PCOS blood test results: what to look for and what the ranges mean

Polycystic ovary syndrome (PCOS) is one of the most common hormonal conditions in women of reproductive age, affecting approximately 1 in 10 in the UK. Despite its prevalence, it is frequently misdiagnosed or dismissed — partly because no single blood test confirms it, and partly because reference ranges for hormonal markers are often poorly calibrated for the PCOS population.

How PCOS is diagnosed

The most widely used diagnostic criteria are the Rotterdam criteria, which require two of the following three features:

• Irregular or absent periods (oligo- or anovulation)
• Clinical or biochemical signs of elevated androgens (excess testosterone or its effects)
• Polycystic ovaries on ultrasound (12 or more follicles in either ovary, or ovarian volume above 10 mL)

Blood tests are used to assess the second criterion and to rule out other conditions that can mimic PCOS.

Key blood markers in PCOS

LH and FSH (luteinising hormone and follicle-stimulating hormone)

In PCOS, LH is often elevated relative to FSH. An LH:FSH ratio above 2:1 or 3:1 is a classic finding, though it is not present in all cases and is not required for diagnosis. Both hormones should ideally be tested on day 2–5 of the menstrual cycle (or at any time if periods are absent).

Testosterone (total and free)

Elevated testosterone is one of the biochemical hallmarks of PCOS. However, total testosterone can be within the reference range even when free testosterone (the biologically active fraction) is elevated. This is because PCOS is often associated with low sex hormone-binding globulin (SHBG), which means more testosterone is unbound and active.

AMH (anti-Müllerian hormone)

AMH is produced by ovarian follicles and is a marker of ovarian reserve. In PCOS, AMH is typically elevated — often two to three times higher than in women without PCOS — because the condition involves an increased number of small antral follicles. AMH above 10–12 pmol/L in a woman of reproductive age is consistent with PCOS, though ranges vary between labs.

Insulin and glucose

Insulin resistance is present in approximately 70% of women with PCOS, regardless of weight. However, standard NHS blood tests do not routinely include fasting insulin. HbA1c and fasting glucose are more commonly tested and can identify impaired glucose regulation, but they are less sensitive for early insulin resistance than a fasting insulin level or an oral glucose tolerance test.

Markers to rule out other conditions

Several conditions can mimic PCOS and should be excluded:

• TSH — thyroid dysfunction can cause irregular periods and elevated androgens
• Prolactin — elevated prolactin (hyperprolactinaemia) can suppress ovulation
• 17-hydroxyprogesterone — to exclude congenital adrenal hyperplasia (CAH)
• DHEAS — elevated DHEAS may indicate adrenal rather than ovarian androgen excess

Why reference ranges are often inadequate for PCOS

Standard reference ranges for hormones are derived from the general population and do not account for the specific patterns seen in PCOS. A testosterone result of 2.0 nmol/L might be within the female reference range (typically 0.3–2.5 nmol/L) but could still be associated with significant symptoms if SHBG is low. Similarly, an AMH of 8 pmol/L might be "normal" but is significantly lower than the 15–25 pmol/L commonly seen in PCOS.

This is why reading PCOS blood results requires looking at the pattern across multiple markers, not just whether each individual result is flagged.

What to ask your GP or gynaecologist

• Can I have testosterone, SHBG, LH, FSH, AMH, prolactin, and TSH tested?
• Should I have a pelvic ultrasound as well?
• Is my insulin resistance being assessed, and if so, how?
• What is my free androgen index?
• If PCOS is confirmed, what management options are available to me?