Reproductive factors, exogenous estrogen use, and risk of Parkinson's disease.

To determine if reproductive factors or exogenous estrogen are associated with risk of Parkinson's disease (PD), we conducted a prospective study with 22 years of follow-up among postmenopausal participants in the Nurses' Health Study. Relative risks (RRs) and 95% confidence intervals (CIs) of PD were estimated from a Cox proportional hazards model adjusting for potential confounders. Risk of PD was not significantly associated with any of the reproductive factors measured or exogenous estrogen use. Use of postmenopausal hormones, however, may modify the associations of smoking and caffeine intake with PD risk. The inverse relation between smoking and PD risk was attenuated among ever users of postmenopausal hormones (P for interaction = 0.05). Similar results were obtained for caffeine (P for interaction = 0.09). In exploratory analyses, women using progestin-only hormones were found to have an increased PD risk, but this result was based on a very small number of cases (n = 4). In this large longitudinal study, we found no evidence of a beneficial effect of exogenous or endogenous estrogens on risk of PD. The use of postmenopausal hormone use may interact with other risk factors, but findings are preliminary and need confirmation in other populations.