Estrogen — what it is, what it does, and what happens when it falls
Estrogen is not one hormone — it is three, with very different properties and roles. Understanding the difference between estradiol, estrone, and estriol changes how you understand your symptoms, your lab results, and your treatment options.
Rose
"When I first had my hormones tested, my estrogen came back as a single number and I had no idea what it meant — or which estrogen had been measured, or whether it was estradiol that matters or some other form. The three-estrogen picture is one of those things that takes ten minutes to understand and changes how you read everything else. This page is those ten minutes."
Key takeaways
✓There are three estrogens: estradiol (E2), estrone (E1), and estriol (E3) — they have very different potencies and roles
✓Estradiol is the one that matters — it is the active estrogen of the reproductive years and the one that falls at menopause
✓The estrogen in systemic HRT is estradiol — always check this on your prescription
✓Estrone becomes the dominant estrogen after menopause — produced in body fat, significantly weaker than estradiol
✓Estriol is the pregnancy estrogen — very weak systemically, but effective locally for vaginal tissue (Ovestin)
✓Transdermal estradiol (patch or gel) bypasses the liver and has a lower clot risk than oral estrogen — this is the preferred modern formulation
✓The conjugated equine estrogens (Premarin) used in the 2002 WHI study are not the same as body-identical estradiol
The three estrogens — E1, E2, E3
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Estradiol (E2)
The primary active estrogen — the one that matters most
Estradiol is the most potent and biologically active estrogen. It is the dominant estrogen during the reproductive years, produced primarily by the ovaries. It drives almost every function we attribute to estrogen — bone protection, cardiovascular health, brain function, skin, libido, temperature regulation, and vaginal health. When doctors say "estrogen" in the context of HRT, they mean estradiol. When ovarian function declines at menopause, it is estradiol that falls.
Key facts
• The estrogen in all systemic HRT formulations is estradiol
• Measured as E2 in blood tests — the most clinically relevant estrogen measurement
• Falls significantly in perimenopause and postmenopause
• Optimal range in premenopause: 100-400 pmol/L depending on cycle phase
• Postmenopause without HRT: typically below 100 pmol/L
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Estrone (E1)
The postmenopausal estrogen — weaker, produced in fat
Estrone becomes the dominant circulating estrogen after menopause, produced by aromatase enzymes in body fat rather than by the ovaries. It is significantly weaker than estradiol — roughly one third of its potency. This is why postmenopausal women with more body fat have somewhat higher estrogen activity than lean women, and why extreme leanness in postmenopause can accelerate bone loss. Estrone has a complex relationship with breast cancer risk — higher estrone levels in postmenopause are associated with modestly increased risk.
Key facts
• Dominant estrogen in postmenopause
• Produced by aromatase in fat tissue, adrenal glands, and liver
• About one third the potency of estradiol
• Higher in overweight and obese postmenopausal women
• Can be converted to estradiol and back — the body uses the forms interchangeably
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Estriol (E3)
The pregnancy estrogen — weak but locally important
Estriol is produced in large amounts during pregnancy by the placenta — it is the dominant estrogen of pregnancy. In non-pregnant women, it is produced in small amounts as a metabolite of estradiol and estrone. It has very weak systemic activity but strong local activity on vaginal and urinary tract tissue — which is why estriol is used in some local vaginal preparations (Ovestin) for genitourinary syndrome of menopause. It has negligible effect on the uterine lining, bone, or cardiovascular system at the doses used.
Key facts
• Dominant estrogen of pregnancy
• Very weak systemic activity
• Strong local activity on vaginal and urinary tissue
• Used in Ovestin cream for vaginal dryness
• Does not protect bone, brain, or cardiovascular system at typical doses
What estrogen does — nine functions
All of these are estradiol functions — which is why its loss at menopause affects so many systems simultaneously.
