High levels of multiple hormones, particularly estrogen and testosterone, increase the risk of breast cancer, according to a study published recently by researchers from the Brigham and Women’s Hospital and Harvard Medical School.

The findings could make it easier to predict an individual’s risk of breast cancer, which affects as many as one in eight American women over the course of their lifetime.

The team’s study, which analyzed the impact of interacting hormones on cancer growth, filled a gap in research on the factors influencing breast cancer, according to co-author Shelley S. Tworoger, assistant professor of medicine and epidemiology at Harvard School of Public Health.

Previous studies have focused on the influence of hormones only in isolation, Tworoger said.

“This research changes how we may look at risk prediction in the future,” she said.

The researchers were able to take advantage of samples from the Nurses’ Health Study, a cohort of 121,700 American female nurses.

In what Tworoger describes as a “huge undertaking,” 32,826 of the study’s participants individually drew their own blood in 1989 and 1990 before sending their sample in for analysis. In follow-ups years later, the study then drew upon 320 confirmed breast cancer patients from the sample, analyzing levels of estrogens, androgens, and prolactin.

“Our ultimate goal would be to be able to identify women who are at a high risk of breast cancer so they can take preventive action,” said Tworoger

While current risk-prediction models, which take into account family and individual health history, can describe trends within a population, Tworoger said that establishing an accurate risk-prediction model for individuals remains a challenge.

More research is necessary to determine the implications of the researchers’ findings, Tworoger said. She said that drawing upon an even larger sample size would enable researchers to answer more key questions about the influence of hormones on risk rates.

In the future, for instance, Tworoger said she would like to confirm indications in the study that estrogens may have a slightly stronger influence on breast cancer risk than androgens.

Tworoger said she is working with researchers at Oxford to obtain access to a much larger, international dataset. But she said the end results of this research is still “several years away.”

“I think the most important thing is that more research is needed about how these hormones influence the risk of breast cancer,” said Tworoger. “If we can identify the best way to predict cancer risk using hormones, women who are at a higher risk could take medication which blocked their effects.”