If insulin helps drive androgen excess, why not order fasting insulin for every patient with PMOS (formerly PCOS)? Because a hormone’s role in disease biology and the usefulness of a routine blood test are different questions. Insulin can amplify ovarian steroid production, yet an insulin level is not recommended as a routine diagnostic or management test.
The terminology has recently changed: the condition long known as polycystic ovary syndrome (PCOS) is now called polyendocrine metabolic ovarian syndrome (PMOS). Many board resources and clinical records will continue to use PCOS during the transition, so recognizing both names is useful.
The pathway explains why insulin matters
In the two-cell model, luteinizing hormone (LH) stimulates ovarian theca cells to produce androgen precursors, including androstenedione and testosterone. Follicle-stimulating hormone (FSH) supports granulosa-cell aromatase, which converts theca-derived androgens to estrogens. Insulin can act alongside LH to amplify theca-cell steroidogenesis. The degree to which these signals contribute varies across patients; one uniform hormone pattern does not describe every case.
Insulin also acts beyond the ovary. Higher insulin can suppress hepatic production of sex hormone–binding globulin (SHBG), leaving a greater fraction of circulating testosterone unbound. Keep the distinction clear: ovarian steroidogenesis can increase androgen production, while reduced SHBG can increase free testosterone without requiring a matching rise in total testosterone.
This pathway helps explain why normal body weight does not rule out PMOS, and why insulin resistance is relevant even when a patient’s glucose is currently normal. But neither insulin resistance nor an elevated LH-to-FSH ratio is, by itself, a diagnostic criterion. A supportive mechanism is not a required finding.
Why a fasting insulin value is not the answer
A fasting insulin assay measures the concentration in one blood sample. It does not directly show how sensitive muscle, liver, or ovarian cells are to insulin, nor does it reveal how strongly ovarian steroidogenic pathways respond in that individual. The available insulin assays have limited clinical relevance for routine care, and a result does not reliably establish PMOS or select treatment.
Normal glucose also does not prove that insulin signaling is normal. A person may compensate for insulin resistance by secreting more insulin while maintaining glucose within the reference range. Conversely, a single insulin measurement cannot establish that compensation is occurring or quantify ovarian insulin effects. Do not substitute a fasting insulin result for a clinical diagnosis of insulin resistance—or infer its absence from a normal glucose value.
Match the test to the clinical question
For routine care, ask whether the patient has dysglycemia, not whether an insulin concentration seems high. International guidance recommends assessing glycemic status at diagnosis in adults and adolescents with PMOS, regardless of body mass index. The preferred test is a 75-g oral glucose tolerance test (OGTT); it assesses the glucose response, not ovarian insulin signaling.
| Test or assessment | What it answers | Important limit |
|---|---|---|
| Fasting insulin | A single circulating insulin concentration | Not recommended for routine assessment of insulin resistance in PMOS |
| 75-g OGTT | Glycemic status, including impaired glucose tolerance | Does not directly measure ovarian insulin action; preferred for assessing glycemia in PMOS |
| Fasting plasma glucose and/or HbA1c | Glycemic status when an OGTT cannot be performed | Less accurate than the OGTT for this assessment |
Reassess glycemic status every one to three years, with the interval guided by individual risk factors. Do not reserve screening for patients with higher weight: the guideline recommends assessment irrespective of BMI.
Pregnancy planning makes the distinction practical
In obstetric practice, the metabolic question becomes especially relevant before conception and during pregnancy. The international guideline recommends considering an OGTT when a patient with PMOS is planning pregnancy or seeking fertility treatment. If testing was not performed before conception, it may be offered at the first prenatal visit; the guideline also recommends offering an OGTT at 24–28 weeks. Coordinate this with the patient’s existing results and local prenatal diabetes-screening protocols.
A useful counseling explanation is: “Insulin can contribute to hormone changes in this condition, but a blood insulin level is not a reliable routine way to measure that effect. We can assess the glucose-related risks with tests that guide care, including an oral glucose tolerance test when appropriate.” This acknowledges the mechanism without turning an uncertain insulin number into a diagnosis.
Common reasoning traps
- Confusing mechanism with measurement: Insulin can contribute to androgen excess without a fasting insulin assay being a useful routine test.
- Treating normal glucose as proof of normal insulin action: Glucose testing evaluates glycemia; it does not directly measure tissue-specific insulin signaling.
- Restricting screening by BMI: Glycemic assessment is recommended at diagnosis regardless of BMI.
- Promoting LH:FSH ratio to a diagnostic criterion: LH can influence ovarian steroidogenesis, but the ratio is not a required PCOS/PMOS finding.
Practical takeaways
- Insulin may amplify LH-stimulated theca-cell androgen production and lower hepatic SHBG, increasing free testosterone.
- Increased production and increased free-androgen availability are related but distinct mechanisms.
- Routine insulin assays are not recommended for assessing insulin resistance in PMOS.
- Assess glycemic status at diagnosis regardless of BMI; the 75-g OGTT is the most accurate recommended test.
- In pregnancy planning, use the appropriate OGTT pathway and coordinate with local prenatal screening practice.
