PCOS and endometriosis are two of the most common gynaecological conditions affecting people of reproductive age, and they are frequently mentioned in the same breath — often as if they were variations on a theme. They are not. They have different underlying causes, different dominant symptoms, and entirely different diagnostic pathways. Confusing them, or assuming one rules out the other, can send someone down the wrong treatment path for years.
The confusion is understandable. Both affect menstrual cycles, both can impair fertility, both are underdiagnosed, and both disproportionately affect young women whose symptoms are too often dismissed. But the conditions diverge sharply once you look beneath the surface — and understanding where they differ is what allows each to be treated properly.
PCOS is a hormonal and metabolic disorder driven by elevated androgens, insulin resistance, and disrupted ovulation. Endometriosis is an inflammatory, structural disease in which tissue similar to the uterine lining grows outside the uterus and causes lesions, adhesions, and pain. The clearest distinguishing feature is the dominant symptom: PCOS centres on irregular cycles and metabolic features; endometriosis centres on pelvic pain. Crucially, you can have both.
Key Takeaways
- PCOS is fundamentally hormonal and metabolic; endometriosis is fundamentally inflammatory and structural — they are different categories of disease, not variants of one
- PCOS affects an estimated 8–13% of reproductive-age women depending on the diagnostic criteria used (Bozdag et al., 2016, Human Reproduction); endometriosis affects roughly 10%
- PCOS is diagnosed using the Rotterdam criteria (two of three: irregular ovulation, hyperandrogenism, polycystic ovaries on ultrasound); endometriosis traditionally requires laparoscopy for definitive diagnosis
- Blood tests are central to diagnosing PCOS but cannot diagnose endometriosis — a key practical distinction
- Severe, progressive period pain points to endometriosis; irregular or absent periods and metabolic features point to PCOS
- The two conditions can coexist, and one diagnosis should never be used to dismiss persistent symptoms that fit the other
Two Different Kinds of Disease
The most important thing to understand is that PCOS and endometriosis sit in different diagnostic categories.
PCOS is, at its core, an endocrine and metabolic condition. It is characterised by an imbalance of reproductive hormones — most notably elevated androgens (male-type hormones such as testosterone) — and is strongly linked to insulin resistance. The insulin resistance and the hormonal disruption feed each other: high insulin drives the ovaries to produce more androgens, and the excess androgens disrupt the normal monthly maturation and release of an egg. The “cysts” in the name are misleading; they are not true cysts but small, immature follicles that have stalled because ovulation is not occurring normally. The downstream effects ripple across the whole body — weight, skin, hair, mood, and long-term metabolic risk.
Endometriosis is a different beast entirely. It is an inflammatory, structural disease in which tissue resembling the endometrium (the uterine lining) grows in places it should not — on the ovaries, the peritoneum, the uterosacral ligaments, the bowel, and beyond. This ectopic tissue responds to hormonal cycles, bleeds, and provokes chronic inflammation, leading to scar tissue, adhesions that fuse organs together, and progressive damage. Its defining feature is pain, not hormonal imbalance.
“PCOS is a problem of hormones and metabolism; endometriosis is a problem of misplaced tissue and inflammation. They are different categories of disease that happen to share an organ.”
This distinction matters because it dictates everything downstream — what the symptoms are, how each is found, and how each is treated.
Where the Symptoms Diverge
Despite the overlap that causes confusion, the symptom profiles point in different directions when examined closely.
PCOS typically presents with:
- Irregular, infrequent, or absent periods — the hallmark, reflecting disrupted or absent ovulation
- Signs of high androgens — acne, excess facial or body hair (hirsutism), and scalp hair thinning
- Weight gain and difficulty losing weight, tied to insulin resistance
- Difficulty conceiving due to irregular ovulation
Endometriosis typically presents with:
- Severe, progressive period pain (dysmenorrhoea) that worsens over time and disrupts daily life
- Chronic pelvic pain that can occur outside menstruation
- Deep pain during sex (dyspareunia), covered in Endometriosis and Sex
- Painful bowel movements or urination, particularly around menstruation
- Difficulty conceiving due to inflammation and anatomical distortion
The single most useful differentiator is the character of the periods. Endometriosis is defined by pain; PCOS is defined by irregularity. Someone whose periods are agonisingly painful but reasonably regular fits endometriosis far better than PCOS. Someone whose periods are infrequent or absent, with acne and difficulty losing weight, fits PCOS. The pattern is not absolute, but it is a reliable starting point.
How Each Is Diagnosed
The diagnostic pathways are perhaps the cleanest line between the two conditions.
