FSH is the abbreviation for "follicle stimulating hormone". As the name suggests, its main function in the female cycle is to stimulate growth of your follicles.
Every month, in the first half of your cycle, several eggs mature in your follicles, one (sometimes several) of which leaves the ovary in the middle of the cycle during ovulation and migrates towards the uterus. In men, the follicle-stimulating hormone stimulates the new production of sperm cells in the testicles.
As you may know, the female cycle is controlled by a complex interaction of hormones in which your brain, namely the pituitary gland (hypophysis), the hypothalamus and the ovaries are also involved. The gonadotropin-releasing hormone (GnRH) is released from the hypothalamus into the pituitary gland. Gonadotropins are sex hormones that stimulate the growth of eggs which include FSH and LH (luteinizing hormone). Certain cells in the pituitary gland react to this by releasing FSH and LH.
FSH plays a crucial role for the first half of your cycle
Together with LH, FSH ensures the maturation of follicles in the ovaries. Several follicles mature in the ovaries in each cycle and each follicle contains an immature egg cell that matures under the influence of FSH. The follicles themselves produce the female sex hormone estrogen. The levels of FSH, LH and estrogen, increase until ovulation and because the follicles grow steadily under the influence of FSH, your estrogen level also rises.
This is important for a possible pregnancy because estrogens are responsible, among other things, for the build-up of the uterine lining in the first phase of the cycle. Shortly before ovulation they also ensure that the cervix opens and the cervical mucus becomes thinner so that the sperm can pass through it more easily. When the estrogen level reaches a certain level, the LH is signalled to trigger ovulation. The LH level now rises sharply.
During ovulation, the follicle bursts open and releases the mature egg which leads to it now being able to "migrate" in the fallopian tubes towards the womb. In the first twelve to 24 hours after ovulation, the egg is ready for fertilization.
Immediately after ovulation, the production of FSH decreases and LH, estrogen and FSH drop sharply. The empty follicle now forms the so-called corpus luteum, which produces more progesterone in the second half of the cycle. Among other things, this hormone causes a possibly fertilized egg cell to be better able to implant itself in your uterine lining (endometrium). If a pregnancy occurs, your progesterone level remains high, if pregnancy does not occur, progesterone and estrogen levels drop towards the end of your cycle.
FSH throughout your cycle
At the beginning of the cycle (the cycle begins with the first day of menstruation) the FSH value is still low. Only those follicles that are suitable for further development and have the necessary FSH receptors respond to FSH. About 15 to 20 of these primary follicles grow in the first half of your cycle and one of these follicles responds so well to the FSH that it produces more estrogen than all the others. In reproductive medicine, this knowledge is used to control the maturity of this follicle, called the dominant follicle, based on its estrogen content.
Until shortly before ovulation, the dominant follicle produces so much estrogen that it eventually inhibits the release of the follicle-stimulating hormone- this way, it further extends its "lead" over the other follicles, as these have a much lower sensitivity to FSH and are therefore less strongly stimulated to grow. For this reason, there is also a slight drop in the FSH value shortly before ovulation, before its value rises again at the time of ovulation itself. In rare cases, more than one follicle matures, so that several eggs can be fertilized.
Did you know?
The Pearl Fertility algorithm “uses” this typical course of FSH by detecting its characteristic drop shortly before ovulation which allows Pearl to make an even more accurate ovulation determination than tests which only test LH.
What is a good FSH value?
The FSH value can be determined from your blood. In women, it fluctuates during the cycle: While it is still relatively low on the first day of the cycle, it first rises continuously, then drops slightly just before ovulation and rises sharply at the time of ovulation. In the second half of the cycle, the FSH value drops again. During pregnancy, the FSH value is naturally very low.
Please note that the standard values for FSH may differ from those mentioned here depending on the laboratory. Results should always be evaluated and assessed by the treating physician.
Follicular phase: 2.5-11 U/l
Ovulation: 8.3-16 U/l
Luteal phase: 2.5-11 U/l
Postmenopause: 27-82 U/l
Low FSH value: Infertility as a possible consequence
Low FSH values indicate an underfunction of the pituitary gland or hypothalamus. If too little or no follicle-stimulating hormone (FSH) is produced, and if this is recognized as a reason for infertility, a chemically produced substance which, like the hormone, stimulates the ovaries can be administered. If hormone treatment for infertility is administered, the FSH can lead to increased egg maturation. In women who tend to form many and/or too small follicles, overstimulation can occur and multiple pregnancies must be expected. Medical supervision is essential during hormone treatment.
Since the hormone prolactin inhibits the release of FSH and LH, an increased prolactin level can lead to very low FSH values. This occurs, for example, in the case of certain tumors, endometriosis or of course when breastfeeding.
A too low FSH level can also be caused by underweight. In this case the production of FSH and LH is stopped and the estrogen level is extremely low. Often the period also fails to appear. In order to bring the levels back to a normal level, it is usually sufficient to return the weight to normal.
FSH value too high: Possible causes
An excessively high FSH value can have various reasons. With increasing age, both FSH and LH increase naturally. Especially during the years before the menopause the value increases - this is a completely normal process and part of aging. The natural cause here is the interaction of hormones: while the production of estrogens decreases, more and more FSH is released to stimulate estrogen production again. In younger women, a high FSH level can indicate the premature onset of menopause.
By the way: FSH also plays a role for male fertility. In men, a high FSH level can for example indicate a disturbance in sperm formation or an underfunction of the testicles.