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Variability of day 3 follicle-stimulating hormone levels in eumenorrheic women.

OBJECTIVE
To assess the intercycle variability of early follicular follicle-stimulating hormone (FSH) and estradiol (E2) levels in eumenorrheic women according to age.


STUDY DESIGN
Forty-eight female volunteers with regular menstrual cycles were studied. Serum samples were obtained on cycle day 3 in multiple cycles over a one-year period. Serum was also obtained on consecutive cycle days 2, 3 or 4 in a single cycle in 20 participants. All samples were assayed for FSH and E2 by radioimmunoassay.


RESULTS
In women under 40 years of age, all initial day 3 FSH levels were < 20 mIU/mL, and 15% of subjects had a subsequently elevated day 3 FSH level (> or = 20 mIU/mL) within the year. In eight women 40-44 years of age, 75% had an initial day 3 FSH level < 20 mIU/mL, but half of them subsequently had an elevated level, > 20 mIU/mL, within the year. Between cycle days 2, 3 and 4, FSH (coefficient of variation [CV] = 14.8%, 95% confidence interval (CI) = 11.4-18.3%) varied less than E2 (CV = 31.1%, 95% CI = 21.5-40.6%).


CONCLUSION
A single day 3 FSH level < 20 mIU/mL is highly predictive of all subsequent values within a year and occurred only in women under 40. An FSH level drawn on cycle day 2 or 4 will probably be within 18% of the day 3 value, but the E2 level may vary by up to 40%.