Month of birth is associated with the subsequent diagnosis of autoimmune hypothyroidism. A nationwide Danish register-based study

Background The triggering of thyroid autoimmunity in the genetically susceptible remains a conundrum. Environmental exposures during gestation and/or early postnatally have proponents, as suggested in diabetes mellitus, with a higher incidence of births during spring and summer. Whether the development of autoimmune hypothyroidism (AIT) is influenced by month or season of birth is less clear. Method Nationwide cohort study of 111,565 individuals diagnosed with AIT and four euthyroid controls per case, matched according to age and sex, were identified from Danish health registers. Differences in month-of-birth across the year was evaluated by the Walter-Elwood test. The risk of AIT patients being born in a certain month or season of the year was calculated using a Cox regression model. Results There was a significant difference in birth month between cases and controls, p<0.001. Individuals with AIT had a significantly increased risk of being born in June (Hazard ratio 1.04; 95% Confidence interval (CI): 1.02-1.08) and in the summer (June-August; HR 1.02; 95%CI: 1.01-1.04). Conclusion In this large-scale nationwide cohort study, we found a higher risk of AIT when born in the summer season or more specifically in June, supporting the hypothesis that seasonal variations in exposures - gestationally and/or early postnatally - may contribute to the development of AIT. This article is protected by copyright. All rights reserved.