Exogenous and endogenous hormones and breast cancer.

Exposure to higher levels of both exogenous and endogenous hormone is associated with breast cancer risk. Because of the association between breast cancer and HRT, only the minimal duration of HRT use is recommended for symptom control, and it is not recommended for chronic disease management. Current research issues include the role of progestins, other types of HRT, duration of unopposed estrogen use, and characteristics of cancers that develop on HRT. Circulating sex steroid levels are associated with breast cancer risk, but multiple issues need to be addressed before they are used routinely in clinical practice. Current research issues include measurement of levels for routine clinical practice, integration with standard breast cancer risk models and genetic polymorphism data, and applicability to estrogen-receptor-negative cancers.