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What Is PCOS? The Complete Guide

Polycystic ovary syndrome (PCOS) — recently renamed Polyendocrine Metabolic Ovarian Syndrome (PMOS) by international consensus, though PCOS remains the more widely used term — is the most common endocrine disorder in women of reproductive age worldwide, as confirmed by StatPearls (NCBI, July 2025). Its prevalence ranges from 5% to 26% depending on which diagnostic criteria are applied, affecting tens of millions of women globally. PCOS is not simply a condition of irregular periods or ovarian cysts — it is a complex endocrine and metabolic disorder whose root pathophysiology involves insulin resistance, androgen excess, and hypothalamic-pituitary-ovarian axis dysregulation that produce a wide and often confusing symptom picture spanning reproductive, metabolic, dermatological, and psychological domains.

StatPearls (2025) states clearly that “the pathophysiology of PMOS primarily involves insulin resistance and a high prevalence of visceral adiposity, even in the absence of obesity, which disrupts hormonal cross-talk among the hypothalamus, pituitary gland, and ovaries.” This metabolic root explains why PCOS is not just a gynaecological problem but a whole-body metabolic condition — and why treating only the reproductive symptoms without addressing the underlying insulin resistance produces incomplete and unsustainable results. PCOS is associated with a significantly elevated risk of type 2 diabetes, cardiovascular disease, metabolic syndrome, endometrial cancer, obstructive sleep apnea, and metabolic fatty liver disease — making early identification and comprehensive management a genuine long-term health priority.

PCOS Symptoms: The Complete List

The symptom profile of PCOS is broader than most people — including many clinicians — expect. Menstrual and reproductive symptoms include: irregular, infrequent, or absent periods (oligomenorrhoea or amenorrhoea); heavy or prolonged bleeding when periods do occur; absent or infrequent ovulation; difficulty conceiving; and recurrent pregnancy loss. Androgen excess symptoms include: hirsutism (excess hair growth in male-pattern distribution — face, chest, abdomen, inner thighs); persistent adult acne, particularly jawline and chin acne that does not respond well to standard acne treatments; and androgenic alopecia (scalp hair thinning in a male-pattern distribution — crown and temples). Metabolic symptoms include: unexplained weight gain or resistance to weight loss, particularly with abdominal fat accumulation; acanthosis nigricans (dark, velvety skin patches on the neck, armpits, or groin — a sign of insulin resistance); multiple skin tags; and post-meal energy crashes and carbohydrate cravings. Psychological symptoms include anxiety (present in approximately 27–50% of women with PCOS), depression (21–23%), and disordered eating. Physical symptoms include pelvic pain, bloating, fatigue, and sleep disruption — PCOS is associated with a significantly elevated rate of obstructive sleep apnea, even in lean women.

The Rotterdam Criteria: How PCOS Is Diagnosed

PCOS is diagnosed using the Rotterdam consensus criteria, established in 2003 and still the internationally accepted standard. The Rotterdam criteria require the presence of two out of the following three features, in the absence of other conditions that could explain the findings: (1) oligo-ovulation or anovulation — irregular, infrequent, or absent ovulation, typically reflected in irregular menstrual cycles; (2) clinical or biochemical signs of hyperandrogenism — either visible signs (hirsutism, acne, androgenic alopecia) or elevated blood levels of androgens (testosterone, DHEA-S, androstenedione); and (3) polycystic ovarian morphology (PCOM) on ultrasound — defined as 20 or more follicles per ovary on standard ultrasound, or increased ovarian volume above 10 mL on either ovary. Importantly, the third criterion (polycystic ovaries on scan) alone does not diagnose PCOS — many women have polycystic-appearing ovaries without the hormonal disorder, and PCOS can be diagnosed without ovarian cysts if the other two criteria are met.

The Rotterdam criteria produce four distinct PCOS phenotypes: Phenotype A (the “classic” full presentation — anovulation + hyperandrogenism + polycystic ovaries); Phenotype B (anovulation + hyperandrogenism without polycystic ovaries); Phenotype C (hyperandrogenism + polycystic ovaries with regular ovulation); and Phenotype D (anovulation + polycystic ovaries without hyperandrogenism). These phenotypes carry different metabolic risk profiles — Phenotypes A and B, which include hyperandrogenism, are associated with the highest insulin resistance and metabolic risk.

