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

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Abstract
Summary: 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 were evaluated by the Walter‐Elwood test. The risk of patients with AIT 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<.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.
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  • Novo Nordisk

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