What Cause Endometrial Thickening and Fibroids: What to Know

September 13, 2025

What Causes Endometrial Thickening and Fibroids: What to Know

If you’ve been told your ultrasound shows a “thickened lining” or uterine fibroids, you’re not alone. Both findings are common and often benign, but they can also signal conditions that deserve careful follow-up. This guide explains what endometrial thickening and fibroids are, why they happen, how they’re evaluated, and which treatments are available—drawing on trusted sources like the NIH and FDA.

Endometrial Thickening: What It Means

The endometrium is the inner lining of the uterus. Its thickness naturally changes through the menstrual cycle, becoming thicker under the influence of estrogen and shedding during a period. “Endometrial thickening” on imaging can be normal (depending on cycle timing) or can reflect a condition called endometrial hyperplasia—an overgrowth of the lining from too much estrogen relative to progesterone.

Common Causes of Endometrial Thickening

Hormonal imbalance (unopposed estrogen): Obesity, polycystic ovary syndrome (PCOS), perimenopause, and estrogen-only hormone therapy can increase estrogen exposure without sufficient progesterone, promoting overgrowth of the lining.
Tamoxifen use: This breast cancer medication has estrogen-like effects in the uterus and can thicken the lining.
Irregular ovulation: Anovulatory cycles (common in PCOS and perimenopause) reduce progesterone exposure.
Normal cyclic variation and early pregnancy: In premenopausal women, a thicker lining can be physiologic depending on cycle timing or early pregnancy.

Why it matters: Persistent endometrial hyperplasia—especially when cells look atypical under the microscope—can raise the risk of endometrial cancer. Risk factors for endometrial cancer overlap with those for hyperplasia and include obesity, unopposed estrogen therapy, PCOS, and tamoxifen use (National Cancer Institute).

Symptoms to Watch For

Heavy, prolonged, or irregular menstrual bleeding
Bleeding between periods
Any bleeding after menopause

How Doctors Evaluate a Thickened Lining

History and physical exam, including menstrual and medication history
Transvaginal ultrasound to measure the lining and look for focal lesions
Endometrial sampling (office biopsy) to check for hyperplasia or cancer
Hysteroscopy (a camera in the uterus) if a polyp, fibroid, or focal area needs direct visualization and treatment

Treatment Options for Endometrial Hyperplasia

Progestin therapy: Oral progestins or a levonorgestrel-releasing intrauterine device (IUD) can thin the lining and treat many types of hyperplasia.
Address underlying factors: Weight management, treating PCOS, and reviewing hormone therapies.
Surgical options: Hysteroscopic removal of polyps or focal lesions; hysterectomy may be recommended for atypical hyperplasia or if medical therapy fails (particularly in those who have completed childbearing).

Management is individualized based on your age, fertility goals, biopsy results, and symptom severity.

Uterine Fibroids: The Basics

Uterine fibroids (leiomyomas) are benign growths of the muscle wall of the uterus. They are extremely common, especially during the reproductive years. Fibroids can be inside the uterine cavity (submucosal), in the wall (intramural), or on the outer surface (subserosal), and their size and location influence symptoms.

What Causes Fibroids?

The exact cause isn’t fully understood, but research points to:

Hormones: Estrogen and progesterone promote fibroid growth.
Genetics: Fibroids often run in families.
Race and age: Black women develop fibroids more often, at younger ages, and with greater symptom burden.
Other factors: Early menarche, obesity, and possibly vitamin D deficiency are associated with increased risk (NIH/NICHD; HHS Office on Women’s Health).

