Menstrual and reproductive history and use of exogenous sex hormones and risk of thyroid cancer among women: a meta-analysis of prospective studies

PURPOSE: Thyroid cancer has a higher incidence in women than in men, and it has been hypothesized that hormonal factors may explain such disparity. We performed a meta-analysis of observational prospective studies to investigate the association between menstrual and reproductive variables and exogenous hormone use and the risk of thyroid cancer among women. METHODS: We calculated summary relative risks and 95 % confidence intervals (95 % CI) using random effect models. RESULTS: Overall, 5,434 thyroid cancer cases from twenty-four papers were included. Increasing age at first pregnancy/birth (SRR 1.56, 95 % CI 1.01-2.42) and hysterectomy (SRR 1.43, 95 % CI 1.15-1.78) were associated with thyroid cancer risk. Women that were in menopause at enrolment had a reduced thyroid cancer risk (SRR 0.79, 95 % CI 0.62-1.01). No other menstrual, reproductive, and hormonal variable was associated with thyroid cancer risk. CONCLUSIONS: Menstrual and reproductive factors may play a role in the etiology of thyroid cancer, possibly through the mediation of estrogen receptors.