Irregular periods, excess hair, acne, or difficulty losing weight — if you're wondering whether it's PCOS, here's exactly what needs to be investigated and why.
June 2026 · 9 min read · Lambert Medical Clinical Team
Polycystic ovary syndrome (PCOS) affects around 1 in 10 women of reproductive age — making it one of the most common hormonal conditions a GP sees. Yet diagnosis is often delayed by years, and many women are told their results are "normal" without receiving a clear explanation of what was actually tested, or why a PCOS diagnosis requires more than one blood test.
This guide explains how PCOS is properly diagnosed, which blood tests are needed, what the results look like, and how to distinguish PCOS from other causes of irregular periods.
PCOS is diagnosed using the Rotterdam criteria — the international standard since 2003. A diagnosis requires two of the following three features:
Irregular Periods
Cycles longer than 35 days, fewer than 8 periods per year, or absent periods (amenorrhoea)
Elevated Androgens
Raised testosterone or DHEAS on blood test, or clinical signs: excess facial/body hair (hirsutism), acne, or scalp hair loss
Polycystic Ovaries
Ovaries containing 20+ follicles or increased ovarian volume on ultrasound
This means PCOS can be diagnosed without abnormal blood tests if you have irregular periods and polycystic ovaries on ultrasound. It can also be diagnosed without an ultrasound if you have irregular periods and elevated androgens. Blood tests are essential but are not the only piece of the puzzle.
A thorough PCOS workup should include the following tests. Ideally blood is drawn on day 2–5 of your menstrual cycle where possible, or at a random time if periods are very infrequent.
| Test | What It Measures | What to Look For in PCOS |
|---|---|---|
| LH & FSH | Pituitary hormones controlling ovulation | Elevated LH:FSH ratio (>2:1) — seen in many but not all PCOS cases |
| Total Testosterone | Total circulating testosterone | May be mildly raised or upper end of normal; check alongside SHBG |
| Free Androgen Index (FAI) | Calculated from testosterone & SHBG | More sensitive for androgen excess than testosterone alone |
| SHBG | Sex hormone-binding globulin | Often low in PCOS — low SHBG means more free testosterone is active |
| DHEAS | Adrenal androgen | Elevated in adrenal PCOS subtype; very high levels suggest adrenal tumour |
| Fasting Insulin & Glucose | Insulin resistance markers | Insulin resistance is present in 50–70% of women with PCOS; critical for management |
| HbA1c | 3-month glucose average | Screens for pre-diabetes and diabetes, common PCOS long-term complication |
| TSH (Thyroid) | Thyroid stimulating hormone | Rules out hypothyroidism — causes irregular periods and overlapping symptoms |
| Prolactin | Pituitary hormone | Elevated prolactin causes irregular periods and can mimic PCOS — must be excluded |
| Oestradiol | Oestrogen level | Helps assess ovarian function; low levels in premature ovarian insufficiency |
| AMH | Anti-Müllerian hormone | Often elevated in PCOS — reflects high follicle count; also used to assess ovarian reserve |
A GP who only tests testosterone and tells you it's normal has not ruled out PCOS. Testosterone may be within the quoted normal range in 20–30% of women with PCOS. The Free Androgen Index (FAI) — calculated from testosterone and SHBG — is a more sensitive marker. If SHBG is low, free testosterone can be significantly elevated even when total testosterone reads as normal. Always ask to see actual numbers, not just a verdict.
Irregular periods are the most common reason women seek investigation for PCOS — but they have many causes. Blood tests help distinguish between them.
| Condition | Key Blood Markers | Other Features |
|---|---|---|
| PCOS | Raised LH:FSH, low SHBG, elevated FAI | Hirsutism, acne, weight gain, polycystic ovaries on USS |
| Hypothyroidism | Elevated TSH, low free T4 | Fatigue, weight gain, cold intolerance, constipation |
| Hyperprolactinaemia | Elevated prolactin | Milky nipple discharge (galactorrhoea), headache |
| Premature Ovarian Insufficiency | Very high FSH, low oestradiol, low AMH | Hot flushes, vaginal dryness — before age 40 |
| Congenital Adrenal Hyperplasia | Elevated 17-OHP | Severe hirsutism, virilisation — rarer, often missed |
| Functional Hypothalamic Amenorrhoea | Low LH, FSH, oestradiol — all low | Excessive exercise, low body weight, stress |
PCOS is not just a reproductive condition — it is a metabolic syndrome. Insulin resistance is present in approximately 50–70% of women with PCOS, regardless of weight. Elevated insulin drives the ovaries to produce more testosterone, which suppresses ovulation and worsens the hormonal imbalance. Identifying insulin resistance changes the management approach.
Women with insulin resistance respond well to lifestyle modifications that improve insulin sensitivity — particularly reducing refined carbohydrates and increasing physical activity. Metformin, a medication that improves insulin sensitivity, is often used to regulate periods and reduce androgen levels. Without testing for insulin resistance, this important management pathway is often missed.
Our PCOS hormone panel covers LH, FSH, testosterone, SHBG, DHEAS, fasting insulin, HbA1c, thyroid, prolactin, and AMH — everything needed for a complete diagnosis, not just a partial picture. Results reviewed with a GP who specialises in hormonal health.
A PCOS diagnosis is not just about periods and fertility — it carries long-term metabolic health implications that need to be monitored. Women with PCOS have a higher lifetime risk of type 2 diabetes, cardiovascular disease, non-alcoholic fatty liver disease, and endometrial cancer (due to infrequent periods and absent ovulation leading to unopposed oestrogen exposure).
This is why a proper PCOS workup includes a metabolic screen — not just hormone levels. Annual monitoring of fasting glucose, HbA1c, lipid profile, and blood pressure is recommended for women with confirmed PCOS, particularly those with insulin resistance or overweight.
Full PCOS hormone panel including metabolic screen — LH, FSH, testosterone, SHBG, DHEAS, insulin, HbA1c, thyroid, prolactin and AMH.
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