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Bone density
Estrogen is the primary hormone protecting bone density in women. It inhibits osteoclasts — the cells that break down bone — and promotes osteoblast activity. The rapid bone loss in the first 5 years after menopause is directly caused by falling estradiol.
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Cardiovascular health
Estrogen protects the cardiovascular system through multiple mechanisms: it maintains arterial elasticity, raises HDL cholesterol, lowers LDL, reduces inflammatory markers, and supports endothelial function. Its loss at menopause is a primary driver of the cardiovascular risk increase in postmenopausal women.
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Brain and cognitive function
Estradiol is neuroprotective — it supports neuronal health, promotes BDNF (brain-derived neurotrophic factor), reduces neuroinflammation, and supports the acetylcholine system involved in memory. Its decline contributes to brain fog, cognitive changes, and is implicated in increased dementia risk in postmenopause.
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Temperature regulation
Estrogen stabilises the hypothalamic thermostat. As it fluctuates and falls in perimenopause, the thermostat becomes hypersensitive — triggering the heat-dissipation responses (hot flashes, night sweats) at lower temperature thresholds than before.
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Sleep architecture
Estrogen influences sleep quality — particularly REM sleep and slow-wave sleep. Its fluctuation in perimenopause disrupts sleep architecture directly, independently of the hot flashes it causes.
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Vaginal and urinary health
Estrogen receptors line the vaginal wall, urethra, and bladder. Falling estrogen causes the tissue thinning, dryness, reduced lubrication, and urinary symptoms of Genitourinary Syndrome of Menopause (GSM).
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Skin and collagen
Estrogen stimulates collagen production and maintains skin thickness, elasticity, and moisture. Skin loses 30% of its collagen in the first 5 years after menopause — directly driven by falling estradiol.
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Serotonin and mood
Estrogen stimulates serotonin synthesis, upregulates serotonin receptors, and modulates dopamine and noradrenaline. Its fluctuation in perimenopause drives the mood instability, anxiety, and depression that many women experience as their first significant symptoms.
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Metabolic function
Estrogen regulates insulin sensitivity, fat distribution, and metabolic rate. Its loss shifts fat storage toward the abdomen, reduces insulin sensitivity, and lowers resting metabolic rate — the constellation of metabolic changes that make weight management harder in perimenopause.
What low estradiol feels like — the symptom list
•Hot flashes and night sweats — the most recognised symptom of low estradiol
•Vaginal dryness, thinning, and discomfort — GSM is entirely driven by local estrogen loss
•Sleep disruption — both from night sweats and direct effects on sleep architecture
•Brain fog, memory changes, and reduced cognitive speed
•Mood instability, anxiety, and depression
•Accelerated bone loss — silent, measurable on DEXA scan
•Joint pain and stiffness — estrogen has anti-inflammatory effects
•Skin thinning and dryness
•Hair thinning — estrogen supports hair follicle health
•Urinary urgency, frequency, and recurrent UTIs
•Reduced libido — partly via serotonin and dopamine effects
•Weight gain and metabolic shift toward abdominal fat
•Palpitations — estrogen affects the cardiovascular autonomic system
•Headaches and migraines — often triggered by estrogen fluctuation in perimenopause
Understanding your estrogen blood test
What to ask for — and what the numbers mean
When you ask for your estrogen to be tested, ask specifically for estradiol (E2) — not just "estrogen." Many labs default to measuring estrone, which is clinically less meaningful for perimenopausal women.
Estradiol fluctuates enormously across the menstrual cycle — it peaks around ovulation and drops before menstruation. A single reading gives limited information. In perimenopause, FSH (follicle-stimulating hormone) is often more useful as a trend — rising FSH over time is more indicative of the transition than any single estradiol reading.
