Are pregnancy, parity, menstruation and breastfeeding risk factors for thyroid cancer? Results from the Korea National Health and Nutrition Examination Survey, 2010‐2015

CONTEXT: It is unclear whether reproductive factors such as pregnancy, parity, menstruation, and breastfeeding are risk factors for thyroid cancer. The aim of this study was to evaluate the association of reproductive factors with thyroid cancer risk. DESIGN/SETTING: Large-scale nation-wide cross-sectional data were analysed from representative samples from the Korea National Health and Nutrition Examination Survey V-VI (2010-2015). RESULTS: In this study, 38 086 people over the age of 18 years were enrolled, of which 241 had a diagnosis of thyroid cancer. Of 21 543 female subjects, 210 had thyroid cancer. In logistic regression analysis, pregnancy, parity and number of reproductive years (period between menarche and menopause, excluding breastfeeding period) were significantly associated with thyroid cancer after adjusting for age, body mass index and smoking. However, the number of pregnancies did not show a linear relationship with thyroid cancer. Among breastfeeding women, total duration of breastfeeding and number of babies breastfed significantly decreased the risk for thyroid cancer. CONCLUSION: Our findings suggest that reproductive factors can be associated with the development of thyroid cancer. A large prospective cohort study is needed to clarify the causality.