Raised levels of several hormones can triple the risk of breast cancer in post-menopausal women, a study has shown.
Scientists looked at the combined effect of multiple sex and growth hormones on a woman's cancer chances.
They found that one hormone at higher than normal levels increased the breast cancer risk by 10% compared with having no elevated hormones.
But the risk for women with five or six hormones at raised levels was doubled, while having seven or eight tripled the odds of getting cancer.
The US researchers compared levels of eight hormones in blood samples collected from 33,000 nurses aged 43 to 69.
They included different types of oestrogen, testosterone and other "male" hormones, or androgens, and prolactin which stimulates the production of breast milk.
Levels of the growth hormone IGF-1 (insulin-like growth factor 1) and c-peptide, a biomarker for insulin, were also measured. Insulin acts as growth hormone as well as regulating the body's use of sugar.
Over a period of nine years, the scientists identified 320 post-menopausal women who were diagnosed with breast cancer and not on hormone-replacement therapy.
Hormone levels in the cancer patients were matched against those of women who did not develop the disease.
The findings, published today in the journal Breast Cancer Research, showed that "ER positive" breast cancers with molecular receptors sensitive to oestrogen were the most influenced by hormone levels.
Study leader Dr Shelley Tworoger, from Brigham and Women's Hospital in Boston, said: "Elevated oestrogens had the biggest effect on risk, especially for ER positive (oestrogen-sensitive) cancer. However, androgens, and prolactin also contribute to increasing risk of breast cancer.
"These hormones are known to stimulate the growth of breast cancer cells in the lab and, while androgens can be converted to oestrogen in the body, these hormones have also been found to stimulate cancer cell growth in the absence of ER (oestrogen receptors). Our results suggest that models used to assess breast cancer risk could be improved by taking into account multiple sex hormone and growth hormone levels."
Women were ranked according to the number of hormones they had above average levels for their age.
On an individual basis, the highest levels of circulating oestrogens, prolactin or androgens roughly doubled the normal risk of breast cancer.
However the number of different hormones at raised levels also had a significant impact on risk.
Women with no hormones above average levels had a "substantially lower" risk of cancer, the study found.