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Advances in Anatomic Pathology:
doi: 10.1097/PAP.0b013e31802ef00f
Review Articles

Postmenopausal Hormone Replacement Therapy: Endometrial and Breast Effects

Yeh, I-Tien MD

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Department of Pathology, University of Texas Health Science Center at San Antonio, 7703 Floyd Curl Drive, San Antonio, TX

Reprints: I-Tien Yeh, MD, Department of Pathology, University of Texas Health Science Center at San Antonio, 7703 Floyd Curl Drive, San Antonio, TX 78229 (e-mail:

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The history of hormone replacement therapy (HRT) dates back to the late 1800s, when animal extracts of ovaries were first used. With the development of synthetic hormones, widespread use in postmenopausal women extended throughout the industrialized world, so that by the late 1900s roughly one-third to one-half of all postmenopausal women in the United States and Europe were taking HRT. Two events changed the course of use of HRT: the association of an increased rate of endometrial carcinoma with estrogen-only HRT and the association of an increased breast cancer rate with combined estrogen and progestin HRT. This review explores the evidence of the effects of HRT on the endometrium and the breast, with emphasis on the pathologic changes.

The story of hormone replacement therapy (HRT) begins in the late 19th century at the Landau clinic in Berlin with the use of extracts of animal ovarian follicles to treat a young woman who had undergone an oophorectomy, with subsequent symptoms of extreme mood swings, hot flushes, and depression. This follicular extract provided relief for her.1 At about the same time, the Merck manual described a tablet called “Ovarin” derived from bovine ovaries for “climacterica.”2

Dr Emil Novak reviewed the state of HRT in 1922 and stated: “incalculable harm has been done by the trashy superficiality, the ignorance, and at times the commercialism associated with the use of ovarian extracts.”3 He encouraged the testing of specific extracts. Following this review, Allen and Doisy4 reported isolation of ovarian extract that caused typical estrus in animals, which was first called amniotin, then called folliculin. The name estrogens for this class of follicular extracts was first used in 1939, which was also the year that Schneider5 reported 500 patients with menopausal symptoms relieved by administration of estrogens. Many formulations became available, including diethylstilbestrol, conjugated equine estrogens (CEE), estrogen patch, and ethinyl estradiol. Some of these products were derived from pregnant horse urine, especially CEE.

The use of HRT was furthered by the lay public and press, with publications ranging from Dr Wilson's 1966 best seller Feminine Forever,6 which portrayed menopausal women as being rejuvenated by HRT, to Marie Jones' hit play Women on the Verge of HRT.7 By the latter part of the 20th century, physicians often recommended long-term HRT. In addition to menopausal symptoms, articles in the medical literature suggested that HRT prevented osteoporosis and reduced coronary-related deaths.

The first major problem with HRT surfaced in the 1970s, when it was shown that endometrial hyperplasia and carcinoma were increased in patients receiving pure estrogen therapy.8,9 In addition, the Coronary Drug Project showed increased risks of blood clot and stroke.10,11 The Federal Drug Administration (FDA) ordered labeling to include these risks. However, investigators subsequently showed that by adding a progestin, the risk of endometrial hyperplasia/carcinoma went back to the baseline level. By the 1990s, there were estimates that up to 50% of postmenopausal women in Western Europe12 and about 35% in the United States13 were on HRT because of the numerous beneficial effects attributed to such therapy.2,14

The second major problem with HRT occurred in 2002, with the publication of the Women's Health Initiative (WHI), which enrolled 161,809 women in 5 studies, including an observational group and 4 clinical trials of postmenopausal women on combined estrogen/progesterone therapy, estrogen-only therapy, low fat diet, or calcium and vitamin D supplementation, respectively.15 The combined estrogen plus progestin therapy was halted because of the increased risk of cardiovascular events among HRT users, increased thromboembolic events, and increased breast cancer risk. Benefits of HRT included reduction of osteoporosis and fracture risk, reduction of colon cancer, and alleviation of postmenopausal symptoms. The relative risk (RR) to benefit ratio of HRT was shifted toward excess risk by this study, and the use of HRT after publication of this trial dramatically declined. A significant minority of postmenopausal women remains on HRT for treatment of menopausal symptoms, osteopenia, or personal preference. The need for safer formulations of HRT is clear, and several trials with lower doses of CEE/medroxyprogesterone acetate (MPA) have been published, establishing the continued endometrial safety and efficacy of lower doses.16 In the 1970s, standard dose of conjugated estrogens was 1.25 mg/d; this dose decreased to 0.625 mg/d in the late 1980s.17 Recent trials are for doses of 0.45 and 0.3 mg/d of CEE.18,19

