Post-menopausal women with high levels of hormones such as estrogen or testosterone have a high risk of breast cancer, a new study has revealed.
The study looked at eight different sex and growth hormones and found that the risk of breast cancer increased with the number of elevated hormones - each additional elevated hormone level increased risk by 16percent.
The researchers from the Brigham and Women's Hospital and Harvard Medical School used blood samples collected from nurses up to nine years before health information, including their breast cancer status, was recorded.
Post-menopausal women who were diagnosed with breast cancer were matched to two controls of a similar age.
The highest levels of circulating estrogens (estrone and estrogen), prolactin, and androgens (testosterone, androstenedione, DHEA, or DHEA-sulfate) were individually associated with between 50 and 200 percent increase in breast cancer risk.
The number of different hormones elevated above normal further increased risk, so that women with one elevated hormone had an increased risk of 10percent (compared to normal levels), but the risk for women with five or six elevated hormone levels was doubled, and that for women with seven or eight was tripled.
All these risks were slightly higher for women with ER positive disease.
"Elevated estrogens had the biggest effect on risk, especially for ER positive cancer," said Dr Shelley Tworoger, from Brigham and Women's Hospital.
"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 estrogen in the body, these hormones have also been found to stimulate cancer cell growth in the absence of ER.
"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," she added.
The study was published in BioMed Central's open access journal Breast Cancer Research.