Estrone and Estradiol Are Not One Number

1. Background

Estrone and estradiol are the two principal circulating estrogens in humans. Conventional “estrogen” testing reports estradiol alone — historically by immunoassay — which quietly equates one estrogen with total estrogen status.

That equivalence does not hold: E1 and E2 differ in potency and origin, they interconvert, and the balance between them changes with reproductive stage and with how estrogen is administered.

This review sets out the case for measuring both estrogens and interpreting their ratio — and states plainly, in §6, the concentrations at which Clinicore’s assay can and cannot answer these questions.

2. Two estrogens, one interconverting pool

Aromatase converts androstenedione to estrone and testosterone to estradiol. The two estrogens are then interconverted by 17β-hydroxysteroid dehydrogenase: reduction favours E2, oxidation favours E1.

Estrone is additionally stored as estrone sulfate — the most abundant circulating estrogen — through the opposing actions of sulfotransferase and steroid sulfatase, forming a deep reservoir that can regenerate active estrogen.^1,2

Functionally, E2 is the potent, high-affinity ligand for estrogen receptors ERα and ERβ, whereas E1 is a comparatively weak estrogen that behaves as a convertible reservoir.

The clinical corollary: E2 reports potent estrogenic drive, while E1 reports the peripheral, reservoir arm of estrogen supply.

3. The ratio inverts across the menopausal transition

Before menopause, ovarian granulosa cells secrete estradiol as the dominant estrogen, and E2 exceeds E1.

After menopause, ovarian estradiol output falls and the dominant source becomes peripheral aromatisation of androgens in adipose and other tissues — which yields estrone. The balance inverts, and E1 becomes the dominant circulating estrogen.

Ultrasensitive LC-MS/MS reference ranges illustrate the postmenopausal pattern: estradiol on the order of 3.8–36 pmol/L against estrone of roughly 22–122 pmol/L.³

A panel reporting a single estrogen cannot represent this stage-dependent shift. Note that the estradiol concentrations in that postmenopausal range sit below Clinicore’s reportable limit — see §6.

4. The ratio shifts with the route of estrogen therapy

The E2:E1 ratio also reports how estrogen is delivered.

Oral estradiol undergoes extensive first-pass hepatic metabolism, a substantial fraction converted to estrone, which lowers the circulating E2:E1 ratio. Non-oral routes preserve a more physiologic ratio. A direct pharmacokinetic comparison found a higher E2-to-E1 ratio with sublingual than with oral 17β-estradiol — approximately 1.1 versus 0.7⁴ — and oral administration raises estrone and its metabolites relative to transdermal delivery.⁵

Because the ratio encodes the route, it can corroborate the prescribed preparation, flag non-adherence, and inform whether a physiologic estrogen profile is being achieved in menopausal hormone therapy or gender-affirming therapy.

This is the application Clinicore’s assay serves best, because therapeutic estradiol concentrations sit well inside the measuring interval.

5. Applications favouring measurement of both estrogens

5.1 Menopausal status and estrogen-therapy monitoring

The E2:E1 ratio helps confirm menopausal transition and reflects the route of administration, supporting individualised adjustment of hormone therapy.^3,4 A single estrogen value cannot distinguish an estrone-dominant postmenopausal profile from an oral-therapy profile; the pair can.

5.2 Ovarian function recovery during aromatase-inhibitor therapy

In younger women treated with an aromatase inhibitor, undetected ovarian reactivation compromises treatment. Estrone may be the more sensitive marker of ovarian function recovery.^6,3

This application requires an ultrasensitive estrogen assay and is outside what Clinicore’s method can support. The concentrations produced during estrogen suppression fall below the reportable limit. It is described here because it is part of the published physiology; it is not an application of this laboratory’s panel. See §6.

5.3 Estrogen-related cancer risk context

In postmenopausal women, higher estradiol and estrone — including the estrone-sulfate reservoir — are associated with breast-cancer risk in pooled and umbrella analyses.^7,8 Characterising both estrogens supports a more complete picture than estradiol alone.

5.4 Aromatisation and estrogen in men

Estrogens circulate at low but physiologically meaningful concentrations in men, generated by aromatisation, where estradiol contributes to bone, vascular and metabolic physiology. Quantifying both estrogens characterises aromatisation status in ranges where immunoassay is unreliable.⁹

6. Analytical considerations, and the limits of this assay

The concentrations that matter most clinically are low. Estrogens commonly fall below 20 pg/mL in postmenopausal women and in men, and far lower under aromatase-inhibitor therapy — ranges in which conventional immunoassays lack the specificity and accuracy to be trusted.^1,9

Published ultrasensitive LC-MS/MS methods achieve sub-picogram-per-millilitre limits of quantitation and measure estrone concurrently.³

What Clinicore’s assay does, and does not, reach

Clinicore reports estradiol and estrone from 10.125 pg/mL. That is a deliberate production limit — the concentration the method reproduces reliably at full daily throughput, not the instrument’s demonstrated floor.

Suitable on this assay:

• Premenopausal assessment
• Male assessment
• Postmenopausal assessment where the clinical decision turns on whether estradiol is below approximately 10 pg/mL
• Monitoring the route and adequacy of estrogen therapy, where concentrations sit comfortably inside the interval — the §4 application

Not suitable on this assay:

• Monitoring estradiol suppression during aromatase-inhibitor therapy
• Detecting ovarian function recovery during AI therapy (§5.2)
• Computing a numeric E2:E1 ratio at postmenopausal concentrations, where estradiol commonly falls below the reportable limit

Those questions require a more sensitive method than this one. The physiology in §3 and §5.2 is real and is described accurately; the numeric ratio at those concentrations is not something this panel can produce, and saying so is more useful than implying otherwise.

See the estradiol and estrone analyte pages for the current reportable ranges.

Table 1. Estradiol versus estrone at a glance

Property	Estradiol (E2)	Estrone (E1)
Biological role	Potent estrogen; high-affinity ER ligand	Weak estrogen; convertible reservoir
Predominates in	Premenopause (ovarian)	Postmenopause (peripheral aromatisation)
Principal source	Ovarian granulosa; aromatisation of testosterone	Aromatisation of androstenedione; adipose tissue
Interconversion	⇌ E1 via 17β-HSD	⇌ E2 via 17β-HSD; estrone-sulfate reservoir
Effect of oral estrogen	Raised less (first-pass lowers E2:E1)	Raised markedly (first-pass conversion)
Preferred assay	LC-MS/MS	LC-MS/MS
Distinct utility	Potent-estrogen drive	Menopausal and route status; reservoir

7. Conclusions and recommendations

A single estradiol value should not be equated with estrogen status. Measure estrone and estradiol together and read the ratio: it separates premenopausal from postmenopausal physiology, reveals the route of estrogen therapy, and adds aromatisation and risk context that one analyte cannot.

Interpret the two as a pair — and match the question to an assay that reaches the concentrations it is decided at. For therapy route and adequacy, that is this panel. For suppression monitoring during aromatase-inhibitor therapy, it is not, and §6 says where the line falls.