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The endometrial findings in patients using HRT depend upon the dose, duration, and chemical formulation of hormones, as well as other factors, such as hormone receptors. This discussion will focus on the three most commonly used HRT regimens: estrogen-only HRT, continuous combined (estrogen and progesterone) HRT, and sequential combined (estrogen followed by progesterone) HRT.

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Estrogen-only HRT (ERT)

Estrogen effects on the endometrium include glandular and stromal proliferation, increased vascularization, and edema. Squamous morule formation can be seen, as well as foamy histiocytes in the stroma. The lipid laden histiocytes may represent the site of lipid storage and conversion to sex hormones (Fig. 1).20,21 Continued administration of standard high dose estrogen (0.625 mg/d) for a year leads to endometrial hyperplasia in up to 10% of patients, and up to 62% after three years of usage.22 This is microscopically evidenced by crowding of endometrial glands, which may be irregular in size and shape. Atypical hyperplasia may occur as soon as 4 months after treatment with unopposed estrogens if there is a preexisting simple hyperplasia.22 With discontinuation of estrogen, hyperplasia usually regresses, but some atypical changes may persist. Excess risk for endometrial carcinoma continues for years after completion of HRT.23,24

Figure 1
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Combined HRT (CHRT)

Many of the endometrial biopsies from patients on combined regimens may show mixed proliferative and secretory activity in the first months of therapy. This is typically followed by atrophy with continued therapy. The particular findings are dependent upon the hormone formulations, as well as patient factors.

Most endometria from patients on a sequential estrogen-progesterone regimen show weak secretory features, with glands containing some vacuoles (Fig. 2).25 The changes are typically irregular, and the stroma may show decidual change. A small percentage (15%) displays proliferative activity, with identifiable epithelial mitoses. This proliferation may be due to the relative inadequacy of the progesterone dose to suppress the estrogenic stimulation. If progesterone is given for 9 or more days per cycle, the proliferative activity is less. About 7% to 8% of endometria are inactive.26

Figure 2
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Women undergoing continuous administration of estrogen and progesterone display atrophy or tissue that is insufficient for diagnosis in approximately 50% of endometrial biopsies (presumably representing endometria so thin that there is minimal tissue). Most of the remainder show weak secretory activity, evidenced by focal glandular vacuolization and sometimes partially decidualized stroma. Proliferative activity is rare.26

Metaplasias are commonly seen in patients on HRT, with tubal and squamous metaplasias the most common (Fig. 3), but papillary, eosinophilic, clear cell, and mucinous metaplasia may also be evident.20,21 These findings may sometimes simulate hyperplasia or even carcinoma, but there should not be “back-to-back” glands, stromal response, or necrosis.

Figure 3
Figure 3
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Endometrial Polyps With HRT

The incidence of endometrial polyps is increased in women on HRT, especially with sequential combined therapy. In addition there is a significantly higher incidence of hyperplasia arising in polyps from women on HRT than nonhormone users.27

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In normal cycling endometrium, estrogen drives proliferation. Unopposed estrogenic stimulation eventually leads to hyperplasia of the endometrium. This association was known prior to widespread use of exogenous hormone therapy from experience with patients with polycystic ovary syndrome.28,29 Most contemporary studies use the WHO classification for endometrial hyperplasia, which divides hyperplasias into simple, complex, and atypical.30

The RR of developing endometrial carcinoma in HRT, as compared with women not on HRT, has been reported to be variably elevated with a range of 2.3 to 10. The risk is proportional to the dose of estrogen and the duration of therapy. A large, randomized, double-blinded clinical trial (Postmenopausal Estrogen/Progestin Intervention, PEPI) showed that estrogen treatment markedly increased hyperplasia as compared to placebo.22 The rates of hyperplasia for estrogen-only treatment (0.625 mg/d CEE) were: 21% after 1 year, 45.4% after 2 years, and a remarkable 62.2% after 3 years. This is in contrast to an endometrial hyperplasia rate of only 1.6% rate after three years in women treated by placebo. Considering only complex and atypical hyperplasia, the rate was still 34.4% after three years of estrogen-only treatment.

