Hyperthyroidism incidence fluctuates widely in and around pregnancy and is at variance with some other autoimmune diseases:a Danish population-based study

Context: Hyperthyroidism in women of reproductive age is predominantly caused by Graves' disease. Pregnancy associated changes in the immune system may influence the onset of disease, but population-based incidence rates in and around pregnancy have not been reported. Objective: To estimate the incidence of maternal hyperthyroidism (defined by redeemed prescription of antithyroid drugs) in and around pregnancy and to compare this with the incidence of other autoimmune diseases such as rheumatoid arthritis (RA) and inflammatory bowel disease (IBD). Design: Population-based cohort study. Setting: Danish nationwide registers. Participants: Women giving birth to singleton liveborn children in Denmark from 1999-2008 (n=403,958). Main outcome measure(s): Incidence rates (IR) of maternal hyperthyroidism during a four-year period beginning two years before and ending two years after the date when the mother was giving birth first time in the study period. Results: Altogether 3,673 women (0.9%) were identified with onset of hyperthyroidism from 1997-2010, and the overall IR of maternal hyperthyroidism was 65.0/100,000/year. The IR of hyperthyroidism in and around pregnancy varied widely and was high in the first three months of pregnancy (incidence rate ratio (IRR) versus the remaining study period: 1.50 (95% CI 1.09-2.06)), very low in the last three months of pregnancy (0.26 (0.15-0.44)), and reached the highest level 7-9 months postpartum (3.80 (2.88-5.02)). The incidence variation in and around pregnancy was different for RA and IBD. Conclusion: These are the first population-based data on the incidence of hyperthyroidism in and around pregnancy. The incidence of hyperthyroidism was high in early pregnancy and postpartum whereas such particular pattern was not observed for other diseases of autoimmune origin.