LH to FSH Ratio in PCOS: Limited Diagnostic Utility
The LH to FSH ratio is NOT recommended as a diagnostic criterion for PCOS and should not be routinely used in clinical practice. Current evidence-based guidelines explicitly exclude this ratio from diagnostic criteria, favoring the Rotterdam criteria instead 1.
Why the LH:FSH Ratio Is Not Used for Diagnosis
The Rotterdam criteria (endorsed by the Endocrine Society) require only two of three features for PCOS diagnosis 1:
- Androgen excess (clinical or biochemical)
- Ovulatory dysfunction
- Polycystic ovaries on ultrasound
Notably absent: any requirement for LH:FSH ratio measurement.
Evidence Against Using LH:FSH Ratio
The most compelling evidence comes from a controlled study showing that the median LH:FSH ratio did not differ significantly between PCOS patients (1.6) and normal controls (1.2), with only 7.6% of PCOS samples showing ratios above 3 2. This demonstrates poor discriminative power for diagnosis.
What the Ratio Actually Reflects (If Measured)
While not diagnostic, when an elevated LH:FSH ratio is present (typically >2), recent research suggests it may identify a neuroendocrine-dominant PCOS phenotype characterized by 3:
- Elevated anti-Müllerian hormone (AMH) levels (strong correlation: r=0.65)
- Prominent polycystic ovarian morphology
- Severe anovulation and oligo/amenorrhea
- Minimal association with metabolic dysfunction (weak correlation with insulin resistance)
- No correlation with hyperandrogenism (serum androgens or hirsutism scores)
Clinical Implications When Ratio Is Elevated
Patients with phenotype D (ovulatory dysfunction + polycystic ovaries, but without hyperandrogenism) show the highest LH:FSH ratios (1.75 ± 0.87) 3. This suggests the ratio, when elevated, may indicate a subgroup where:
- Ovulation induction is the primary therapeutic need
- Metabolic interventions may be less critical
- Anti-androgenic treatments are unnecessary
Common Pitfalls to Avoid
Do not order LH:FSH ratio for PCOS diagnosis - it adds no diagnostic value and may cause confusion 2
Recognize assay variability - Different measurement systems require different cut-offs (e.g., 1.21 vs 1.51 for the same clinical significance) 4, making standardization problematic
Understand timing issues - The ratio varies significantly throughout the menstrual cycle in both PCOS and normal women 2, reducing reliability
Avoid over-interpretation - Even when elevated, the ratio does not confirm or exclude PCOS; it may only suggest phenotypic characteristics 3
What to Measure Instead
For PCOS diagnosis, focus on 1:
- Total or free testosterone (for androgen excess)
- Clinical assessment (hirsutism, acne, androgenic alopecia)
- Menstrual history (for ovulatory dysfunction)
- Pelvic ultrasound (for polycystic ovarian morphology)
- Exclusion of mimics: TSH, 17-hydroxyprogesterone, prolactin
For metabolic assessment:
- Fasting glucose and 2-hour OGTT
- Lipid profile
- Consider AMH if available (better discriminative power than LH:FSH) 5
The LH:FSH ratio represents outdated diagnostic thinking and should be abandoned in favor of the evidence-based Rotterdam criteria.