FSH (Follicle stimulation hormone)
FSH (Follicle stimulation hormone) is a hormone that stimulates the ovary to produce oocytes. FSH is produced from the pituitary (or hypophysis). The pituitary is an endocrine gland in the base of the brain that secretes hormones for the control of the function of the thyroid gland, the adrenal glands, the gonads (testes-ovaries) and it also secretes growth hormone and prolactin. For the function of the gonads, the pituitary secretes two hormones that are called gonadotropins. The follicle stimulating hormone (FSH) promotes the development of the follicles, and the luteinising hormone (LH) promotes steroidogenesis in the ovary and is responsible for the final maturation of the oocyte, the ovulation and the beginning and continuation of the function of the corpus luteum.
The two gonadotropins FSH and LH are produced and stored in the gonadotroph cells of the pituitary and they exert their biological action on the gonads (testes-ovareis) via their receptors located on the target organs.
The FSH and LH secretion is controlled by the GnRH hormone, which is secreted in pulses by the pituitary. Thus, the FSH and LH gonadotropin secretion also takes place in pulses. As we have seen, the pituitary is under control of the hypothalamus, through the GnRH. On the contrary, the hormone secretion from the pituitary and the hypothalamus is affected by the levels of ovarian hormones. Therefore, there is a hypothalamic-pituitary-ovarian hormone axis, with a mechanism of negative feedback. Oestradiol (E2) produces in the ovary has a negative feedback on the on the FSH secretion and a positive feedback on the LH secretion.
Measurements have shown that the GnRH levels are lower in children and higher in adults. GnRH levels are also increased during the middle of the menstrual cycle (ovulation). GnRH levels are higher in a post-menopausal woman that in a woman during her reproductive age. Similar to that is the course of the gonadotropin levels. The post-menopausal period in a woman is characterized by almost the cancellation of the ovarian function and an excessive rise in the FSH and LH secretion by the pituitary.
The rise of FSH and LH levels during menopause is not sudden, as when the ovaries are removed. A period of gradual reduction of the ovarian function, i.e. a period of reduced fertility whose last stop is menopause, has preceded. During this time increased FSH levels are observed.
It thus appears as an effort of the pituitary through the continuously increasing gonadotropin levels to stimulate the ovary by recruiting and developing follicles in order to preserve fertility. The ovary however has a reduced reserve and cannot effectively respond to the pituitary commands.
In conclusion, increased FSH levels show reduced ovarian reserve and increased biological ovarian age.