Are irregular cycles always due to PCOS?

Note on the name: in May 2026, polycystic ovary syndrome (PCOS) was formally renamed polyendocrine metabolic ovarian syndrome (PMOS), following a global consensus process. The old name was inaccurate — the condition affects multiple hormone and metabolic systems, not just the ovaries, and "polycystic" wrongly implies ovarian cysts. Nothing about the condition, its diagnosis or its treatment has changed. Both names will be used side by side for the next few years, and we use PCOS below since that is the term most people know.

No — and assuming PCOS without proper assessment is a common way for other diagnoses to be missed. PCOS is the most frequent cause, but the list also includes thyroid disorders, raised prolactin, premature ovarian insufficiency, functional hypothalamic amenorrhoea from stress or low body weight or heavy exercise, perimenopause, and less commonly adrenal conditions or structural problems. Two are worth particular mention because they get mistaken for PCOS. Functional hypothalamic amenorrhoea is essentially the opposite situation — cycles stop because energy availability is too low — and the management is close to the reverse of PCOS advice, so getting this wrong matters. Premature ovarian insufficiency needs recognising early, both for fertility planning and because the associated low oestrogen affects bone and cardiovascular health. A reasonable assessment: FSH, LH, AMH, prolactin, thyroid function and testosterone, plus a pelvic ultrasound. Timing in the cycle matters for some of these. Polycystic-looking ovaries on a scan alone do not diagnose PCOS — a substantial proportion of women without PCOS have that appearance.