Association of breastfeeding with thyroid function and autoimmunity in postmenopausal women.

OBJECTIVE Whether breastfeeding influences thyroid function and autoimmunity is not elucidated. We examined the association of history of breastfeeding with thyroid hormones and thyroperoxidase antibody (TPOAb). DESIGN Cross-sectional study of data from the 2013-2015 Korea National Health and Nutrition Examination Survey. METHODS A total of 816 postmenopausal women were stratified into three groups according to the duration of breastfeeding and number of breastfed children. Thyroid hormones levels, TPOAb titers, and the prevalence of hypothyroidism and TPOAb positivity were evaluated in each group. RESULTS Subjects with a history of prolonged breastfeeding had lower levels of thyrotropin (TSH) and TPOAb than the others. After adjusting for multiple confounding factors, the estimated means of TSH and TPOAb were associated with cumulative duration of breastfeeding. Duration of breastfeeding per child was associated with TSH levels, and number of breastfed children was associated with TPOAb titers. The odds ratio (OR) of hypothyroidism was significantly lower in group for ≥36 months of cumulative duration of breastfeeding, and the OR of TPOAb positivity was significantly lower in group with ≥3 breastfed children. In addition, duration of breastfeeding and the number of breastfed children were linearly correlated with log TSH and TPOAb each other in the multivariate model. CONCLUSIONS The present study showed that prolonged breastfeeding was inversely associated with TSH and the prevalence of hypothyroidism. Moreover, TPOAb and the prevalence of TPOAb positivity were inversely associated with number of breastfed children. These results indicate that breastfeeding may exert a protective effect on thyroid function and autoimmunity.