Endometrial hyperplasia is caused by too much estrogen acting on the uterine lining without enough progesterone to counterbalance it. This hormonal imbalance, often called “unopposed estrogen,” triggers the cells lining the uterus to keep multiplying instead of shedding in a normal menstrual cycle. The result is a thickened endometrium that can cause abnormal bleeding and, in some cases, progress toward cancer.
How Unopposed Estrogen Drives the Problem
During a normal menstrual cycle, estrogen thickens the uterine lining in the first half, preparing it for a potential pregnancy. After ovulation, the ovary produces progesterone, which stabilizes the lining and eventually triggers it to shed as a period. When ovulation doesn’t happen, or when estrogen comes from an outside source, progesterone never rises enough to stop the growth signal. The lining keeps proliferating.
At the cellular level, estrogen switches on genes that drive cell division while also suppressing the natural self-destruct process cells use to clear out old or damaged tissue. Without progesterone stepping in to halt that cycle, the endometrial glands grow irregularly and the lining becomes abnormally thick. This is the core mechanism behind every cause of endometrial hyperplasia, whether the excess estrogen comes from body fat, a medical condition, or a medication.
Obesity: The Most Common Cause
Fat tissue is not just storage. It actively produces estrogen by converting hormones from the adrenal glands into a form of estrogen called estrone. The more fat tissue you carry, the more efficient this conversion becomes, and it gets even more pronounced with age. In postmenopausal women, this process actually replaces the ovaries as the body’s main source of estrogen, which is why obesity raises hyperplasia risk even after menopause.
Women who carry weight primarily around the hips and thighs (a gynoid fat distribution) tend to have higher conversion rates than those with abdominal fat. The continuous estrogen exposure from fat tissue, without the cyclical progesterone that premenopausal ovaries provide, creates exactly the conditions that allow the endometrial lining to grow unchecked.
Polycystic Ovary Syndrome and Irregular Cycles
PCOS is one of the most common hormonal conditions in women of reproductive age, and it carries a well-documented link to endometrial hyperplasia. The connection is straightforward: women with PCOS frequently skip ovulation. Without ovulation, the ovary doesn’t form the structure (the corpus luteum) that produces progesterone. The result is months or even years of low-level estrogen exposure with virtually no progesterone opposition.
PCOS adds a second layer of risk. The excess androgens that characterize the condition can be converted into estrogen in fat and other tissues, further increasing the estrogen load on the uterus. Women with PCOS who go long stretches without a period are at particularly high risk, because each skipped cycle means another month of unopposed stimulation.
Estrogen-Only Hormone Therapy
Postmenopausal women who take estrogen without a progestogen face a significantly elevated risk. A large Cochrane review found that unopposed estrogen therapy increases the odds of developing endometrial hyperplasia at all doses, even within just one to three years of use. Compared to women taking combined estrogen-plus-progestogen therapy, those on estrogen alone had 3.8 to 9.4 times higher odds of developing hyperplasia after one year, depending on the specific regimen.
Adding a progestogen to the regimen dramatically reduces this risk, which is why combined therapy is now standard for any woman with an intact uterus. Women who have had a hysterectomy can safely take estrogen alone, since there is no uterine lining to overstimulate.
Tamoxifen and Medication-Related Causes
Tamoxifen, widely used to treat and prevent breast cancer, blocks estrogen’s effects in breast tissue. But it does the opposite in the uterus. The drug behaves differently depending on which tissue it enters. In breast cells, it recruits molecules that silence estrogen’s growth signals. In endometrial cells, it recruits molecules that amplify those signals, effectively mimicking estrogen and promoting cell proliferation.
This tissue-specific behavior is driven by differences in the molecular machinery available in each cell type. Endometrial cells have higher levels of the co-activating proteins that tamoxifen latches onto, which is why the uterine lining responds to the drug as though it were estrogen. Women taking tamoxifen are typically monitored for endometrial changes, and any abnormal bleeding should be evaluated promptly.
Less Common Causes
Certain ovarian tumors, particularly granulosa cell tumors, can produce large amounts of estrogen on their own. These are rare, but when they occur, the excess estrogen can cause rapid endometrial thickening. Hyperplasia in these cases often resolves once the tumor is removed.
Any condition that leads to chronic anovulation, not just PCOS, can set the stage for hyperplasia. This includes thyroid disorders, significant stress, extreme exercise, and the perimenopause transition, when cycles become irregular and ovulation becomes unpredictable. Late menopause (after age 55) and early onset of periods (before age 12) also extend the total years of estrogen exposure and modestly increase risk.
Two Types, Very Different Risks
The current WHO classification divides endometrial hyperplasia into two categories: hyperplasia without atypia and atypical hyperplasia. The distinction matters enormously for prognosis.
Hyperplasia without atypia is a benign overgrowth without significant genetic changes in the cells. The glands are irregularly shaped and sized, but the cells themselves look normal under a microscope. This type responds well to treatment and carries a low risk of becoming cancerous.
Atypical hyperplasia is a different story. The cells show abnormal features that place them on the spectrum toward cancer. A National Cancer Institute study found that 8% of women with atypical hyperplasia progressed to cancer within four years, and that number climbed to 28% over 19 years of follow-up. This type often requires more aggressive management, sometimes including surgery.
How It’s Detected
Endometrial hyperplasia most commonly shows up as abnormal uterine bleeding: periods that are heavier than usual, bleeding between periods, or any bleeding after menopause. In postmenopausal women with bleeding, a transvaginal ultrasound is typically the first step. The American College of Obstetricians and Gynecologists considers an endometrial thickness of 4 mm or less reassuring, with a greater than 99% negative predictive value for cancer. Anything thicker, or a lining that can’t be clearly visualized, warrants a tissue sample.
A biopsy is the only way to confirm hyperplasia and determine whether atypical cells are present. Persistent or recurrent bleeding should be evaluated with a biopsy regardless of how thin the lining appears on ultrasound, because rare forms of endometrial cancer can develop even with a lining under 3 mm.
How Hyperplasia Is Treated
For hyperplasia without atypia, treatment centers on delivering progesterone directly to the uterine lining. A progesterone-releasing intrauterine device achieves a 93% regression rate, typically within about 10 months. Oral progesterone pills are an alternative but less effective, with a regression rate around 66% over a similar timeframe. Among women who don’t respond to oral progesterone initially, switching to the intrauterine device pushes the success rate back up to 93%.
For atypical hyperplasia, hysterectomy is often recommended given the meaningful risk of progression to cancer. Women who want to preserve fertility may be managed with intensive progesterone therapy and close monitoring, but this requires frequent biopsies to confirm the abnormal cells are resolving. Addressing the underlying cause, whether that’s weight loss, treating PCOS, or adjusting hormone therapy, is essential to prevent recurrence regardless of which type is diagnosed.