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There are several reproductive hormones in the human body, including testosterone, estrogen, and progesterone. Human chorionic gonadotrophin (HCG), follicle stimulating hormone (FSH), gonadotrophin releasing hormone (GnRH), and luteinizing hormone (LH) are also essential to the reproductive cycle. Testosterone is often viewed as a male hormone, but is also found in females. Similarly, estrogen is seen as a female hormone, but is also a necessary for males. Some of the other reproductive hormones stimulate the release of estrogen, testosterone, and progesterone, while HCG is necessary during pregnancy.
Even though testosterone is necessary for women, many people think of it as the male hormone. In women, this hormone is responsible for tasks such as maintaining muscle mass. In men, it is responsible for sperm production and many male characteristics. For example, the surge in testosterone at puberty causes males to grow facial and pubic hair. Testosterone release is controlled by FSH, a hormone that aids in sperm production. FSH release, in turn, is controlled by GnRH. Rising levels of testosterone stop the production of GnRH, preventing the release of FSH and thus, testosterone. This helps keep testosterone levels from climbing too high.
Normally considered the female reproductive hormone, estrogen stimulates sexual characteristics, such as breast, uterine, and vaginal development. It is also necessary, however, for the maintenance of the male reproductive system and the healthy development of sperm. In women, the rise in estrogen levels at puberty causes the development of breasts, and prepares the vagina and uterus for pregnancy. In response to follicle development, estrogen stimulates the uterine lining, called the endometrium, to start thickening. The follicle is a structure within the ovaries where eggs grow.
Like testosterone, estrogen is stimulated by FSH, which in turn is stimulated by GnRH. Rising estrogen levels inhibit the production of GnRH, thus preventing estrogen levels from surging too high. Low estrogen levels also allow endometrium to shed if a woman does not get pregnant. This shedding is called menstruation.
GnRH stimulates the production of luteinizing hormone (LH), another of the reproductive hormones. In men, LH helps in testosterone production and release. In women, the hormone transforms the follicle into a corpus luteum after the egg — or oocyte — has been released. Once it has been transformed, the corpus luteum can start to secrete progesterone in accordance with LH stimulation.
Progesterone is a female reproductive hormone that prevents the uterus from contracting and shedding the endometrium too soon. The hormone also prevents a new follicle from developing once an ooycte has been released. Like estrogen, rising levels of progesterone reduces the production of GnRH, which reduces LH, preventing progesterone levels from surging too high.
The presence of reproductive hormones, such as progesterone, is essential to a successful pregnancy. Since the rise of many of these hormones inhibits their continued release, another hormone must be in place for pregnancy to continue. Human chorionic gonadotrophin (HCG) is a hormone that is released by the trophoblast, cells that will eventually become the placenta and the umbilical cord, after the embryo implants itself into the endometrium. Unlike many of the other reproductive hormones, HCG production is not affected by the release or inhibition of GnRH. HCG takes on the role of FSH and LH, allowing estrogen and progesterone to still be released even after GnRH is no longer being released.