Comorbidity of autoimmune thyroid disorders and psychiatric disorders during the postpartum period: a Danish nationwide register-based cohort study

Background: The postpartum period is well-known risk period for the first onset of autoimmune thyroid disorders (AITDs) as well as first onset of psychiatric disorders. These two disorders are some of the most prevalent medical conditions postpartum, often misdiagnosed and disabling if left untreated. Our study was designed to explore the possible bidirectional association between AITDs and psychiatric disorders during the postpartum period. Methods: A population-based cohort study through linkage of Danish national registers, which comprised 312 779 women who gave birth to their first child during 1997–2010. We conducted Poisson regression analysis to estimate the incidence rate ratio (IRR) of psychiatric disorders among women with first-onset AITDs, the IRR of AITDs among women with first-onset psychiatric disorders as well as the overlap between these disorders using a comorbidity index. Results: Women with first-onset AITDs postpartum were more likely to have first-onset psychiatric disorders than women who did not have postpartum AITDs (IRR = 1.88, 95% confidence interval (CI): 1.25–2.81). Women with first-onset postpartum psychiatric disorders had a higher risk of AITDs than women with no psychiatric disorders (IRR = 2.16, 95% CI: 1.45–3.20). The comorbidity index 2 years after delivery was 2.26 (95% CI: 1.61–2.90), indicating a comorbidity between first-onset AITDs and psychiatric disorders. Conclusions: First-onset AITDs and psychiatric disorders co-occur in the postpartum period, which has relevance to further studies on the etiologies of these disorders and why childbirth in particular triggers the onset.