Association between iron deficiency and prevalence of thyroid autoimmunity in pregnant and non-pregnant women of childbearing age: a cross-sectional study

Background: Thyroid autoimmunity (TAI) is prevalent among women of reproductive age and associated with adverse pregnancy outcomes. This study aimed to investigate the association between iron nutritional status and the prevalence of TAI in women during the first trimester of pregnancy and in non-pregnant women of childbearing age. Methods: Cross-sectional analysis of 7463 pregnant women during the first trimester of pregnancy and 2185 non-pregnant women of childbearing age nested within the sub-clinical hypothyroid in early pregnancy study, a prospective collection of pregnant and non-pregnant women's data, was conducted in Liaoning province of China between 2012 and 2015. Serum thyrotropin, free thyroxine, thyroid peroxidase antibodies (TPOAbs), thyroglobulin antibodies (TgAbs), serum ferritin, and urinary iodine were measured. Iron deficiency (ID) was defined as serum ferritin <15 mu g/L and iron overload (IO) was defined as ferritin >150 mu g/L. TPOAb-positive was defined as >34 U/mL and TgAb-positive was defined as >115 U/mL. Multilevel logistic regression was conducted to examine the association between TAI and different iron nutritional status after adjusting for potential confounders. Results: The prevalence of isolated TPOAb-positive was markedly higher in women with ID than those without ID, in both pregnant and non-pregnant women (6.28% vs. 3.23%, chi(2) = 10.264, P = 0.002; 6.25% vs. 3.70%, chi(2) = 3,791, P = 0.044; respectively). After adjusting for confounders and the cluster effect of hospitals, ID remained associated with TPOAb-positive in pregnant and non-pregnant women (odds ratio [OR]: 2.111, 95% confidence interval [CI]: 1.241-3.591, P = 0.006; and OR: 1.822, 95% CI: 1.011-3.282, P = 0.046, respectively). Conclusion: ID was associated with a higher prevalence of isolated TPOAbs-positive, but not with isolated TgAb-positive, in both pregnant women during the first trimester of pregnancy and non-pregnant women of childbearing age, while IO was not associated with either isolated TPOAb-positive or isolated TgAb-positive.