title: Premenopausal endogenous oestrogen levels and breast cancer risk: a meta-analysis.
abstract: BACKGROUND
  Many of the established risk factors for breast cancer implicate circulating hormone levels in the aetiology of the disease. Increased levels of postmenopausal endogenous oestradiol (E2) have been found to increase the risk of breast cancer, but no such association has been confirmed in premenopausal women. We carried out a meta-analysis to summarise the available evidence in women before the menopause.
  
  
  METHODS
  We identified seven prospective studies of premenopausal endogenous E2 and breast cancer risk, including 693 breast cancer cases. From each study we extracted odds ratios of breast cancer between quantiles of endogenous E2, or for unit or s.d. increases in (log transformed) E2, or (where odds ratios were unavailable) summary statistics for the distributions of E2 in breast cancer cases and unaffected controls. Estimates for a doubling of endogenous E2 were obtained from these extracted estimates, and random-effect meta-analysis was used to obtain a pooled estimate across the studies.
  
  
  RESULTS
  Overall, we found weak evidence of a positive association between circulating E2 levels and the risk of breast cancer, with a doubling of E2 associated with an odds ratio of 1.10 (95% CI: 0.96, 1.27).
  
  
  CONCLUSIONS
  Our findings are consistent with the hypothesis of a positive association between premenopausal endogenous E2 and breast cancer risk.
authors.affiliation: Faculty of Epidemiology and Population Health, London School of Hygiene and Tropical Medicine, London WC1E 7HT, UK. kate.walker@lshtm.ac.uk
authors.forename: K
authors.initials: K
authors.lastname: Walker
authors.forename: D J
authors.initials: DJ
authors.lastname: Bratton
authors.forename: C
authors.initials: C
authors.lastname: Frost
mesh_terms: D000328:Adult; D001943:Breast Neoplasms / Q000453:epidemiology*; D004958:Estradiol / Q000097:blood*; D005260:Female; D006801:Humans; D008875:Middle Aged; D017697:Premenopause; D011446:Prospective Studies; D012307:Risk Factors
chemical_list: D004958:Estradiol