When women with an intact uterus use systemic estrogen therapy, protecting the endometrium (uterine lining) is an important part of treatment. For this reason, systemic estrogen is commonly prescribed together with an appropriate progesterone or progestogen.
Below are several well-known, peer-reviewed clinical studies that evaluated endometrial safety.
Why Is Progesterone Important?
Estrogen stimulates the uterine lining. Without adequate progesterone or progestogen, prolonged exposure to systemic estrogen can increase the risk of endometrial hyperplasia, which may increase the risk of endometrial cancer over time.
The goal of adding progesterone is to help protect the uterine lining while allowing women to receive the benefits of systemic estrogen therapy.
PEPI Trial (Postmenopausal Estrogen/Progestin Interventions)
The PEPI Trial is one of the landmark studies evaluating endometrial safety during hormone therapy.
Study Overview
3-year randomized clinical trial
Compared estrogen alone with several estrogen-plus-progestogen regimens
Included postmenopausal women with an intact uterus
Key Findings
The study found that:
Women receiving estrogen alone experienced significantly higher rates of endometrial hyperplasia.
Women receiving estrogen together with progesterone/progestogen maintained endometrial protection, with substantially lower rates of hyperplasia.
This study helped establish the importance of combining progesterone with systemic estrogen in women who have a uterus.
Study Link
https://jamanetwork.com/journals/jama/fullarticle/395611
REPLENISH Trial
The REPLENISH Trial evaluated a combined oral bioidentical estradiol and progesterone therapy in postmenopausal women.
Study Overview
Phase 3 randomized clinical trial
Evaluated oral 17β-estradiol combined with progesterone
Included postmenopausal women with an intact uterus
How Uterine Safety Was Measured
Researchers evaluated endometrial safety using:
Endometrial biopsies at baseline
Repeat endometrial biopsies after 12 months of treatment
Key Findings
Rates of endometrial hyperplasia remained within FDA-accepted safety standards, supporting the endometrial safety of the combined estradiol/progesterone regimen studied.
Study Publication
https://pmc.ncbi.nlm.nih.gov/articles/PMC7147409/
ClinicalTrials.gov Listing
https://clinicaltrials.gov/study/NCT01942668
ELITE Trial (Early versus Late Intervention Trial with Estradiol)
The ELITE Trial is another frequently referenced menopause study.
It is important to understand that ELITE was not designed primarily as an endometrial safety study. Instead, its primary objective was to evaluate the cardiovascular effects of initiating estradiol therapy earlier versus later after menopause.
Study Overview
Independent, peer-reviewed clinical trial
Conducted by outside researchers
Not conducted by Inner Balance
Published Study
https://www.nejm.org/doi/full/10.1056/NEJMoa1505241
Because Inner Balance was not involved in conducting this research, we encourage reviewing the original publication and its supplementary materials for details about the study design, methodology, and outcomes. The published paper provides the most accurate explanation of how uterine safety was monitored throughout the trial.
Understanding These Studies
These studies were conducted by independent investigators and were not performed by Inner Balance.
Collectively, they support the well-established principle that:
Systemic estrogen alone is not considered uterine protective in women with an intact uterus.
Appropriate progesterone or progestogen is used alongside systemic estrogen to help protect the endometrium.
Different clinical trials use different methods to evaluate endometrial safety, including endometrial biopsies, clinical monitoring, and other predefined safety assessments depending on the study design.
When reviewing clinical research, we encourage reading the original publications whenever possible, as they provide the most complete description of each study's methodology and findings.
If you're considering hormone therapy and have questions about how clinicians apply the available scientific evidence when developing treatment protocols, we encourage discussing those questions directly with your provider.