Irregular periods, extra hair, and an ovary full of stalled follicles, all driven by one engine running on insulin and luteinizing hormone. Learn the Rotterdam criteria, the unopposed estrogen risk, and how treatment changes the moment she wants to conceive.
The Setup
One Engine, Three Symptoms
PCOS looks like three unrelated problems, irregular periods, extra hair, and cysts, but they all flow from one engine: too much androgen from an insulin-driven ovary that cannot finish ovulating. Trace it once and the whole syndrome lines up.
A 22-year-old woman has periods only a few times a year, dark coarse hair on her chin and upper lip, and stubborn acne. Her BMI is 32 and she has velvety darkening in the folds of her neck. A pregnancy test is negative and her thyroid and prolactin are normal.
Which single underlying problem best ties her symptoms together?
Start with insulin. Many women with PCOS are insulin resistant, so the pancreas pumps out extra insulin to keep blood sugar normal. That high insulin does two things: it pushes the ovary to make more androgens (male-type hormones) and it lowers SHBGSex hormone binding globulin, the protein that mops up testosterone in the blood. When it drops, more free, active testosterone is left circulating., the protein that mops up testosterone. Less binding protein means more free, active testosterone. The velvety dark skin in the folds (acanthosis nigricans) is the visible fingerprint of that insulin resistance.
Now the hormone loop. The pituitary releases luteinizing hormone (LH) out of proportion to FSH, so the classic pattern is a high LH to FSH ratio. High LH drives the ovary to make even more androgen, while the relatively low FSH means follicles start to grow but never mature or release an egg. They stall and pile up at the edge of the ovary as small follicles, the string of pearls on ultrasound. No mature follicle means no ovulation, which means irregular or absent periods and trouble conceiving.
The dangerous downstream. Because she does not ovulate, her ovary keeps making estrogen but almost no progesterone (progesterone normally comes from the corpus luteum after ovulation). The uterine lining is bathed in estrogen with nothing to oppose it, month after month. Flip each card for the consequences.
High LH to FSHTap to flip
Follicles that stallHigh LH drives ovarian androgen; the relatively low FSH leaves follicles unable to mature. They arrest as many small follicles, the string of pearls, and ovulation fails.
Unopposed estrogenTap to flip
Endometrial cancer riskNo ovulation means no progesterone, so estrogen stimulates the lining unchecked. Over years this drives endometrial hyperplasia and endometrial cancer. This is why endometrial protection is mandatory in PCOS.
Metabolic loadTap to flip
More than the ovaryInsulin resistance brings type 2 diabetes, dyslipidemia, fatty liver, obstructive sleep apnea, and metabolic syndrome. PCOS is a whole-body problem, not just a period problem.
One engine: insulin and LH push ovarian androgen, which gives the skin signs and stalls the follicles. The failure to ovulate then leaves estrogen unopposed and the lining at risk.
Clinical Images
Ultrasound: string of pearls follicles · tap
Acanthosis nigricans: insulin resistance · tap
At the Bedside
The Look and the Story
PCOS is the most common cause of androgen excess in young women. The story is irregular cycles plus signs of extra androgen, often with a metabolic backdrop. Tap each finding.
Oligomenorrhea
Tap to reveal
Few or absent periods (often fewer than 8 or 9 a year) from failure to ovulate. A common reason a young woman cannot conceive.
Hirsutism and acne
Tap to reveal
Coarse dark hair in a male pattern (chin, upper lip, chest) and oily skin with acne. These are the clinical signs of high androgens; you do not always need a lab to count them.
Acanthosis nigricans
Tap to reveal
Velvety dark thickening in the neck and skin folds. A visible marker of insulin resistance, often with obesity, though not every patient is overweight.
Slow virilization vs rapid
Tap to reveal
PCOS androgen excess is mild and gradual. Rapid virilization with a deepening voice, balding, and clitoral enlargement points away from PCOS and toward an androgen-secreting tumor.
From the Attending
PCOS is a diagnosis of exclusion, but it is also the most common one you will land on. The history that fits is gradual: irregular cycles since the teens, slowly worsening hair, a creeping weight gain. The story that does not fit PCOS is sudden and severe. Slow and mild is PCOS. Fast and dramatic is a tumor until proven otherwise.
Confirming It
Two of Three, After You Exclude
The diagnosis is the Rotterdam two-of-three rule, but only after you have ruled out the look-alikes. Work the criteria, then the exclusions.