The addition of progesterone appears to decrease the risk for endometrial neoplasia, as compared with estrogen alone. In sequential combined therapy, the PEPI trial documented a 5.1% rate of hyperplasia, including a 1.7% rate of complex hyperplasia and no cases of atypical hyperplasia. A multicenter UK trial of sequential combined HRT, with a median duration of treatment of 2.5 years, had a 6.1% rate of either complex or atypical hyperplasia.31 This trial also examined continuous combined therapy in the same women following the sequential therapy, and there were no cases of endometrial hyperplasia in 1,196 women completing the study.

In earlier studies of small numbers of patients, the percentage of women developing hyperplasia was as high as 36% after just one year, even with combined therapy.32 However, these data are difficult to interpret, as pathologic terminology has changed since that time, and some of the so-called hyperplasias may be considered “disordered proliferative” or cystic atrophy in modern terms. Also, doses and formulations have changed since the earlier studies. Simple and complex hyperplasias that develop in patients undergoing HRT are usually reversible with progesterone therapy, which inhibits the proliferative stimulus of estrogen. In 34 of 36 patients in the PEPI trial, there was no evidence of hyperplasia in repeat biopsies after the administration of 10 mg/d of MPA given for 3 months.

Two recent large trials of HRT versus placebo, the Heart and Estrogen/Progestin Replacement Study (HERS)33,34 and the WHI study found no increase in endometrial carcinoma with combined HRT.15

In the event that endometrial carcinoma does develop with HRT, the tumors usually arise in a background of endometrial hyperplasia. The carcinomas are typically well differentiated (FIGO grade 1) endometrioid adenocarcinomas with minimal myometrial invasion (Fig. 4).35 This form of carcinoma has been called type I, in contrast to type II endometrial carcinoma which shows high-grade nuclear features and is not associated with estrogen stimulation (serous-papillary and clear cell histology).25

Figure 4
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Estrogen stimulation alone probably does not induce carcinoma, but the proliferative activity induced by estrogen may increase the possibility of errors in DNA transcription. Additional cofactors may be required as well. Molecular studies of the type I endometrial carcinomas have demonstrated microsatellite instability and K-ras mutations, as well as mutations of the PTEN gene.36,37 Type II serous endometrial carcinomas are more often associated with p53 mutations.25,38

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Response to Estrogen and Progesterone in the Normal Breast

Mammary duct development is driven by estrogen, whereas differentiation of the lobules is driven by progesterone. Full differentiation of the breast occurs during pregnancy and lactation (so-called type 3 and 4 lobules39). In turn, the effects of estrogen and progesterone are mediated by nuclear receptors. In normal human breast, both estrogen receptors (ERs) and progesterone receptors (PRs) are expressed by 15% to 30% of luminal epithelial cells.40,41 The number of ER-positive cells is higher during the follicular phase of the menstrual cycle and decreases in the luteal phase, whereas the number of PR-positive cells stays relatively constant. In postmenopausal breast, the number of ER-positive cells within lobules is increased to about 50% in the absence of hormone therapy.42

In animal studies, long-term estrogen or combination estrogen/progesterone therapy increases cell proliferation and the percentage of glandular tissue in the breast.43–45 Tissue culture shows proliferative effects of estradiol as well.46 Human studies are sparse,47,48 and the evidence of increased glandular tissue is mostly indirect, using increased mammographic density as an indicator.49–51 Mammographic density is increased in 11% to 73% of HRT users compared with nonusers or with pretreatment films of users. Increases in density mostly occurred within the first year and were higher with the use of CHRT than with ERT.

