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.
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