The Rotterdam criteria. Diagnose PCOS when two of these three are present: (1) oligo-ovulation or anovulation (irregular or absent periods), (2) clinical or biochemical hyperandrogenism (hirsutism, acne, or a high androgen level), and (3) polycystic ovaries on ultrasound (many small follicles or an enlarged ovary). Two of three is enough; you do not need all three.
Exclude the mimics first. PCOS is a diagnosis of exclusion, so check the studies that rule out other causes of irregular cycles and androgen excess before you commit. Try each fork.
A 24-year-old with irregular cycles and hirsutism. Which baseline labs help exclude common mimics?
Thyroid disease and high prolactin both cause irregular cycles, and pregnancy is the first thing to exclude in any missed period. These come before you call it PCOS. Rule out pregnancy, thyroid, and prolactin first.
She has irregular cycles since her teens, mild hirsutism, but you want to exclude a congenital cause. Which test?
Nonclassic congenital adrenal hyperplasia (a partial 21-hydroxylase deficiency) mimics PCOS with hirsutism and irregular cycles. A high 17-hydroxyprogesterone flags it. An elevated 17-hydroxyprogesterone points to CAH, not PCOS.
Another patient has rapid hair growth, a deepening voice, and new clitoral enlargement over a few months. Best next step?
Rapid virilization is a red flag, not PCOS. A very high testosterone suggests an ovarian tumor and a very high DHEAS suggests an adrenal tumor. Image to find it. Fast virilization plus very high androgens equals tumor hunt, not PCOS treatment.
The look-alikes. The table separates PCOS from its common mimics.
Mimic
What separates it
Thyroid disease
Abnormal TSH; hypothyroidism and hyperthyroidism both disturb cycles. Easy to check and correct.
Hyperprolactinemia
High prolactin, sometimes with galactorrhea and headaches or visual changes from a pituitary adenoma.
Nonclassic CAH
Elevated 17-hydroxyprogesterone from partial 21-hydroxylase deficiency; hirsutism since childhood.
Cushing syndrome
Central obesity, striae, easy bruising, hypertension; abnormal dexamethasone suppression or high cortisol.
Androgen-secreting tumor
Rapid virilization with a very high testosterone (ovary) or DHEAS (adrenal). A mass on imaging.
Board Trap
A high LH to FSH ratio supports PCOS but is not required for the diagnosis, and a normal ratio does not exclude it. Likewise, do not anchor on cysts: the Rotterdam rule is two of three, so a woman with irregular cycles and clear hirsutism has PCOS even if her ovaries look normal on ultrasound. It is two of three, not all three, and not the LH ratio alone.
The Plan
It Depends on Her Goal
Treatment forks on one question: does she want to get pregnant now? Lifestyle helps everyone, but the medicine changes completely depending on the answer.
First for everyone: lifestyle. Weight loss and exercise are first-line and improve every part of PCOS at once: cycles get more regular, androgen falls, insulin resistance eases, and fertility improves. Even a modest loss can restart ovulation.
Not trying to conceive. The workhorse is a combined oral contraceptive. It does several jobs together: it regularizes cycles, raises SHBG so free testosterone falls (improving hair and acne), and, most importantly, it protects the endometrium by supplying progestin. If hirsutism persists, add the antiandrogen spironolactone. Metformin helps the metabolic picture and can ease insulin resistance. If she cannot take estrogen, a cyclic progestin alone still protects the lining.
Trying to conceive. Here the contraceptive comes off and the goal flips to ovulation. First-line is letrozole, an aromatase inhibitor that lowers estrogen feedback and boosts FSH to mature a follicle; it gives higher live-birth rates than clomiphene, especially with obesity. Metformin can be an adjunct. Letrozole and clomiphene induce ovulation; combined contraceptives prevent it, so you never use them to help her conceive.
From the Attending
Never leave an anovulatory PCOS patient with an unprotected uterus. Even if she declines contraception, she needs a progestin source, a combined pill, a progestin IUD, or cyclic progestin, to shed the lining and prevent endometrial hyperplasia and cancer. Anovulation means unopposed estrogen. Protect the lining, every time.
Board Trap
Spironolactone is an antiandrogen, but it is teratogenic and can feminize a male fetus, so it is only used alongside reliable contraception, never in a woman trying to conceive. The classic wrong answer is starting spironolactone for hirsutism in a patient who wants to get pregnant. Spironolactone needs contraception on board. It is never the fertility answer.
Prove It
Board Walkthrough
Original clinical vignettes, 5 dealt per round, answer choices shuffled, never repeating within a round. Tap a wrong answer first to see why it almost works, then read the glowing clues.
Medically reviewed by Kaitlyn Cocuzzo, MD and Fatima Ali, DO · Last reviewed June 2026
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