PCOS is diagnosed using the Rotterdam criteria, which require at least two of three features: irregular or absent ovulation; clinical or biochemical evidence of high androgens; and polycystic-appearing ovaries on ultrasound. Critically, much of this can be established through blood tests and ultrasound — testosterone levels, the LH:FSH ratio, and markers of insulin resistance leave measurable fingerprints. A diagnosis can often be reached in primary care or a single endocrinology or gynaecology visit.
Endometriosis has no equivalent. There is no blood test for it. Standard ultrasound frequently appears normal even in significant disease. The historical gold standard for definitive diagnosis is laparoscopy — keyhole surgery to visualise and biopsy lesions directly — although specialist transvaginal ultrasound and pelvic MRI performed by experienced clinicians can now provide strong supporting evidence, particularly for deep infiltrating disease and endometriomas. The diagnostic journey is notoriously long, averaging several years. For the full pathway, see How to Get an Endometriosis Diagnosis.
“PCOS leaves measurable hormonal and metabolic fingerprints in blood and on ultrasound. Endometriosis leaves none — which is why it so often takes years and surgery to confirm.”
This asymmetry has a practical consequence: PCOS is comparatively quick to diagnose, and once labelled, it can overshadow a coexisting endometriosis whose pain gets attributed to “just bad PCOS periods.”
Can You Have Both?
Yes — and this is where the distinction stops being academic. PCOS and endometriosis are not mutually exclusive, and a meaningful subset of people live with both. Having one does not confer protection against the other.
The problem is diagnostic overshadowing. When someone already carries a PCOS diagnosis, new or worsening pelvic pain is easily folded into the existing label rather than investigated as a separate issue. But PCOS does not typically cause severe, progressive period pain or deep dyspareunia — and when those symptoms appear in someone with PCOS, endometriosis should be on the table. The reverse is also true: someone diagnosed with endometriosis who has persistently irregular cycles, hirsutism, or metabolic features may also have undiagnosed PCOS.
The clinical principle is simple. Each condition should be assessed on its own symptom profile. A PCOS diagnosis does not explain endometriosis-type pain, and pain that fits endometriosis deserves investigation regardless of what other diagnosis is already on the chart.
Why Getting It Right Matters
The treatments diverge as much as the diseases do. PCOS management focuses on the hormonal and metabolic drivers: insulin sensitivity through diet, exercise, and sometimes metformin; cycle regulation; managing androgen-related symptoms; and ovulation induction when fertility is the goal. The long-term concern is metabolic — type 2 diabetes and cardiovascular risk.
Endometriosis management focuses on suppressing the inflammatory, hormone-responsive lesions and removing them where appropriate: hormonal suppression of the cycle, pain management, pelvic floor physiotherapy, and excision surgery. The long-term concerns are progressive tissue damage, adhesions, and pain that can become chronic if untreated.
Treating one as if it were the other helps no one. Metformin and a low-glycaemic diet will not address endometriosis lesions; excision surgery will not fix insulin resistance. And because both conditions affect fertility through different mechanisms, the right diagnosis shapes the right reproductive plan — explored further in Endometriosis and Fertility.
How Tracking Helps Distinguish the Two
The clearest way to separate these conditions in practice is to look at patterns over time — and that is exactly what symptom tracking captures. The defining questions are about timing and character: Are the periods painful or merely irregular? Does pain cluster cyclically around menstruation, or is it absent? Are cycles wildly unpredictable, or regular but agonising?
EndoTracking records cycle length and regularity alongside pain severity, location, dyspareunia, and bowel and bladder symptoms, then maps them against cycle phase. For someone trying to work out whether their symptoms fit endometriosis, PCOS, or both, this produces the precise data a clinician needs: a record showing whether the dominant problem is cyclical pain (pointing to endometriosis) or cycle irregularity (pointing to PCOS), and whether the two patterns coexist. The app’s exportable report turns months of lived experience into a structured summary you can bring to a gynaecologist or endocrinologist — see Endometriosis Symptom Tracker for more on building that picture.
The Bottom Line
PCOS and endometriosis are commonly confused, but they are distinct conditions with different causes, symptoms, and diagnoses. PCOS is hormonal and metabolic, defined by irregular cycles and androgen excess, and largely diagnosable through blood tests and ultrasound. Endometriosis is inflammatory and structural, defined by progressive pelvic pain, and diagnosed through imaging and surgery rather than bloodwork. And because the two can coexist, neither diagnosis should ever be used to wave away symptoms that fit the other.
If your symptoms do not fit neatly into one box — or if a PCOS diagnosis has not explained pain that keeps getting worse — that mismatch is worth taking seriously. Track the pattern, document it, and ask your clinician directly whether the other condition could also be present.
EndoTracking is a personal health tracking app and does not provide medical advice. Diagnosing PCOS or endometriosis requires clinical evaluation. If your symptoms fit either condition, consult a qualified healthcare provider.