Parity and Incidence of Thyroid Autoimmunity: A Population-Based Tehran Thyroid Study

Autoimmune thyroid disease is the most common autoimmune disorder. Evidence regarding causes of the high prevalence and incidence of thyroid autoimmunity in women, and especially women of reproductive age, is still inconclusive and previous studies have suggested genetic, environmental, and existential factors to play a role in its pathogenesis. In this study, we aimed to investigate the effect of parity and other reproductive factors on the incidence of thyroid autoimmunity within the framework of the Tehran Thyroid Study (TTS). This study was conducted within the framework of the TTS and 1999 nonpregnant euthyroid thyroid peroxidase antibody (TPOAb) negative women were followed up for an average of 8.3 years. A pooled logistic regression model was used to assess the association (odds ratio) between time-dependent covariates parity, menopause, and abortion, and incidence of TPOAb positivity. The total incidence rate of TPOAb positivity was 8.65 [7.35-10.18] per 1000 person-years. We found no significant association between changes in the number of parity and risk of developing TPOAb using multiple pooled logistic models both as crude effect and after adjustment for age, body mass index, and smoking. Similarly, there was no association between changes of parity, menopause, and abortion status, and incidence of TPOAb positivity. Parity does not seem to have an independent role in triggering thyroid autoimmunity, but vast immunological and physiological changes during pregnancy may act as a precipitating factor in the context of other genetic and environmental modifiers.