What are the best days of the menstrual cycle to check Follicle-Stimulating Hormone (FSH) and estradiol levels for evaluating ovarian function and reserve?

Optimal Timing for FSH and Estradiol Testing During Menstrual Cycle

FSH and estradiol levels should be measured during days 2-5 of the menstrual cycle (early follicular phase) for most accurate assessment of ovarian function and reserve. 1

General Principles for Hormone Testing

• For regular menstrual cycles:

  • Test FSH and estradiol between days 2-5 of the cycle (early follicular phase)
  • Day 3 is traditionally considered optimal, but days 2-5 provide reliable results 2
  • Morning testing is preferred, especially for prolactin measurements 1

• For irregular cycles:

  • If amenorrhea: measure FSH and estradiol randomly
  • If oligomenorrhea: measure during early follicular phase (days 2-5) 3

Specific Testing Recommendations by Condition

For Ovarian Reserve Assessment

• Test FSH and estradiol on days 2-5 of cycle
• FSH values increase with age (approximately 0.11 IU for each year of age) 4
• FSH levels >20 mIU/mL suggest diminished ovarian reserve 5
• Consider adding Anti-Müllerian Hormone (AMH) testing for women ≥25 years 1
  • AMH can be measured on any day of the cycle (not cycle-dependent)
  • AMH represents the best endocrine marker for age-related decline in ovarian reserve 3

For Premature Ovarian Insufficiency (POI)

• Diagnosis requires:
  • Amenorrhea for at least 4 months
  • Two elevated FSH measurements >35-40 IU/L at least one month apart
  • Estradiol levels <50 pmol/L 1
• Testing should be performed after stopping oral contraceptives/hormone therapy for at least 2 months 3

For Polycystic Ovary Syndrome (PCOS)

• Test on days 3-6 of the cycle when possible 1
• Look for:
  • LH/FSH ratio >2 (suggestive of PCOS)
  • Elevated testosterone (>2.5 nmol/L)
  • Low progesterone (<6 nmol/L) indicating anovulation 3, 1

Interpreting Test Results

Normal Values (Early Follicular Phase)

• FSH: Values vary by age; typically <10 IU/L in reproductive-age women
• Estradiol: 20-160 pg/mL (73-587 pmol/L)
• LH: 2-10 IU/L

Abnormal Values

• FSH >35 IU/L suggests ovarian failure 1
• LH/FSH ratio >2 suggests PCOS 1
• FSH >20 mIU/mL suggests diminished ovarian reserve 5

Important Considerations

• FSH shows less day-to-day variability (CV=14.8%) than estradiol (CV=31.1%) between days 2-4 5
• A single FSH measurement <20 mIU/mL is highly predictive of subsequent values within a year for women under 40 5
• For women over 40, FSH levels may fluctuate significantly between cycles, requiring repeat testing 5
• Combination of FSH and LH on day 1 provides the best prediction of ovarian reserve differences between age groups 6

Common Pitfalls to Avoid

• Testing during hormonal contraceptive use (wait at least 2 months after discontinuation)
• Relying solely on a single FSH measurement in women over 40
• Testing prolactin levels after physical activity or stress (should be measured in resting state)
• Failing to consider age-specific reference ranges when interpreting FSH values
• Measuring hormones outside the early follicular phase when evaluating ovarian reserve

By following these guidelines for timing FSH and estradiol measurements, clinicians can obtain the most accurate assessment of ovarian function and reserve to guide diagnosis and management of various reproductive disorders.