Lack of correlation between maximum early follicular phase serum follicle stimulating hormone concentrations and menstrual cycle characteristics in women under the age of 35 years

The gradual increase in follicle stimulating hormone (FSH) concentrations in women approaching menopause results from the depletion of the ovarian follicular pool, a process referred to as 'ovarian ageing'. This study investigates whether variable endogenous FSH concentrations, as have been observed in normo-ovulatory young women, are related to menstrual cycle characteristics, including predictors of ovarian ageing. Serum concentrations of immunoreactive FSH, oestradiol, and inhibin-A and inhibin-B were measured, and follicular growth was assessed by transvaginal ultrasound throughout the follicular phase in 39 healthy volunteers (20-35 years) with regular menstrual cycles. Median serum FSH concentration on cycle day 3 was 5.1 IU/l (range 3.6-11.2), and median maximum follicular phase FSH was 6.2 IU/l (range 4.3-11.2), observed on cycle day 6 (range 2-15). Maximum FSH concentrations were not correlated with age or cycle length, nor with maximum inhibin-B. The number of small (<10 mm) antral follicles on cycle day 3 was 11 (range 4-21) and was not correlated with age, nor with maximum FSH. Inhibin-A remained low until a significant rise on cycle day 9 (range 3-12), which was significantly correlated with the late follicular rise in oestradiol (r = 0.56, P = 0.01). These observations indicate a lack of correlation between maximum follicular phase serum FSH concentrations and parameters of ovarian ageing in women under the age of 35 years. In addition, FSH concentrations assessed on cycle day 3 represent an underestimation of maximum early follicular phase FSH. Distinct individual differences in intra-ovarian modification of FSH action, resulting in differences in the FSH threshold for stimulation of ovarian function, may be operative.