What is the incidence of endometrial hyperplasia in postmenopausal women receiving estrogen‑containing hormone replacement therapy without progestogen?

Incidence of Endometrial Hyperplasia with Unopposed Estrogen Therapy

Unopposed estrogen therapy in postmenopausal women with an intact uterus causes endometrial hyperplasia in approximately 20% of women after one year, rising to 62% after three years of moderate-dose therapy, making it unacceptably dangerous without concurrent progestogen protection. 1, 2

Quantified Risk by Duration and Dose

Short-Term Exposure (6–12 Months)

• Moderate to high-dose unopposed estrogen increases hyperplasia risk 5.4-fold at 6 months compared to placebo 2
• After 12 months, approximately 20% of women develop endometrial hyperplasia on unopposed estrogen 3
• Low-dose unopposed estrogen shows a 3% incidence of hyperplasia at 6–12 months, though this did not reach statistical significance 2

Medium-Term Exposure (24 Months)

• At 24 months, the odds ratio for hyperplasia rises to 9.6 (95% CI 5.9–15.5) with moderate-dose unopposed estrogen 2
• The absolute incidence continues to climb with each additional month of exposure 2

Long-Term Exposure (36 Months)

• After 36 months of moderate-dose unopposed estrogen, 62% of women develop some form of endometrial hyperplasia compared to only 2% in the placebo group 2
• The odds ratio reaches 15.0 (95% CI 9.3–27.5) at 36 months 2
• The risk of endometrial cancer increases 2- to 10-fold with unopposed estrogen therapy lasting 5 years or more 1, 4, 3

Dose-Dependent Effects

• High-dose unopposed estrogen produces the highest rates of hyperplasia and irregular bleeding 2
• Moderate-dose unopposed estrogen (e.g., conjugated equine estrogen 0.625 mg daily) carries substantial risk, as demonstrated in the PEPI trial 2
• Low-dose unopposed estrogen shows a trend toward increased hyperplasia (3% vs 0% in placebo), though the absolute numbers remain small in short-term studies 2

Endometrial Cancer Risk

• The relative risk of endometrial cancer with unopposed estrogen ranges from 2.3 (95% CI 2.1–2.5) after one year to approximately 9.5-fold after 10 years of continuous use 1, 5
• Unopposed estrogen increases endometrial cancer risk 10- to 30-fold after 5 or more years of therapy 1, 5
• This elevated risk persists for 5 or more years even after discontinuing unopposed estrogen 1

Protective Effect of Adding Progestogen

• Adding progestogen to estrogen therapy reduces endometrial cancer risk by approximately 90% compared to unopposed estrogen 1, 5, 2
• Sequential progestogen regimens (12–14 days per month) reduce hyperplasia incidence to approximately 5.4%, with atypical hyperplasia occurring in only 0.7% of cases 3
• Continuous combined estrogen-progestogen therapy is not associated with the development of hyperplasia or carcinoma and may normalize endometrium in women who developed complex hyperplasia on sequential therapy 3

Comparison of Progestogen Regimens

Sequential (Cyclic) Regimens

• Monthly sequential progestogen (12–14 days per cycle) provides adequate endometrial protection when the duration meets the minimum threshold 1, 2
• Long-cycle sequential therapy (progestogen every 3 months) shows higher incidence of hyperplasia compared to monthly sequential therapy 2
• Approximately 15% of endometrial biopsies from women on sequential HRT show proliferative activity, including atypical hyperplasia in up to 1% of cases 4, 3

Continuous Combined Regimens

• Continuous combined therapy produces endometrial atrophy in the majority of women 6, 4, 3
• Only 2–3% of women on continuous combined HRT show proliferative activity, usually without atypical hyperplasia 4
• At longer durations of treatment, continuous therapy is more effective than sequential therapy in reducing the risk of endometrial hyperplasia 2
• Continuous combined HRT is not associated with increased risk of hyperplasia or carcinoma 3

Clinical Implications

• Unopposed estrogen should never be prescribed to women with an intact uterus because the hyperplasia rate is unacceptably high (20% at 1 year, 62% at 3 years) 1, 2, 3
• Progestogen must be administered for at least 12–14 days per cycle in sequential regimens; shorter durations fail to prevent endometrial proliferation 1
• Micronized progesterone 200 mg orally at bedtime for 12–14 days per month is the preferred progestogen due to superior breast safety while maintaining adequate endometrial protection 1, 5
• Continuous combined regimens (e.g., micronized progesterone 100–200 mg daily) offer the most robust endometrial protection and eliminate withdrawal bleeding 1, 3