Common Symptoms

Heavy or prolonged menstrual bleeding (which can cause anemia)
Pelvic pressure or pain, bloating
Frequent urination or constipation (from pressure on bladder or bowel)
Pain with intercourse
Fertility or pregnancy issues in some cases, especially with submucosal fibroids

Diagnosing Fibroids

Pelvic exam and transvaginal ultrasound
Saline infusion sonohysterography or hysteroscopy to assess the uterine cavity
MRI when detailed mapping is needed (e.g., pre-surgery)

Fibroid Treatment Options

Treatment depends on your symptoms, fibroid size/location, and reproductive plans. Options include:

Watchful waiting: If you have mild or no symptoms.
Medications for bleeding and pain: Nonsteroidal anti-inflammatory drugs (NSAIDs) and tranexamic acid for heavy menstrual bleeding.
Hormonal therapies: Combined hormonal contraceptives and progestin-only methods (including the levonorgestrel IUD) can reduce bleeding in many patients.
Targeted fibroid medicines: FDA-approved oral therapies for heavy menstrual bleeding due to fibroids include relugolix-estradiol-norethindrone acetate (Myfembree) and elagolix-estradiol-norethindrone acetate (Oriahnn). Gonadotropin-releasing hormone (GnRH) agonists such as leuprolide may also shrink fibroids temporarily (often used preoperatively).
Procedures:
- Myomectomy (surgical removal of fibroids) preserves the uterus and may be done hysteroscopically, laparoscopically, or via open surgery.
- Uterine artery embolization (UAE) shrinks fibroids by limiting blood supply.
- MRI-guided focused ultrasound (MRgFUS) is a noninvasive option for selected patients.
- Hysterectomy definitively treats fibroids in those who do not wish to preserve fertility.

How Are Endometrial Thickening and Fibroids Related?

Shared symptom: Both can cause heavy or irregular bleeding, which is why they’re often evaluated together.
Different biology: Endometrial hyperplasia involves the lining; fibroids are muscle tumors of the uterine wall. Fibroids don’t directly cause hyperplasia, but submucosal fibroids can distort the cavity and contribute to bleeding or a “thicker-looking” stripe on ultrasound.
Common hormonal drivers: Estrogen and progesterone influence both conditions; situations of high estrogen exposure can play a role in each.

When to See a Clinician

Any bleeding after menopause
Heavy periods soaking through pads/tampons hourly or causing anemia symptoms (fatigue, dizziness)
Bleeding between periods or after sex
Pelvic pain/pressure, urinary frequency, or difficulty conceiving

Prompt evaluation helps rule out cancers, confirm the diagnosis, and tailor treatment to your goals.

Key Takeaways

Endometrial thickening can be normal or due to hyperplasia; evaluation focuses on ruling out precancer and treating hormonal imbalance.
Fibroids are common, benign tumors that may cause heavy bleeding and pressure symptoms; many effective medical and procedural treatments exist, including FDA-approved medicines for fibroid-related bleeding.
Discuss your symptoms, biopsy results (if any), and reproductive plans with your clinician to choose the safest, most effective option.

This article is for educational purposes and does not replace personalized medical advice. If you have concerning symptoms, seek care promptly.

Trusted Sources and Further Reading

MedlinePlus: Endometrial Hyperplasia – https://medlineplus.gov/ency/article/001302.htm
National Cancer Institute (NCI): Endometrial Cancer Prevention (risk factors) – https://www.cancer.gov/types/uterine/patient/endometrial-prevention-pdq
NIH/NICHD: Uterine Fibroids – https://www.nichd.nih.gov/health/topics/uterine-fibroids
HHS Office on Women’s Health: Uterine Fibroids – https://www.womenshealth.gov/a-z-topics/uterine-fibroids
MedlinePlus: Uterine Fibroids – https://medlineplus.gov/uterinefibroids.html
FDA: Myfembree approval for heavy menstrual bleeding due to fibroids – https://www.fda.gov/drugs/news-events-human-drugs/fda-approves-therapy-management-heavy-menstrual-bleeding-associated-uterine-fibroids
FDA: First oral treatment for heavy menstrual bleeding associated with fibroids (Oriahnn) – https://www.fda.gov/drugs/drug-approvals-and-databases/fda-approves-first-oral-treatment-prolonged-heavy-menstrual-bleeding-fibroids