In one study, proliferative rate, as measured by Ki67 and proliferating cell nuclear antigen, was higher in CHRT users than nonusers, as was % epithelial area, measured by morphometry.47 However, another study did not show an increase in proliferative rate by Ki67 with either ERT or CHRT although the treatment period with HRT was less.52 The pathology of the breast with HRT has not been well described. Increased proliferative fibrocystic changes with HRT may be present.53

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The multiple studies of the relationship between breast cancer and HRT have been predominantly epidemiologic. Meta-analysis of 51 epidemiologic studies by the Collaborative Group on Hormonal Factors in Breast Cancer estimated the RR of breast cancer to be increased by 2.3% per year of HRT use, and 5.8% per year of CHRT use.54–56 This is a RR of 1.35. With cessation of treatment, the excess risk is normalized in 5 to 10 years. Subsequently published observational trials also found similar RRs: the British Million Women Study, RR 1.6657; the Australian Melbourne Collaborative Cohort Study, hazard ratio 1.5158; the Nurses Health Study, 1.42 (for >20 y of hormone use).59 Likewise, the WHI clinical trial showed a hazard ratio of 1.26 for CHRT.15

To put the increase in RR of breast cancer with CHRT in perspective, a similar RR of 1.1 to 2.0 for breast cancer is found with these factors: menarche before age 12, high socioeconomic status, nulliparity, never nursing, first full-term pregnancy after age 30, alcohol consumption, and usual type ductal hyperplasia in a breast biopsy.

The WHI study included a CHRT clinical trial of 16,608 healthy postmenopausal women randomly assigned to either CHRT or placebo, originally planned for 8.5 years. This trial was halted in 2002, 3 years before its scheduled completion because of an increased risk benefit ratio, as evidenced by the increased incidence of strokes and deep venous thrombosis, and an increase in breast cancer.

The estrogen-only versus no treatment clinical trial of the WHI study included 10,739 postmenopausal women with prior hysterectomy. This trial was continued until 2004 with a mean follow-up of 6.8 years and found the relative incidence of breast cancer to be 0.77, which was lower than expected, though the risk of stroke was increased.60

Following WHI's lead, the Women's International Study of long Duration Oestrogens after Menopause (WISDOM) trial, involving 22,000 women in the United Kingdom, Australia, and New Zealand also stopped prematurely for “practical and economic reasons.”61 The Swedish trial Hormonal Replacement Therapy After Breast Cancer-Is it Safe? (HABITS) was halted in 2004 because of the high number of recurrent or new breast cancers in the HRT arm: 26/174 versus 7/171 in the placebo arm.62

The Nurses' Health Study (NHS) is an observational study of 121,700 women that has now been followed for 30 years, and analysis of estrogen use for less than 20 years showed no statistically significant increase in breast cancer rates. With longer than 20 years use, there was a linear increase in ER+/PR+ carcinomas in leaner women.59

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The CHRT WHI trial found 8 additional invasive breast carcinomas per 10,000 women annually, an absolute increase in risk of 0.08%. The average age at screening in the WHI trial was 63; in younger perimenopausal women taking HRT for relief of menopausal symptoms, the absolute risk may be even less.

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The increased number of breast cancer with HRT may be due either to detection of preexisting small breast cancers growing more rapidly under HRT stimulation or to de novo tumors caused by increased mutations. In terms of tumor growth, it is estimated that it would take 5 to 10 years for a malignant cell to reach a clinically detectable tumor size of 1 cm.63 This estimation does not take apoptosis into account and assumes that every cell survives. Thus the increase in breast cancers seen in such studies as the WHI, which had only a median follow-up time of 5.2 years, may not be detecting breast cancers that were directly caused by HRT, but more likely detecting tumors growing more rapidly under the influence of HRT. The breast carcinomas that were recorded by the NHS, which were increased only after 20 years, are more likely to be de novo tumors.

