The Importance of Testing Both Estradiol and Estrone Sulfate

By Donnie Yance

Estrogens are a group of hormones essential for various bodily functions, especially female reproductive health. The two primary types of estrogens tested are estradiol and estrone sulfate.

Estrogens influence nearly every aspect of energy homeostasis and metabolism. Testing both estradiol and estrone sulfate offers a comprehensive picture of estrogen levels, helping clinicians diagnose hormonal imbalances and related health conditions. Estradiol dominates in nonpregnant women of childbearing age, while estrone sulfate becomes the predominant form after menopause.

Reasons for Testing Both Hormones

Estradiol Levels and Correlation to Breast Cancer

Estrogens regulate reproductive and metabolic processes in females, playing a critical role in lipid metabolism. They work through estrogen receptors to communicate with target cells. Breast cancer is predominantly hormone-dependent, with about 70% of cases expressing estrogen and/or progesterone receptors—key predictive markers.

Estradiol (E2) is the primary estrogen in nonpregnant women of childbearing age, produced mainly by the ovaries.¹

Estrogen metabolism and estrogen metabolites are associated with breast cancer, as well as other hormone-sensitive cancers including endometrial, prostate, and lung cancer. Additionally, we explore other factors associated with estrogen-driven breast cancer.²

Circulating estrogen levels correlate with estrogen-responsive gene expression in ER+ breast tumors, suggesting E2 measurements may serve as valuable markers for endocrine sensitivity and personalized treatment.^{3 4 5}

Plasma estrogens show the strongest correlations with breast cancer risk, particularly when measuring free estradiol (unbound to SHBG), which represents the most active fraction. These relationships potentially explain virtually all associations between breast cancer and body mass index in postmenopausal women, likely due to estrogen synthesis in subcutaneous fat. Plasma androgen levels also correlate with risk, even after adjusting for estrogen levels, suggesting local estrogen production plays an important etiological role.^{6 7}

Nine prospective case-control studies (1990-2000) demonstrated that breast cancer risk increases 1.29-fold (95% CI, 1.15 to 1.44; P_.001) for every doubling of E2 concentration. ^{8 9 10 11 12 13 14 15}

Aromatase Inhibitors and Resistance Mechanisms

Evidence suggests aromatase inhibitors (AIs) may not fully suppress estrogen levels in some obese women despite effective aromatase inhibition.^{16 17} Single nucleotide polymorphisms in CYP19A1 have been associated with differential letrozole benefits in metastatic breast cancer.¹⁸

Aromatase-independent pathways may contribute to aromatase inhibitor resistance. Research on 45 ER-positive breast cancers revealed that steroid-metabolizing enzymes (SRD5A1, HSD3B1, STS, and HSD17B1) produce estrogenic compounds from androgens and estrone sulfate. Expression of these enzymes correlated with ER activity, particularly in postmenopausal patients, suggesting alternative pathways for ER activation that bypass aromatase.¹⁹

Baseline Estradiol Levels Predict Anastrozole Benefit

Anastrozole is a prescription medicine used to treat and prevent hormone receptor-positive breast cancer in postmenopausal women. The IBIS-II prevention trial (3,864 high-risk postmenopausal women, 16 countries, 2003-2012) examined baseline hormone levels and breast cancer risk over 131 months. Key findings:

• Anastrozole reduced breast cancer incidence by 51% (4.4% vs. 8.5% placebo)
• In placebo group, each quartile increase in estradiol-SHBG ratio increased risk by 25%
• Anastrozole eliminated this hormone-related risk
• Greatest benefit occurred in women with moderate to high estradiol-SHBG ratios (quartiles 2-4), with 45-46% risk reduction
• Women in the lowest quartile showed minimal benefit
• Similar but weaker patterns were observed with testosterone-SHBG ratios (21% risk increase per quartile)

The researchers concluded that routine hormone testing can identify which high-risk postmenopausal women will benefit most from aromatase inhibitor therapy.²⁰

