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

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. We calculated summary relative risks and 95 % confidence intervals (95 % CI) using random effect models. 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. Menstrual and reproductive factors may play a role in the etiology of thyroid cancer, possibly through the mediation of estrogen receptors.