Hormone Replacement Therapy (HRT) - Comprehensive Overview

1. Definition and Background

• Medical Physiology (Boron & Boulpaep), p. 1185

2. Hormones Used

Estrogens

Progestogens

• Lippincott Illustrated Reviews: Pharmacology, p. 833

3. Mechanism of Action

Mechanism of action - Lippincott Illustrated Reviews: Pharmacology

• Lippincott Illustrated Reviews: Pharmacology, p. 832

4. Routes of Administration

5. Indications

Primary Indication

• Vasomotor symptoms (hot flashes/flushes, night sweats) - the most effective treatment available. Estrogen treatment reestablishes negative feedback on hypothalamic control of norepinephrine secretion, reducing the frequency and severity of hot flashes.

Other Indications

• Genitourinary syndrome of menopause (GSM): vaginal atrophy, dryness, dyspareunia, recurrent UTIs, urgency
• Premature ovarian insufficiency (POI) / surgical menopause before age 40-45: HRT is strongly indicated to replace hormones until the natural age of menopause; risk-benefit ratio is more favorable
• Prevention of osteoporosis: reduces bone resorption and fracture risk (hip, vertebral); however, bisphosphonates are now preferred as first-line; HRT may be considered in women with coexisting severe vasomotor symptoms
• Mood/psychological symptoms during perimenopause: improved mood, sexual function
• Female hypogonadism/Turner syndrome/gonadal dysgenesis: HRT for feminization and maintenance of secondary sexual characteristics

System-level benefits of estrogen therapy - Lippincott Illustrated Reviews: Pharmacology

• Lippincott Illustrated Reviews: Pharmacology, p. 833-834
• Guyton & Hall Medical Physiology, p. 1028

6. Regimens

Combined HRT (Estrogen + Progestogen)

• Cyclic (sequential): Estrogen daily + progestogen for 12-14 days/month → scheduled withdrawal bleed
• Continuous combined: Estrogen + progestogen daily → aims for amenorrhea (common choice >12 months post-menopause; unexpected bleeding at any time warrants evaluation)

Estrogen-only HRT

• Women who have had a hysterectomy (no uterus = no endometrial cancer risk from unopposed estrogen)

Local (topical/vaginal) estrogen

• Low-dose; treats urogenital symptoms only; minimal systemic absorption with low-dose preparations; generally does not require progestogen

7. Benefits Summary

8. Risks and Adverse Effects

Breast Cancer

• Combined estrogen + progestogen HRT (especially MPA) increases breast cancer risk with prolonged use (>5 years). The risk appears more modest with micronized progesterone and shorter durations of use. Estrogen-alone therapy (in hysterectomized women) has a less clear association - the WHI estrogen-alone arm actually showed a possible decrease in breast cancer incidence.
• Bailey & Love's Surgery, p. 1075

Cardiovascular Disease

• The Women's Health Initiative (WHI) trial was pivotal:
  • CEE + MPA (combined arm): increased risk of MI and CVD events, especially in the first year of use; also increased risk of ischemic stroke
  • CEE alone (hysterectomized women): neither increased nor decreased CVD risk; but increased stroke risk
  • Younger women (50-59) with prior bilateral oophorectomy: a >30% reduction in all-cause mortality with CEE; older women did not see this benefit
• The KEEPS trial (Kronos Early Estrogen Prevention Study): starting HRT shortly after menopause does not reduce atherosclerotic progression or CVD outcomes, but does significantly improve vasomotor symptoms, mood, sexual function, and bone density - especially when started soon after menopause
• The "timing hypothesis" or "window of opportunity": HRT initiated before age 60 or within 10 years of menopause is more likely to have a neutral or favorable cardiovascular profile; initiating later (>10 years post-menopause) may worsen CVD
• Harrison's Principles of Internal Medicine 22e, p. 3208

Venous Thromboembolism (VTE)

• Oral estrogen increases DVT and PE risk by increasing hepatic synthesis of procoagulant factors
• Transdermal estrogen carries a significantly lower (possibly negligible) VTE risk compared to oral - this is a key reason to prefer transdermal delivery in women at elevated VTE risk
• Coexisting thrombophilias (Factor V Leiden, antiphospholipid syndrome) further amplify VTE risk with oral HRT
• A 2025 Climacteric systematic review (PMID: 40488293) confirmed that transdermal HRT appears safer than oral in women at higher VTE risk.

