What is Estrone?
Estrone (E1) is a weaker estrogen hormone that becomes the predominant circulating estrogen after menopause, and it plays a complex role in hormone replacement therapy—particularly because oral estrogen administration dramatically increases estrone levels (up to 51-fold) compared to transdermal routes, with potentially important clinical implications for breast tissue stimulation and therapeutic effects.
Biochemistry and Physiologic Role
Estrone is one of three main estrogens in the body, alongside estradiol (E2) and estriol. In premenopausal women, estradiol is the dominant and most potent estrogen. After menopause, estrone becomes the primary circulating estrogen as ovarian estradiol production ceases 1.
Key Characteristics:
- Weaker estrogenic activity compared to estradiol
- May act as a partial estradiol antagonist at high concentrations, potentially interfering with estradiol's effects at the estrogen receptor 2
- Interconvertible with estradiol through 17β-hydroxysteroid dehydrogenase (HSD17B) enzymes
Impact of Route of Administration
The route of estrogen administration profoundly affects estrone levels:
Oral Estrogen
- Produces 10-fold higher estrone levels and 10- to 20-fold higher estrone sulfate compared to transdermal administration at comparable doses 1
- Creates an estrone/estradiol (E1/E2) ratio of approximately 9.28 for oral estradiol 3
- After 3 years of oral micronized estradiol 1 mg daily, estrone levels increase 10-fold and estrone sulfate increases 51-fold from baseline 4
Transdermal Estrogen
- Results in E1/E2 ratio of approximately 2.22, much closer to physiologic ratios 3
- Avoids first-pass hepatic metabolism, producing lower estrone levels 1
- The 1.25g transdermal gel produces median estrone levels of 41-49 pg/mL, within normal postmenopausal range 5
Injectable Estrogen
- Produces the lowest E1/E2 ratio of 0.84 3
- Results in markedly higher estradiol levels (mean 1557 pmol/L) with relatively lower estrone 3
Clinical Significance in Hormone Therapy
Breast Tissue Effects
Increases in estrone sulfate levels during menopausal hormone therapy are significantly associated with increased mammographic density (+1.3% increase in density per 1 ng/mL increase in estrone sulfate, P<0.0001) 6. This relationship is important because:
- Increased mammographic density is a marker of breast tissue stimulation
- The biological effects of markedly elevated estrone and estrone sulfate from oral therapy remain incompletely understood 1
- At high concentrations, estrone may act as a partial estradiol antagonist in breast tissue 2
Bone Protection
Estrone sulfate at doses of 0.625 mg daily combined with 1000 mg calcium effectively prevents postmenopausal bone loss, producing a +1.9% increase in spinal bone mineral density after 12 months 7.
Cardiovascular and Other Effects
The guideline evidence shows that combined estrogen-progestin therapy (conjugated equine estrogen 0.625 mg/d plus medroxyprogesterone acetate 2.5 mg/d) does not benefit coronary heart disease and likely increases risk, while also increasing stroke, dementia, gallbladder disease, and urinary incontinence 8.
Clinical Implications
When Estrone Levels Matter:
- Monitoring is generally not recommended for routine hormone therapy 1
- Consider measuring when symptoms don't improve with escalating doses, particularly after changing from oral to transdermal administration 1
- In younger women with premature ovarian insufficiency or surgical menopause, who require higher baseline estradiol levels, monitoring may be more relevant 1
Route Selection Considerations:
Transdermal estrogen is preferable for women with venous thromboembolism risk, hypertension, hypertriglyceridemia, obesity, metabolic syndrome, diabetes, or history of gallbladder disease 1, in part because it avoids the dramatic estrone elevation and first-pass hepatic effects of oral administration.
Common Pitfalls:
- Assuming all estrogen formulations produce similar estrone levels—they don't
- Not recognizing that oral estrogen's high estrone production may interfere with estradiol effects through partial antagonism
- Measuring estrone levels routinely when clinical response is adequate
The biological significance of supraphysiologic estrone levels from oral therapy remains an area requiring further research, particularly regarding long-term breast cancer risk and therapeutic efficacy 1, 2.