If serum estradiol levels are < 5, taking an AI may not provide any additional benefits for treating or preventing breast cancer, as AI’s primarily work by blocking the conversion of androgen and other hormones into estrogen.  In such cases the risk of side effects and the increased potential osteoporosis and cardiovascular disease, may outweigh the benefits.²¹

Prostate Cancer

17β-estradiol may be an initiating driver of advanced prostate cancer and prostate cancer stem cell activation.²² 17β-estradiol isformed mainly by conversion from testosterone through the aromatase enzyme. One study showed that the serum level of 17β-estradiol is raised significantly in patients with prostate cancer and thus emphasizes the role of 17β-estradiol in prostate carcinogenesis in the milieu of normal testosterone. The study further showed a strong positive correlation between PSA and 17β-estradiol among prostate cancer patients which supports the relationship between aromatization and prostate proliferative activities. Serum 17β-estradiol can be included as an additional serum marker for prostate cancer diagnosis after due standardization and validation.²³

Estrone Sulfate

Estrone sulfate (E1S) is the most abundant circulating estrogen in the body, yet it remains one of the most underutilized hormone measurements in clinical practice.

After menopause, the ovaries stop producing estrogen in the traditional sense, but that doesn’t mean estrogen disappears entirely from a woman’s body. Instead, postmenopausal women produce estrogens locally in peripheral tissues—places like fat, muscle, and bone—through a process called intracrinology. This localized production relies heavily on DHEA, a precursor hormone that acts as the raw material for estrogen synthesis. Here’s where it gets interesting: DHEA levels vary dramatically from woman to woman, ranging from barely detectable amounts to levels approaching what we see in premenopausal women. Despite this variation in precursor availability, serum estrogen concentrations in postmenopausal women remain consistently low—so low, in fact, that they’re considered biologically inactive in the bloodstream. This phenomenon helps explain why roughly 25% of postmenopausal women experience few or no menopausal symptoms: their tissues are producing just enough estrogen locally to maintain function without elevating blood levels.²⁴ Understanding these nuances matters tremendously for clinical care, but there’s a catch.

Testing estradiol alone provides only a snapshot of circulating hormone levels at a single moment—an incomplete picture that misses the body’s true estrogen burden. E1S represents the body’s estrogen reservoir, a vast storage pool that continuously feeds tissue-level estrogen activity and reveals what’s actually happening at the cellular level.^{25 26}

This distinction is clinically critical. Many physicians have been puzzled when administering estrogen fails to elevate serum estradiol or estrone levels. Without measuring E1S, practitioners often respond by increasing estrogen dosages—a decision based on incomplete data. This approach frequently produces excess estrogen-related side effects and deepens the diagnostic confusion. However, when E1S is finally measured in these cases, levels are often extremely elevated, revealing significant tissue estrogen exposure that standard testing missed entirely. Measuring E1S is essential for accurately monitoring estrogen status and appropriately adapting hormone dosages, particularly in premenopausal women where E1S is quantitatively the most important circulating estrogen.^{27 28}

Production & Interconversion

The production rate of E1S fluctuates during the menstrual cycle similar to other estrogens. All of its production occurs in peripheral tissues (mainly the liver) from the conversion of estrone and estradiol. Approximately 90% is bound to albumin and has a very low metabolic clearance rate. 65% of estradiol and 54% of estrone are converted to E1S. Conversely the conversion of E1S to estradiol is only 1.4% and 21% of E1S gets converted back to estrone. The interconversion is continuous and dynamic, attempting to achieve estrogen equilibrium. E1S and its conversion by steroid sulfatase (STS) to E1 and thereafter to E2 by reducing isozymes of the 17β-hydroxysteroid-dehydrogenase (17β-HSD) can serve as major estrogen hormone precursors in peripheral tissues such as mammary gland, bone, and hair roots. This pathway is seen as a key factor of tumor growth in estrogen dependent tumors of the mammary gland and as an important source of estrogen in the bone.²⁹