Reference ranges — context matters
Follicular phase (days 1-14)77–921 pmol/LWide range — varies significantly
Around ovulationUp to 1,469 pmol/LPeaks here
Luteal phase (days 14-28)77–1,145 pmol/LFalls toward end of phase
Postmenopause (no HRT)Below 183 pmol/LTypically below 100
On HRT (target range)200–600 pmol/LSymptomatic relief usually in this range
On HRT, the target is symptom control rather than a specific number. Many women feel well with estradiol in the 200-400 pmol/L range. If symptoms persist on a given dose, a higher dose or different formulation may be needed. See the lab guide for the full biomarker picture.
Estrogen in HRT — what you are actually getting
Estradiol — transdermal (patch or gel)
Preferred formulation
The gold-standard delivery method. Estradiol absorbed through the skin enters the bloodstream directly, bypassing the liver. This avoids the first-pass liver effect that increases clotting factor production with oral estrogen.
Lower VTE (clot) risk, more stable blood levels, suitable for women with migraines or liver conditions. Current NICE and BMS preferred formulation.
Oral estradiol is metabolised through the liver on first pass, activating clotting factors. Effective but carries higher VTE risk than transdermal. Still widely prescribed but generally no longer first-line for new starts.
Higher VTE risk, more variable blood levels. Reasonable for women without clot risk who prefer tablets.
Conjugated equine estrogens (Premarin)
Derived from pregnant horse urine. Contains a mixture of estrogen compounds — not the same molecular profile as human estrogens. The estrogen used in the 2002 WHI study alongside MPA progestin. Still used but no longer the preferred formulation.
Not body-identical. Oral route with liver first-pass effects. Modern guidelines prefer estradiol.
Estriol — local vaginal (Ovestin)
Preferred formulation
Local vaginal estriol treats Genitourinary Syndrome of Menopause with minimal systemic absorption. Highly effective for vaginal dryness, pain, and urinary symptoms. Safe for most women including many with breast cancer history.
Addresses local tissue — does not protect bone or cardiovascular system. Can be used alongside or instead of systemic HRT for vaginal symptoms.
The estrogen-progesterone balance
Why estrogen alone is not enough (if you have a uterus)
Estrogen stimulates the growth of the uterine lining (endometrium). Without progesterone to regulate and shed that lining, it continues to thicken — a condition called endometrial hyperplasia that can progress to endometrial cancer.
This is why women with a uterus must take a progestogen alongside estrogen in HRT. The progestogen protects the endometrium. Women who have had a hysterectomy can take estrogen alone — no progestogen required.
The choice of progestogen matters — body-identical micronised progesterone has the best safety profile. See the progesterone guide and the HRT types guide for the full picture.
Rose on this
"Estrogen is the word that comes up in every menopause conversation — but most women have never been told which estrogen, in what form, at what dose, or why the delivery route matters. The difference between transdermal estradiol and the conjugated equine estrogens in the 2002 WHI study is not a small thing. It is the difference between a modern, safe, body-identical hormone and the formulation that generated twenty years of fear. You deserve to know the difference."
From Rose
"Understanding your hormones — what each one does, which ones are falling, what replacing them means — is not medical overreach. It is basic self-knowledge. Estrogen built your bones, protected your heart, supported your brain, and maintained your skin for decades. Understanding what happened when it started to fall is the beginning of being able to do something about it."
What we do not know yet
?The precise optimal estradiol level for different women on HRT — symptom control is currently used as the guide, but individualised hormonal targets are not yet established in guidelines
?Whether the timing hypothesis for cardiovascular and cognitive protection is specific to estradiol or applies to estrone replacement — almost all research is on estradiol
?The long-term effects of the estrogen exposure from aromatase activity in body fat — the relationship between postmenopausal estrone levels and breast cancer risk is complex and not fully characterised
Written by
Rose
Navigating perimenopause · Researcher · Founded rosemyfriend.com
Research basis
PubMed · Cochrane reviews · NICE guidelines · British Menopause Society · The Menopause Society
Read methodology →
Rose provides evidence-graded educational information — not medical advice. Always discuss health decisions with a qualified healthcare provider.
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