Autoimmune Thyroid Diseases and Microscopic Colitis: A Nationwide Matched Case-Control Study in Sweden.

Background: Microscopic colitis (MC), comprising collagenous colitis (CC) and lymphocytic colitis (LC), is an inflammatory condition of the colon, characterized by watery diarrhoea. Previous studies suggest an association between autoimmune thyroid diseases (AITDs) (Hashimoto's thyroiditis and Graves' disease) and MC; however, large-scale histology studies are lacking.

Methods: We conducted a nationwide, matched case-control study. Patients with biopsy-confirmed MC diagnosed between 2006 and 2017 were identified through the population-based histopathology cohort ESPRESSO. Data on AITDs and covariates were retrieved from Swedish national healthcare registers. Odds ratios (ORs) for MC associated with prior AITDs were estimated using conditional logistic regression. Sibling comparisons were performed to minimize shared genetic and environmental confounding.

Results: Among 10,301 MC cases and 48,712 controls, AITDs were significantly more prevalent in MC patients (12.0%) than in controls (7.8%), yielding an adjusted OR of 1.65 (95% confidence interval (CI): 1.54-1.77). This association was attenuated but remained significant in sibling analyses (OR: 1.26; 95%CI: 1.11-1.43) The association was stronger in patients diagnosed with MC before age 50 (OR: 2.41; 95%CI: 2.02-2.89). Subgroup analyses revealed no difference between CC and LC or between sexes.

Conclusion: Individuals with AITDs are at an increased risk of developing MC. That this association was robust across various subgroups may be indicative of shared underlying mechanisms. Our findings highlight the importance of being vigilant of gastrointestinal symptoms in patients with AITDs and that patients with persistent symptoms despite achieving euthyroidism should be evaluated for MC.