Significance of FSH to LH Ratio in Evaluating Reproductive Disorders
An LH to FSH ratio greater than 2 is a significant diagnostic indicator of polycystic ovary syndrome (PCOS), while a ratio less than 1 is typically seen in functional hypothalamic amenorrhea (FHA). 1
Diagnostic Value of FSH:LH Ratio
PCOS Diagnosis
- LH:FSH ratio >2 is considered abnormal and strongly suggestive of PCOS 2, 1
- Reflects the underlying pathophysiology of PCOS:
- Results in clinical manifestations including:
- Hyperandrogenism
- Hirsutism
- Follicular arrest
- Ovarian acyclicity 2
Functional Hypothalamic Amenorrhea (FHA)
- LH:FSH ratio ≤1 is observed in approximately 81.5% of FHA patients 3
- Only 2.2% of FHA patients have an LH:FSH ratio ≥2.1 3
- Helps differentiate FHA from PCOS, which is critical for appropriate management 2
Proper Measurement Technique
- LH and FSH levels should be measured between days 3-6 of the menstrual cycle 2, 1
- Calculation should be based on an average of three estimations taken 20 minutes apart 2
- Laboratory variations in reference ranges and units may affect interpretation 1
Clinical Applications
Reproductive Aging
- Progressive increase in FSH levels begins as early as age 29-30 years 4
- LH levels show significant increase at age 35-36 years 4
- These changes represent early endocrine markers of reproductive aging
Fertility Assessment
- High FSH:LH ratio (>3) has been associated with:
- Fewer mature oocytes during IVF
- Lower implantation rates
- Lower clinical pregnancy rates 5
- May serve as an early biomarker of poor ovarian response even with normal basal FSH levels 5
Menstrual Cycle Characteristics
- Low FSH:LH ratios (<1.34) are associated with:
- Increased follicular phase length (+2.4 days)
- Lower ovulatory rate 6
- Low FSH levels (<2.94 mIU/mL) correlate with:
- Longer follicular phases (+2.6 days)
- Shorter luteal phases (-1.1 days) 6
Important Caveats and Limitations
- The FSH:LH ratio alone is insufficient for diagnosis and must be considered alongside other clinical features 1
- There is marked overlap between sexes after infancy and throughout adulthood in healthy individuals 7
- The ratio has limited predictive value on luteal phase characteristics 6
- Laboratory methods and reference ranges may vary between facilities 2, 1
- In patients with both FHA and polycystic ovarian morphology (PCOM), the LH:FSH ratio remains similar to those with FHA without PCOM 3
Clinical Algorithm for Using FSH:LH Ratio
- Measure FSH and LH between days 3-6 of menstrual cycle (or randomly in amenorrheic patients)
- Calculate LH:FSH ratio
- Interpret results:
- Ratio >2: Suggestive of PCOS (evaluate for other PCOS criteria)
- Ratio ≤1: Consistent with FHA (evaluate for causes such as excessive exercise, underweight, caloric deficiency, stress)
- Ratio 1-2: Indeterminate (requires additional clinical and laboratory assessment)
- Consider additional parameters:
- Presence of hirsutism, acne, or other signs of hyperandrogenism
- Menstrual pattern (oligomenorrhea, amenorrhea)
- Endometrial thickness (thin in FHA)
- Polycystic ovarian morphology on ultrasound
- Testosterone levels
This structured approach to interpreting the FSH:LH ratio provides valuable diagnostic information for reproductive disorders, particularly in distinguishing between PCOS and FHA, which have significant implications for patient management and outcomes.