PCOS and Irregular Periods: The Most Common Presenting Symptom

Menstrual irregularity is the most commonly reported PCOS symptom and the one most likely to prompt initial medical consultation. In PCOS, the disruption to the hypothalamic-pituitary-ovarian (HPO) axis — driven by insulin resistance and androgen excess — prevents the normal hormonal cascade that produces regular ovulation. Without consistent ovulation, the corpus luteum does not form, progesterone is not produced in the second half of the cycle, and periods become irregular, infrequent, or absent. Approximately 84–94% of women diagnosed by Rotterdam criteria have ovulatory dysfunction, according to research in Reproductive Medicine and Biology (2025) by Baba et al. Clinically, cycles longer than 35 days or fewer than 8 periods per year are considered the threshold for oligo-menorrhoea that warrants PCOS assessment.

It is important to note that regular periods do not exclude PCOS. A subset of women with PCOS (“ovulatory PCOS”) have regular menstrual cycles but still meet Rotterdam criteria based on hyperandrogenism and polycystic ovarian morphology — and still carry the same metabolic risks. Period regularity alone is not sufficient to rule out PCOS, particularly in women with prominent acne, hirsutism, weight management difficulties, or insulin resistance signs.

PCOS and Hirsutism: Excess Hair Growth Explained

Hirsutism — excess male-pattern hair growth in women — is present in approximately 60–70% of women with PCOS and is one of the most diagnostically reliable signs of androgen excess. It affects the face (upper lip, chin, cheeks), chest, abdomen, inner thighs, and lower back — areas where hair follicles are androgen-sensitive and respond to elevated testosterone or dihydrotestosterone (DHT) with terminal hair growth. The modified Ferriman-Gallwey score is the clinical tool used to quantify hirsutism severity, with scores above 8 indicating clinically significant excess. Ethnic variation in baseline hair growth means the threshold for what constitutes clinically significant hirsutism varies — what is normal for a woman of South Asian or Mediterranean background may differ from a woman of East Asian or Northern European background — but any new or increasing male-pattern hair growth beyond a woman’s personal and ethnic baseline warrants evaluation.

The androgen excess driving hirsutism in PCOS is primarily testosterone and its derivatives, produced both by the ovaries and the adrenal glands in response to elevated LH and insulin. Insulin directly stimulates ovarian theca cell androgen production — explaining why insulin-sensitising treatments (metformin, lifestyle changes, inositol) reduce hirsutism in PCOS by reducing the insulin signal that drives androgen overproduction.

PCOS and Weight Gain: Why Ordinary Diet Advice Does Not Work

Weight management in PCOS is disproportionately difficult compared to women without PCOS — not because of lack of effort, but because of the hormonal and metabolic environment that drives fat storage and resistance. The primary drivers are insulin resistance (which promotes fat storage particularly in visceral adipose tissue), androgen excess (which shifts fat distribution toward the abdomen), low sex hormone-binding globulin (SHBG) levels (which increase the availability of active testosterone), and compensatory hyperinsulinaemia (which suppresses fat oxidation and drives fat anabolism). Many women with PCOS are lean or normal-weight — PCOS is not defined by obesity — but even lean women with PCOS show visceral fat accumulation and insulin resistance on clinical testing. The weight management challenge in PCOS requires addressing the hormonal root causes — insulin sensitivity and androgen levels — not simply restricting calories, which is why standard dieting advice is so often ineffective for women with undiagnosed or undertreated PCOS.

PCOS and Insulin Resistance: The Root Cause Connection

Insulin resistance is now considered the central pathophysiological driver of PCOS rather than merely a comorbidity. StatPearls (2025) identifies insulin resistance and visceral adiposity as the primary mechanisms disrupting the HPO axis in PCOS. Elevated insulin levels directly stimulate ovarian androgen production by activating theca cells — the ovarian cells that produce testosterone. They also suppress SHBG production in the liver, increasing the proportion of free (biologically active) testosterone. Additionally, insulin inhibits ovarian follicle maturation in a way that promotes the polycystic morphology characteristic of PCOS. A 2025 PMC study found that more than 80% of women who present with hyperandrogenism have PCOS, and that hyperandrogenic women have per se a higher risk of insulin resistance regardless of whether they meet full Rotterdam criteria. Addressing insulin resistance through dietary modification, exercise, and if appropriate metformin or inositol supplementation, produces improvement across virtually all PCOS symptom domains — periods, androgen signs, weight, mood, and fertility — because it addresses the upstream hormonal driver rather than each downstream symptom individually.

