Estrogen

Estrogen is often described as "the female hormone," but that shorthand undersells its reach. While the ovaries are the main production site during the reproductive years, estrogen is also made in smaller amounts by the adrenal glands, fat tissue, and the placenta during pregnancy — and it has receptors in tissues throughout the body, from the bones and skin to the cardiovascular system and brain. Men also produce estrogen, though in much smaller quantities.

The three naturally occurring forms are estradiol (the most potent, dominant during reproductive years), estriol (predominant during pregnancy), and estrone (the primary form after menopause, produced mainly in fat tissue). Collectively, estrogens promote the development of breast and uterine tissue, regulate the menstrual cycle, support vaginal health, maintain bone density by limiting the cells that break down bone, and protect the lining of blood vessels. They also influence mood, memory, and sleep patterns, which is why estrogen fluctuations — at puberty, during pregnancy, and at menopause — often come with psychological and cognitive changes.

In the context of breast cancer, estrogen is important because many breast tumors have estrogen receptors (they are called estrogen receptor–positive, or ER-positive) and use estrogen as fuel for growth. Therapies that block estrogen production or prevent it from binding to its receptor — such as tamoxifen and aromatase inhibitors — are among the most effective treatments for ER-positive breast cancer.

Understanding estrogen helps make sense of many health decisions you may face across your lifetime. At menopause, as estrogen levels fall, you may experience hot flashes, sleep disruption, vaginal dryness, and accelerated bone loss — the very changes that led to the widespread use of hormone replacement therapy (HRT). HRT can relieve symptoms effectively, but it also carries risks, including a modest increase in breast cancer risk with some regimens, which is why the decision to use it is deeply individual and worth discussing carefully with your doctor.

Estrogen's relationship to cancer is nuanced. Lifetime exposure to higher estrogen levels — through early puberty, late menopause, never having been pregnant, or long-term HRT use — is associated with a modestly elevated risk of breast and endometrial cancers. Yet estrogen is also essential for bone and heart health. There is rarely a simple answer, and that is precisely why knowing the hormone's role in your body gives you a stronger foundation for those conversations.