Endometrial Cancer

• Unopposed estrogen significantly increases the risk of endometrial hyperplasia and carcinoma (relative risk 2-10x depending on duration and dose)
• Adding a progestogen (for at least 12-14 days/cycle) eliminates this excess risk
• Unexpected bleeding with continuous combined HRT or recurrence of bleeding after amenorrhea is established requires evaluation (endometrial biopsy/ultrasound)

Other Adverse Effects

• Breast tenderness, bloating, irregular uterine bleeding (especially in first 3-6 months)
• Nausea (oral preparations)
• Headache, mood changes
• Fluid retention
• Possible increased risk of gallbladder disease (oral route)

Absolute Contraindications

• Unexplained abnormal uterine bleeding
• Known or suspected estrogen-sensitive cancers (breast, endometrial)
• Active or recent thromboembolic disease (DVT, PE, stroke, MI)
• Active liver disease with impaired function
• Known thrombophilic disorders (relative; transdermal may be considered with specialist input)
• Pregnancy

9. The "Timing Hypothesis" (Window of Opportunity)

• Initiation before age 60 or within 10 years of menopause: Evidence suggests favorable or neutral cardiovascular effects; vasomotor, skeletal, and quality-of-life benefits predominate
• Initiation after age 60 or >10 years post-menopause: Cardiovascular and VTE risks increase; arterial plaques may already be established, and estrogen may destabilize rather than prevent atherosclerosis
• Guyton & Hall Medical Physiology, p. 1028

10. Special Populations

Premature Ovarian Insufficiency (POI, menopause before age 40)

• HRT is strongly recommended until at least age 51, regardless of VTE/breast cancer risk concerns applicable to older women - the risks of long-term estrogen deficiency (osteoporosis, CVD, cognitive decline) outweigh the risks of HRT in this group

Women with Hysterectomy

• Estrogen-only HRT preferred; no progestogen needed; slightly different risk profile (no endometrial cancer risk; possibly reduced breast cancer risk vs. combined HRT)

Surgical Menopause (bilateral oophorectomy)

• Acute, severe estrogen deficiency; HRT indication is strong especially before natural age of menopause

Breast Cancer Survivors

• HRT is generally contraindicated in hormone receptor-positive breast cancer survivors; non-hormonal alternatives for vasomotor symptoms are used (SSNRIs, gabapentin, clonidine)

11. Selective Estrogen Receptor Modulators (SERMs)

• Medical Physiology (Boron & Boulpaep), p. 1186

12. Non-Hormonal Alternatives for Vasomotor Symptoms

• SNRIs (venlafaxine, desvenlafaxine): first-line non-hormonal options
• SSRIs (paroxetine - FDA-approved for hot flashes as Brisdelle; escitalopram)
• Gabapentin/pregabalin: effective, especially for nocturnal symptoms
• Clonidine: modest efficacy; side effect profile limits use
• Oxybutynin: emerging evidence
• Fezolinetant (Veozah): NK3 receptor antagonist; FDA-approved 2023 specifically for vasomotor symptoms; non-hormonal; targets the hypothalamic KNDy neuron pathway directly

13. Current Recommendations

1. Individualize therapy - no blanket for or against recommendation; risks and benefits differ based on age, time since menopause, type of menopause (natural vs. surgical), symptom severity, and personal risk factors
2. Lowest effective dose for the shortest duration needed to control symptoms
3. Prefer transdermal over oral estrogen in women with elevated VTE risk, hypertriglyceridemia, or history of migraine with aura
4. Add progestogen in all women with an intact uterus
5. Vaginal estrogen for isolated urogenital symptoms, with minimal systemic risk
6. Annual review of continued need, dose, and route
7. HRT is not routinely recommended for primary or secondary cardiovascular prevention

• Guyton & Hall Medical Physiology, p. 1028
• Lippincott Illustrated Reviews: Pharmacology, p. 833-834
• Harrison's Principles of Internal Medicine 22e, p. 3208

Recent Evidence Highlights (2024-2025)