PCOS and Fertility: What You Need to Know

PCOS is one of the leading causes of female infertility, primarily because infrequent or absent ovulation reduces the opportunities for conception. However, PCOS fertility outcomes are generally more positive than many women expect: the majority of women with PCOS can conceive with appropriate medical support, and many conceive naturally — particularly once PCOS is diagnosed and managed. The NICE guideline for PCOS recommends weight management as the first-line fertility intervention in overweight women with PCOS, as even 5–10% weight loss produces significant improvements in ovulation frequency and spontaneous conception rates. Clomifene citrate (Clomid) and letrozole are first-line ovulation induction medications, with letrozole showing higher ovulation and live birth rates in the landmark PPCOS II trial. Metformin improves ovulation rates in anovulatory PCOS and is frequently combined with clomifene for women who do not respond to clomifene alone. IVF is effective for PCOS when other interventions have not achieved pregnancy, though careful ovarian stimulation protocols are required to reduce the risk of ovarian hyperstimulation syndrome (OHSS), which is more common in PCOS.

PCOS vs Thyroid Disease vs Perimenopause: Overlapping Symptoms

One of the primary reasons PCOS remains undiagnosed for so long is that its symptoms overlap substantially with several other common conditions — most importantly hypothyroidism, perimenopause, and hyperprolactinaemia. Hypothyroidism shares fatigue, weight gain, depression, hair loss, irregular periods, and skin changes with PCOS — and both conditions can co-exist. TSH testing is essential in any PCOS assessment to exclude or identify concurrent thyroid dysfunction. Perimenopause in women over 38–40 shares irregular periods, mood changes, weight gain, fatigue, and brain fog with PCOS — and both conditions can occur simultaneously. The key distinguishing features are age (PCOS typically presents in the teens and twenties, though diagnosis often comes later), androgen signs (hirsutism and androgenic acne suggest PCOS rather than perimenopause), and hormonal testing (elevated testosterone in PCOS vs declining estrogen in perimenopause). Hyperprolactinaemia — elevated prolactin from a pituitary adenoma — produces irregular periods and can mimic PCOS on basic testing; serum prolactin testing is part of the standard PCOS exclusion panel. The Rotterdam diagnostic process requires excluding these other conditions before PCOS is confirmed.

PCOS Blood Tests: What to Ask Your Doctor For

The recommended blood test panel for PCOS assessment, as recommended by current clinical guidelines, includes: total testosterone and free androgen index (to assess androgen excess); SHBG (sex hormone binding globulin — typically low in PCOS); LH and FSH with LH/FSH ratio (elevated LH relative to FSH is characteristic of PCOS, though not universally present); DHEA-S (adrenal androgen — elevated in some PCOS presentations); AMH (anti-Müllerian hormone — typically elevated in PCOS, useful marker of ovarian reserve and PCOS diagnosis); prolactin (to exclude hyperprolactinaemia); TSH and free T4 (to exclude thyroid dysfunction); fasting glucose and HbA1c (to assess current blood sugar status); fasting insulin and HOMA-IR (to assess insulin resistance); and full lipid panel (triglycerides, HDL — the dyslipidaemia pattern of high triglycerides and low HDL is common in PCOS). Pelvic ultrasound to assess ovarian morphology is the standard imaging investigation. Not all tests are required in every presentation — a clinician will select the most relevant panel based on the clinical picture.

PCOS Treatment Options: Medication, Supplements, and Lifestyle

PCOS treatment is individualised based on the predominant symptoms and the woman’s reproductive goals. For women not currently trying to conceive, the combined oral contraceptive pill (COCP) is the most widely used first-line treatment for menstrual regulation, androgen suppression (reducing acne and hirsutism), and endometrial protection. Anti-androgen medications including spironolactone and cyproterone acetate reduce hirsutism and acne when the contraceptive pill alone is insufficient. Metformin — the insulin-sensitising diabetes medication — is used in PCOS to improve insulin sensitivity, reduce androgen levels, promote ovulation regularity, and support weight management; it is particularly effective in the insulin-resistant phenotype. Inositol (specifically myo-inositol combined with D-chiro-inositol) has a growing evidence base as a natural insulin-sensitiser for PCOS: a 2022 meta-analysis in International Journal of Molecular Sciences found inositol improved menstrual regularity, androgen levels, and insulin sensitivity in women with PCOS with a favourable safety profile. Lifestyle intervention — low-glycaemic diet, regular resistance and aerobic exercise, and stress management — addressing the insulin resistance root cause produces the broadest benefits and is recommended as a core component of PCOS management at all stages.