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Similar to endometrial carcinomas, the breast cancers that develop in women on prolonged HRT tend to have an overall favorable prognosis. A higher proportion of lobular carcinomas is present in patients on HRT (Table 1), particularly in patients who are taking combined estrogen and progestin therapy.64–71 This finding is fairly consistent in most studies; moreover, the increase incidence of lobular carcinomas since the 1970s mirrors the increase in HRT use.72 These studies are epidemiologic studies that mainly consider only ductal (the wastebasket category) and lobular subtypes of breast carcinomas. These are coded diagnoses, not pathologically reviewed. An updated study of histologic types, which combines data from a previously published study,66 breaks out another histologic type, those tumors with ductal and lobular features, which appear increased (Fig. 5).70 The authors also note that tubular and comedo carcinomas are increased in current users of combined estrogen and progestin therapy.70 Other tumor parameters are more controversial, but tumors are generally reported to be better differentiated, of lower stage, and the patients have a lower overall death rate. These findings may simply reflect the relatively low grade of lobular carcinomas, which are generally ER/PR positive, and have low nuclear and histologic grades. The WHI trial showed an earlier appearance of larger tumors with more positive nodes, perhaps indicative of growth stimulation of preexistent tumors.73 Most breast cancers after HRT are positive for both ER and PR. Tumors may be detected at a smaller size due to more frequent clinical examinations in patients on HRT.

Figure 5
Figure 5
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Table 1
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The standard dose of CEE was 0.625 mg/d in the WHI study, and doses of 0.45 and 0.3 mg have been tested in a large trial. These doses were given alone or in combination with MPA of 2.5 (standard dose) or 1.5 mg/d. Treatment with the lower doses was clinically efficacious in providing vasomotor relief16 and protection against loss of mineral density and total mineral content in bone.74 After 2 years, the CEE/MPA groups had no cases of endometrial hyperplasia, including all dose levels. The CEE alone groups showed significant increases in the rate of hyperplasia, from 3% to 27%, with increasing rates in higher doses.18,19

Besides CEE, 17β-estradiol is the other commonly used estrogen compound that is FDA approved in the United States. Other formulations of estrogens besides oral tablets include transdermal patches, creams, gels, topical emulsions, and vaginal rings of estrogen compounds.2 These compounds most likely have comparable rates both of efficacy and of adverse effects.75

Nonhormonal agents have been tested for vasomotor and other symptomatic relief of menopausal symptoms,76,77 and ongoing testing is occurring for synthetic hormones such as bazedoxifene, a selective ER modulator. Tibolone, which has been used in Europe for decades,78 has not been approved by the FDA for use in the United States. Calcium supplements, raloxifene, biphosphonates, parathyroid hormone, and many others may be of assistance in decreasing the osteoporosis that accompanies menopause.79

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A consensus panel judged that it is appropriate to use standard HRT in women with intolerable menopausal symptoms and standard risks of cardiovascular disease, venous thromboembolism, and stroke.80 In other clinical scenarios, in which there is an elevated cardiovascular disease or venous thromboembolism risk, consensus panelists felt either uncertain or that it would be inappropriate to use HRT. The FDA recommends the shortest interval of time for HRT use possible, at the lowest possible dosage. The North American Menopause Society and the American College of Obstetricians and Gynecologists both recommend treating with the lowest effective dose and the shortest time consistent with treatment goals for HRT use.81,82

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HRT has been in use for over 100 years and has a storied past. Currently, HRT is still the most efficacious method of relieving many perimenopausal and postmenopausal symptoms, such as hot flushes and osteoporosis. Undesirable effects of estrogen-only HRT include a higher incidence of endometrial hyperplasia and well-differentiated carcinoma. By adding a progestin, the endometrial carcinoma rate is brought to baseline level; however, progestin may modestly increase the incidence of breast carcinoma, particularly lobular carcinoma. Currently available HRT formulations include lower dosages of estrogen and progesterone used in the past, and new drugs are being investigated for relief of menopausal symptoms.

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The author thanks her colleagues, Dr Francis E. Sharkey, who read this paper and made helpful comments, and Dr Philip Valente, who allowed her to borrow some glass slides to photograph.

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postmenopausal; hormone replacement therapy; endometrium; breast

© 2007 Lippincott Williams & Wilkins, Inc.


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