Breast Cancer Risk in Younger Women May be Influenced by Hormone Therapy

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NIH scientists have found that two common types of hormone therapy may alter breast cancer risk in women under age 55.

Researchers discovered that women treated with unopposed estrogen hormone therapy (E-HT) were less likely to develop the disease than those who did not use hormone therapy. They also found that women treated with estrogen plus progestin hormone therapy (EP-HT) were more likely to develop breast cancer than women who did not use any hormone therapy. Together, these results could help guide clinical recommendations for hormone therapy use among younger women.

The two hormone therapies analyzed in the study are often used to manage menopause symptoms or following hysterectomy (removal of uterus) or oophorectomy (removal of one or both ovaries). E-HT is recommended only for women who have had a hysterectomy because of its known association with uterine cancer risk.

The researchers conducted a large-scale analysis that included data from more than 459,000 women under 55 years old across North America, Europe, Asia and Australia. Women who used E-HT had a 14% reduction in breast cancer incidence compared to those who never used hormone therapy. This protective effect was more pronounced in women who started E-HT at younger ages or who used it longer. In contrast, women using EP-HT experienced a 10% higher rate of breast cancer compared to non-users, with an 18% higher rate seen among women using EP-HT for more than two years relative to those who never used the therapy.

The association between EP-HT and breast cancer was particularly elevated among women who had not undergone hysterectomy or oophorectomy. Doctors may consider gynecological surgery status when evaluating the risks of starting hormone therapy.

The results align with previous large studies that documented similar associations between hormone therapy and breast cancer risk among older and postmenopausal women and extends these findings to younger women experiencing menopause.