AITD compared with those without. The level of DAS28 was 5.2 vs 5.1. By 
contrast, AITD had little influence on EULAR response to methotrexate at 3 
months (OR of non/moderate response=0.95, 95% CI 0.8 to 1.1), nor at 6 months. 
When stratified by age, however, AITD was more common among EULAR non/moderate 
responders at 3 and 6 months in patients below 45 years resulting in ORs of 
non/moderate response of 1.44 (0.76-2.76) and 2.75 (1.04-7.28).
CONCLUSION: At diagnosis, RA patients with concomitant AITD score worse on 
patient reported but not on objective RA disease activity measures, while DAS28 
was only marginally elevated. The overall chance of achieving a EULAR good 
response at 3 or 6 months remains unaffected, although among a limited subgroup 
of younger patients, AITD may be a predictor for an inferior primary response.