Three interacting drivers
Polycystic ovary syndrome is best understood as a loop linking insulin resistance, excess androgens, and disrupted ovulation. Insulin resistance prompts higher circulating insulin, which can stimulate ovarian theca cells to make more testosterone and reduce sex-hormone-binding globulin, raising free androgen levels. Elevated androgens in turn interfere with normal follicle maturation, so follicles stall and ovulation becomes irregular or absent.
The classic ultrasound appearance of many small follicles reflects this arrested development, not true cysts. The features reinforce one another, which is why the syndrome is variable and chronic rather than a single defect.
Reading the Rotterdam framework
The widely used Rotterdam criteria diagnose PCOS when at least two of three features are present: irregular or absent ovulation, clinical or biochemical signs of high androgens (such as acne, excess hair growth, or raised testosterone), and polycystic ovarian morphology on ultrasound. Because two of three suffice, presentations differ widely between individuals.
Other conditions that mimic these signs must be excluded before the label is applied. This is general educational information, not medical advice; diagnosis and any management of PCOS should be guided by a qualified clinician.