Reproductive and menstrual factors and risk of differentiated thyroid carcinoma: the EPIC study.

Differentiated thyroid carcinoma (TC) is threefold more common in women than in men and, therefore, a role of female hormones in the etiology of differentiated TC has been suggested. We assessed these hypotheses in the European Prospective Investigation into Cancer and Nutrition (EPIC) cohort. Among 345,157 women (mean age 51) followed for an average of 11 years, 508 differentiated TC cases were identified. Hazard ratios (HRs) and 95% confidence intervals (CIs) were estimated using Cox proportional hazards regression models. No significant associations were observed between differentiated TC risk and number of pregnancies, breast feeding, menopausal status, and age at menarche and at menopause. Significant associations were found with history of infertility problems (HR 1.70; 95% CI 1.12-2.60), a recent pregnancy (HR for ≤ 5 vs. >5 years before recruitment 3.87; 95% CI 1.43-10.46), menopause type (HR for surgical vs. natural menopause: 2.16; 95% CI 1.41-3.31), oral contraceptive (OC) use at recruitment (HR: 0.48; 95% CI 0.25-0.92) and duration of OC use (HR for ≥ 9 vs. ≤ 1 year: 0.66; 95% CI: 0.50-0.89). An increased risk was also found with hormone replacement therapy use at recruitment (HR = 1.30, 95% CI 1.02-1.67), but this was not significant after adjustment for type of menopause (HR = 1.22, 95% CI 0.95-1.57). Overall, our findings do not support a strong role of reproductive and menstrual factors, and female hormone use in the etiology of differentiated TC. The few observed associations may be real or accounted for by increased surveillance in women who had infertility problems, recent pregnancies or underwent surgical menopause.