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) concentrationsin women approaching menopause results from the depletion of the ovarianfollicular pool, a process referred to as 'ovarian ageing'. This studyinvestigates whether variable endogenous FSH concentrations, as have beenobserved in normo-ovulatory young women, are related to menstrual cyclecharacteristics, including predictors of ovarian ageing. Serumconcentrations of immunoreactive FSH, oestradiol, and inhibin-A andinhibin-B were measured, and follicular growth was assessed by transvaginalultrasound throughout the follicular phase in 39 healthy volunteers (20-35years) with regular menstrual cycles. Median serum FSH concentration oncycle day 3 was 5.1 IU/l (range 3.6-11.2), and median maximum follicularphase FSH was 6.2 IU/l (range 4.3-11.2), observed on cycle day 6 (range2-15). Maximum FSH concentrations were not correlated with age or cyclelength, nor with maximum inhibin-B. The number of small (<10 mm) antralfollicles on cycle day 3 was 11 (range 4-21) and was not correlated withage, nor with maximum FSH. Inhibin-A remained low until a significant riseon cycle day 9 (range 3-12), which was significantly correlated with thelate follicular rise in oestradiol (r = 0.56, P = 0.01). These observationsindicate a lack of correlation between maximum follicular phase serum FSHconcentrations and parameters of ovarian ageing in women under the age of35 years. In addition, FSH concentrations assessed on cycle day 3 representan underestimation of maximum early follicular phase FSH. Distinctindividual differences in intra-ovarian modification of FSH action,resulting in differences in the FSH threshold for stimulation of ovarianfunction, may be operative.