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A gonadotropin is one of a group of protein hormones considered vital to human reproduction. Gonadotropins are produced by cells called gonadotropes located in the pituitary gland. The two principal types of gonadotropin are follicle-stimulating hormone (FSH) and luteinizing hormone (LH). Human chorionic gonadotropin (hCG) is a third type that is produced during pregnancy. As these hormones are an important part of the reproductive process, gonadotropins are often used as a treatment for infertility in both men and women.
Both FSH and LH production are controlled by another hormone called gonadotropin-releasing hormone (GnRH). The action of GnRH is triggered during puberty. GnRH is secreted by the hypothalamus and released into the pituitary in pulses. The secretion of LH and FSH is controlled by how large these pulses of GnRH are, and how frequently they occur. Men tend to experience pulses of GnRH at a fairly constant rate, while women usually experience the release of the hormone at varying rates, with a peak that occurs just before ovulation.
In women, this follicle-stimulating hormone causes the ovarian follicles to grow and mature. Specifically, FSH acts on cells called granulosa cells that surround the immature egg, or oocyte, inside the ovarian follicle, causing them to grow and increase in number. FSH also stimulates the growth of germ cells in men, causing immature sperm cells called spermatogonia to ripen into mature sperm cells called spermatozoa.
When an ovarian follicle reaches the height of its maturity, also termed the late tertiary phase of development, heightened levels of estrogen in a woman's body activate a large surge of luteinizing hormone. This surge typically lasts for a day to two days before the high levels of LH trigger ovulation. LH also begins a process that converts the follicle into a corpus luteum, a structure in the ovary that produces the hormone progesterone and is vital to the maintenance of pregnancy. In men, LH also plays an important role by encouraging the Leydig cells found in the testes to produce the hormone testosterone.
Gonadotropin injection therapy is sometimes used as a fertility treatment, acting directly on the ovaries of women who wish to become pregnant. Men may also receive gonadotropin injections as a treatment for an abnormally low sperm count. GnRH, FSH, and hCG can all be used to treat infertility, as can another type of hormone called human menopausal gonadotropin (hMG). This hormone is extracted from the urine of women who have undergone menopause, and is essentially a combination of FSH, LH, and hCG. Gonadotropin injections do not always result in a successful pregnancy, with women who take the injections becoming pregnant 20% to 60% of the time.