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

Abstract

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%.