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Parity and Risk of Thyroid Autoimmunity Based on the NHANES (2001‐2002, 2007‐2008, 2009‐2010, and 2011‐2012)

Context: Autoimmune thyroid disease is more common in women than in men. Fetal microchimerism has been implicated as a potential explanation for this disparity. Objective: The objective of this study was to evaluate the relationship between parity and thyroid autoimmunity in the US population. Design, Setting, Patients: The National Health and Nutrition Examination Survey was used to identify females with antithyroperoxidase (TPOAb) and antithyroglobulin antibody (TgAb) measurements and parity data. Subjects (n = 4864) were categorized as never pregnant (n = 909) or previously pregnant (n = 3955). The association of parity with thyroid autoantibodies was examined both qualitatively and quantitatively. Thyroid autoimmunity was defined as TPOAb and/or TgAb titers above the reference limits. Results: Previous pregnancy carried an odds ratio (OR) of 1.55 [95% confidence interval (CI): 1.26 to 1.91] for thyroid autoimmunity compared with never pregnant. Number of pregnancies was associated with thyroid autoimmunity: OR = 1.37 (95% CI: 1.02 to 1.84); 1.4 (95% CI: 1.08 to 1.81); 1.52 (95% CI: 1.18 to 1.96); and 1.73 (95% CI: 1.38 to 2.18) for 1, 2, 3, and ≥4 pregnancies, respectively. Because ever‐pregnant women differed in several variables—age, race, smoking status, history of thyroid disease, and urinary iodine level—from never‐pregnant women (P < 0.001), a multivariate regression analysis was performed, which showed no association of pregnancy with thyroid autoimmunity. The association was further examined utilizing an age‐matched analysis, which confirmed the absence of an association between thyroid autoimmunity and parity. Conclusion: Although we initially observed a strong association between parity and thyroid autoimmunity, after controlling for age and other variables, we were unable to identify an association.