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Adrenal glands are star-shaped glands above your kidneys that secrete hormones.
The hormone estrogen is created naturally in a woman’s body. Most estrogen is produced in the ovaries, but smaller amounts are produced in the liver, the adrenal glands, the breasts, and fat cells. Once a woman reaches menopause, the ovaries no longer manufacture estrogen.
Estrogen is essential to the development of the female reproductive system. It sustains the menstrual cycle and makes childbearing possible. But it has many non-reproductive functions as well. For example, it helps lower cholesterol, regulate mood, and keep your bones and heart strong. However, some studies suggest that a high lifetime exposure to estrogen may contribute to the growth of breast cancer.
Estrogen isn’t one hormone but three closely related ones: estriol, estrone, and estradiol. Estriol is the most prevalent and accounts for between 60% to 80% of the hormones circulating in an adult woman’s body, depending on the phase of her menstrual cycle.
Many of the cells throughout your body — both healthy cells and potentially cancerous ones — contain estrogen receptors. These receptors are a type of protein molecule that stimulate cell growth when they come in contact with estrogen. As estrogen circulates through your bloodstream, it attaches to the estrogen receptors in cancerous cells, causing them to divide and accumulate in your body. In the absence of estrogen, these same cells would stop growing and eventually die.
That’s why circumstances that raise your lifetime estrogen levels or lengthen the amount of time your body is exposed to these hormones may increase your breast cancer risk. These may include:
Being overweight can lead to increased estrogen exposure because the body makes some of its estrogen in fat. So, the more body fat you have, the higher your potential risk for developing breast cancer. This is especially the case after menopause, when your ovaries no longer produce estrogen. In fact, some studies suggest that being overweight in menopause can increase your risk by 30% to 60%.
Some researchers link the development of breast cancer to environmental estrogens — sometimes called artificial estrogens — synthetic substances that behave like the hormone estrogen when they’re absorbed by your body. You may be exposed to environmental estrogens in some plastics, certain skin creams and sunscreens, some cleaning products, pesticides, and weed killers. However, it’s not yet clear whether and to what extent these artificial estrogens play a role in the development of breast, and other reproductive, cancers.
If you have a biopsy or other surgery for breast cancer, your doctor will send the tissue to a lab to test whether it contains significant numbers of estrogen-receptive cells. The majority of breast cancers do contain these cells and are considered estrogen-receptor positive.
You may be surprised to learn that estrogen is a factor in male breast cancer as well. In fact, the majority of breast cancers in men are estrogen receptor-positive.
The higher the number of estrogen-receptive cells, the more likely it is that the cancer will respond to hormone therapy. Hormone therapy, which is sometimes called anti-estrogen therapy, is designed to starve the cancer of the estrogen it needs to grow.
Some breast cancers have progesterone receptors as well. Progesterone is another female hormone that is necessary for childbearing. Cancers that test positive for progesterone receptivity may also respond to hormone therapy.
Learn more about hormone therapy in breast cancer prevention and treatment.
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