Endometrial cancer has rising incidence — now the 4th most common cancer in UK women. Irregular bleeding (especially postmenopausal or IMB in women over 45) must trigger a 2WW referral. Ectopic pregnancy kills — always exclude pregnancy in any woman of reproductive age with irregular bleeding and pain. Prolactinoma is rare but treatable, and delayed diagnosis leads to visual loss from optic chiasm compression. These red flags represent the medicolegal minimum in the GP assessment of irregular periods.

A menstrual diary is the single most useful clinical tool — it converts a subjective complaint into objective, measurable data. Contraception is the most common cause of irregular periods in primary care and is often under-explored. PCOS affects 5–15% of women of reproductive age and presents as oligomenorrhoea — it is the RCGP curriculum's most common gynaecological topic. The menstrual history must always include fertility wishes, as this determines whether treatment aims to regulate cycles for conception or manage symptoms.

PCOS is the most common endocrine disorder in women of reproductive age and the most common cause of oligomenorrhoea requiring investigation. POI (premature ovarian insufficiency) is frequently missed — it affects 1 in 100 women under 40 and has significant implications for bone health, cardiovascular risk, and fertility (NICE NG23 2023). Thyroid disease is the most common treatable systemic cause — it is present in 10% of women with menstrual irregularity. Classification drives investigation and determines whether gynaecology, endocrinology, or primary care management is appropriate.

BMI is often the most informative single examination finding. Hypothalamic amenorrhoea (from low weight, over-exercise, or stress) is diagnosed clinically — FSH/LH are low-normal, oestrogen low. Virilisation signs (clitoromegaly, deep voice, rapidly progressive hirsutism) suggest an androgen-secreting tumour — rare but requires urgent endocrinology referral. The Ferriman-Gallwey score provides objective documentation of hirsutism severity, important for monitoring treatment response.

TFTs and prolactin are the two most common, most treatable, and most frequently missed causes of irregular periods — they should be checked in virtually every case. FSH elevation (>25 IU/L) on two separate occasions establishes POI — a diagnosis with major implications for bone health, cardiovascular risk, and fertility that requires urgent specialist input. Testosterone >5 nmol/L (markedly elevated) suggests an androgen-secreting tumour requiring urgent endocrinology referral. Routine day 21 progesterone confirms ovulation — important if fertility is a concern.

POI requires urgent specialist input — untreated POI leads to rapid bone loss (osteoporosis within 2–3 years), elevated cardiovascular risk, and menopausal symptoms. Hormone replacement is mandatory, not optional, until the natural age of menopause (51 years). Prolactin should be measured twice before referral (stress, recent breast examination, and exercise all elevate prolactin transiently). A prolactin consistently >1000 mU/L requires pituitary MRI to exclude prolactinoma.

Endometrial protection is mandatory in women with oligomenorrhoea or amenorrhoea — unopposed oestrogen (without progesterone withdrawal) causes endometrial hyperplasia, which can progress to endometrial cancer over years. Women with PCOS and infrequent periods must have withdrawal bleeds induced at least every 3 months if not on hormonal contraception. Letrozole has overtaken clomifene as the preferred ovulation induction agent for PCOS (NICE 2023 guidelines update). Metformin improves insulin sensitivity and reduces androgen levels in PCOS — particularly effective in women with BMI >30 and insulin resistance features.

Weight loss is the single most effective intervention in overweight women with PCOS — more effective than metformin for cycle regularisation, ovulation induction, and metabolic benefit. A 5% weight loss reduces fasting insulin by 30% and androgen levels by 20%. Lifestyle change should be first-line for 3–6 months before pharmacological treatment in overweight PCOS. In hypothalamic amenorrhoea, weight restoration must come before pharmacological cycle induction — external hormones will not override a suppressed hypothalamus and fertility treatment is less effective until weight is restored.

Annual metabolic screening in PCOS is important — 50% of women with PCOS develop type 2 diabetes by age 40, and cardiovascular risk is significantly elevated. Long-term amenorrhoea or oligomenorrhoea without endometrial protection increases endometrial hyperplasia risk (relative risk ~3x for endometrial cancer). Women with POI need lifelong HRT review — the decision to stop HRT at 51 (natural menopause age) is a specialist discussion. Safety-netting for visual field change prevents delayed diagnosis of growing pituitary adenoma.