Metabolism

Remember E1S represents a slowly metabolized estrogen reservoir. Sulfated hormones have a 10-fold lower clearance rate than glucuronides. The metabolic clearance rate of estrone sulfate is the lowest by far of any estrogen. Oral estradiol, conjugated equine estrogens, or esterified estrogens are metabolized to estrone and E1S in the intestinal mucosa and in the liver, resulting in total serum estrone levels 3-6 times higher than estradiol.³⁰

Tissue-Level Estrogen Activity

Understanding E1S is critical because serum estradiol measurements only reflect circulating hormone levels, not actual tissue exposure. Target tissues such as breast, endometrium, and prostate contain sulfatase enzymes that actively convert E1S to estrone and estradiol locally.^{31 32} This means E1S functions as a circulating reservoir that delivers active estrogens directly to tissues where they exert their biological effects.

This local conversion is why tissue estrogen concentrations can be dramatically higher than serum levels would suggest. Clinicians who rely solely on serum estradiol measurements may have a false sense of security about total estrogen exposure. The patient may appear to have “normal” estradiol levels while tissues are being exposed to significantly elevated estrogen activity through local E1S conversion. This is precisely why measuring E1S is essential for understanding true estrogen exposure and optimizing hormone therapy safely.³³

Cancer Risk Marker

E1S accumulates preferentially in target tissues, where estrogen levels reach concentrations many times higher than those measured in blood. This tissue-level accumulation explains why serum estradiol measurements alone provide an incomplete picture of cancer risk.³⁴ In breast and endometrial cancer tissue, the ratio of E1S to estrone is significantly elevated compared to normal tissue,^{35 36} and this ratio may serve as a valuable marker for breast cancer risk assessment.³⁷ Similarly, E1S concentrations are markedly elevated in both benign prostatic hyperplasia and prostate carcinoma cells.³⁸ The mechanism appears related to E1S and estradiol upregulating nitric oxide formation, which directly promotes tumor growth in prostatic cancer.³⁹ These findings underscore the critical importance of measuring E1S as part of comprehensive cancer risk evaluation.

Estrone sulfate measurement is essential—not optional—for comprehensive estrogen assessment. Relying solely on serum estradiol is inadequate and potentially misleading, as it fails to capture the tissue-level estrogen exposure that E1S reveals. This sulfated reservoir, with its remarkably slow metabolic clearance, serves dual roles: maintaining estrogen homeostasis through continuous interconversion with estrone and estradiol, while simultaneously accumulating to concentrations many times higher in target tissues than in blood. The significantly elevated E1S-to-estrone ratios observed in breast cancer, endometrial cancer, and prostate disease underscore its value as a cancer risk marker. Optimal patient care demands—not suggests—routine E1S monitoring in hormone replacement therapy, disease management, and cancer risk assessment. Without E1S measurement, clinicians operate with an incomplete and potentially dangerous understanding of their patients’ true estrogen status.

Conclusion

Estrogen testing is far more complex than measuring serum estradiol alone. While estradiol remains an important marker of circulating estrogen activity, it provides only a limited snapshot of a much larger and more dynamic hormonal system. Estrone sulfate (E1S) serves as the body’s primary estrogen reservoir, supplying tissues with a continuous source of estrogen that can be converted into active forms when needed.

Research shows that both estradiol and estrone sulfate play important roles in hormone balance, menopause management, hormone replacement therapy, and the development of hormone-sensitive cancers such as breast, endometrial, and prostate cancer. Because E1S can accumulate in tissues at levels much higher than those found in the bloodstream, relying on estradiol testing alone may underestimate a person’s true estrogen exposure.

Measuring both estradiol and estrone sulfate provides a more complete picture of estrogen status and can help clinicians make better-informed decisions regarding prevention strategies, hormone therapy, and cancer risk assessment. As our understanding of estrogen metabolism continues to evolve, comprehensive testing that includes both hormones must become a standard part of evaluating hormonal health and optimizing patient care.