Follicle-stimulating hormone (FSH) is essential to reproduction. It is produced by the pituitary gland situated just below the brain (behind the palate) and is released into the bloodstream. The hormone regulates the ovaries and is involved in the processes dealing with the growth and selection of the follicle which will later release the egg during ovulation.
Fluctuations in FSH levels are responsible for menstrual cycles and also for the fact that only one egg is produced during each cycle. It is essential, therefore, that there is constant dialogue between the ovaries and the pituitary gland so that the latter can consistently produce the correct quantity of FSH for a normal cycle.
This permanent exchange means that, in cases of poor ovarian function, the pituitary gland will try to compensate for this situation by increasing FSH excretion levels. It is for this reason that FSH levels in women going through the menopause are 20 times greater than in women whose ovaries are functioning normally. Therefore, when the ovary does not respond correctly, FSH increases significantly and systematically.
This relationship between FSH levels and ovarian failure has been used in ovarian reserve evaluation. The latter is understood to be the number of eggs remaining in the ovaries and also the quantity of eggs that can be retrieved when ovarian stimulation is carried out for in vitro fertilisation (IVF).
For many years, measuring FSH has been the most reliable marker in evaluating ovarian reserve, for predicting ovarian response to stimulation and for deciding which stimulation protocol is the most suitable for patients undergoing IVF treatment. However, measuring FSH as an ovarian response predictor has serious limitations:
- It needs to be carried out at a specific point during the menstrual cycle (between the second and fourth day of the cycle) because the levels vary greatly throughout. Care needs to be taken since measurements taken at the mid-point of the cycle can lead to a patient being incorrectly categorised as a poor responder.
- Cycles can vary greatly. Therefore, a single, isolated value can lead to incorrect diagnosis as a poor responder.
- Last of all, even with constant measurements taken at the right time during the cycle, the ability to predict ovarian response is very limited and, therefore, its reliability in terms of diagnosis is low.
The development of other, more reliable ovarian reserve markers, such as measuring anti-Müllerian hormone levels in blood and antral follicle counts by ultrasound, have replaced FSH levels for the evaluation of ovarian reserve. Using these markers, we are not only able to get a more reliable prediction of ovarian response but it is also more convenient and cheaper for the patient.