PCOS and your hormones: what a blood test can and can't show.
Blood tests show the hormone patterns typical of PCOS: raised androgens, a relatively high LH, often a high AMH, and frequently signs of insulin resistance. What they cannot do is diagnose it. PCOS is a clinical diagnosis a doctor makes from your symptoms, your cycles and your results together.
Polycystic ovary syndrome affects roughly one in ten women of reproductive age, and a large share of them go undiagnosed for years. Part of the problem is the name: it points at the ovaries, when PCOS is really a whole-body hormonal and metabolic condition.
How PCOS is diagnosed
The international guideline, led from Australia, uses three features. An adult needs two of the three, with other causes ruled out:
- Irregular or absent ovulation, which usually shows up as irregular, infrequent or absent periods.
- Signs of high androgens, either on a blood test (raised testosterone or free androgen index) or clinically (acne, excess hair growth, hair thinning on the scalp).
- Polycystic ovaries on ultrasound, or a raised AMH used in its place in adults.
The hormone patterns, marker by marker
Testosterone, SHBG and the free androgen index
Total testosterone tells you how much androgen is circulating. SHBG, sex hormone binding globulin, is the protein that carries it; when SHBG is low, more testosterone is free and active. Divide one by the other and you get the free androgen index, which is more useful than testosterone alone and is the reason both markers are measured together. Free testosterone is calculated from total testosterone and SHBG rather than measured directly, because direct assays are unreliable at female levels. DHEAS, an androgen from the adrenal glands, is a second-line marker: it is read mainly where testosterone and the free androgen index are normal, and it adds limited extra information.
LH and FSH
LH is often relatively higher than FSH in PCOS, and you will see the "LH to FSH ratio" quoted online as though it settles the question. It does not. A raised ratio is a supporting clue, plenty of women with PCOS have a perfectly ordinary ratio, and the ratio is not part of the diagnostic criteria at all.
AMH
AMH is made by the small follicles in the ovaries, and in PCOS there are more of them, so AMH tends to run high. In adults it can stand in for the ultrasound criterion, though it is not used that way in teenagers and the cut-off is specific to the laboratory assay. A high AMH by itself means little; in a woman with irregular cycles and raised androgens it adds weight. AMH is not part of the standard KnowLuna PCOS panel, so a doctor requests it separately when it would help.
Fasting glucose, HbA1c and lipids
This is the part the name leaves out. Many women with PCOS have some degree of insulin resistance, whatever their weight, and it drives a good deal of what they feel. Fasting glucose and HbA1c (a three-month average of blood sugar) show where you sit, and lipids give the long-term cardiovascular baseline. Fasting insulin is sometimes discussed, but the 2023 International PCOS Guideline recommends against it for assessing an individual, so it is not in the panel. Our article on insulin resistance testing in women covers what those numbers mean.
Ruling other things out
TSH and prolactin are commonly checked, because thyroid disorders and raised prolactin both cause irregular periods and can look a lot like PCOS from the outside.
| Marker | Typical PCOS pattern | What it does and does not prove |
|---|---|---|
| Testosterone and free androgen index | Normal to moderately raised | Meets the androgen criterion when raised. A normal result does not rule PCOS out if you have clinical signs. |
| LH to FSH ratio | Often above 1, sometimes about 2 or more | A supporting clue only. Not part of the criteria. |
| AMH | Often high for age | Can replace ultrasound in adults, but not in adolescents. Not diagnostic alone. |
| Fasting glucose and HbA1c | Normal or raised | Describes metabolic risk, not the diagnosis. |
| TSH and prolactin | Normal in PCOS | Abnormal results mean another cause has to be considered and dealt with first. They do not rule PCOS out, and the two can coexist. |
Ranges vary by laboratory; your report shows the range your lab uses. Typical ranges on this page are drawn from Pathology Tests Explained (RCPA) and the sources listed below.
What a blood test cannot show
- It cannot confirm PCOS on its own. Two of three criteria must be met, and one of them is about your cycles, not your blood.
- It cannot rule PCOS out. Normal androgens are common in PCOS, particularly if you are on the combined pill.
- It cannot see your ovaries. That is an ultrasound, and in adults AMH can often stand in for it.
- It cannot tell you which "type" you have. The type labels circulating online are not a recognised medical classification. Doctors do describe four research phenotypes, which is a different thing.
Why timing and preparation matter
- Fasting. Insulin and lipids need a fasting sample, usually nothing but water for eight to twelve hours, so a morning collection is easiest.
- Cycle day. Where you have cycles, early in the cycle is best for LH and FSH. If your cycles are very irregular or absent, the doctor reads the results in that light.
- Contraception. The combined pill lowers androgens and raises SHBG, which can mask the very pattern you are looking for. See contraception and hormone tests for the detail.
- Morning collection. Testosterone is highest in the morning, so an early sample is the most interpretable.
Why getting the picture matters
PCOS is manageable, and managing it well changes long-term health. Understanding your own pattern, whether it is more about androgens, more about insulin, or both, is what lets a doctor tailor advice to you rather than hand you a generic leaflet. It is also what turns "irregular" from a mystery into a plan.
Common questions
Is AMH high in PCOS?
Often, yes. PCOS ovaries carry more small follicles, and each one makes AMH, so the level tends to run higher than expected for age. In adults a raised AMH can stand in for the ultrasound criterion. On its own it proves nothing, because AMH is also naturally higher in younger women and varies between laboratory assays.
What is the LH to FSH ratio in PCOS?
A ratio above 1, and sometimes around 2 or more, is commonly described in PCOS. It is a supporting clue rather than a test: many women with PCOS have a normal ratio, the ratio shifts across the cycle, and it is not part of the diagnostic criteria. Your doctor reads it alongside androgens, cycles and AMH.
Can PCOS be diagnosed with a blood test alone?
No. PCOS needs two of three features: irregular ovulation, high androgens (on blood tests or clinical signs), and polycystic ovaries on ultrasound or a raised AMH, with other causes excluded. Blood tests cover part of that picture and rule out thyroid and prolactin causes. A doctor puts the pieces together.
What are the three markers for PCOS?
If you mean blood markers, the ones that matter most are testosterone with SHBG (giving the free androgen index) and a metabolic marker such as fasting glucose or HbA1c, with LH, FSH, TSH and prolactin rounding out the panel. AMH is an optional add-on in adults rather than part of the standard panel. The three diagnostic features, though, are cycles, androgens and ovaries, not three blood tests.
Does the pill hide PCOS on a blood test?
It can. The combined pill lowers circulating androgens and raises SHBG, so the free androgen index often reads as normal even when the underlying pattern is not. It also masks the cycle irregularity that forms one of the criteria. Tell your doctor what you take, and they will read the results with that in mind.
Sources and further reading
- Monash University: International Evidence-based Guideline for the Assessment and Management of PCOS 2023
- Teede HJ et al. Recommendations from the 2023 International Evidence-based Guideline for the Assessment and Management of PCOS. J Clin Endocrinol Metab 2023
- Pathology Tests Explained (RCPA): testosterone
- Pathology Tests Explained (RCPA): sex hormone binding globulin (SHBG)
- Pathology Tests Explained (RCPA): luteinising hormone (LH)
- Jean Hailes for Women's Health: polycystic ovary syndrome (PCOS)
General information only, not medical advice. If your symptoms are severe or sudden, see a doctor promptly. This page does not replace a consultation with a doctor who knows your history; read our medical disclaimer.