Hormones function as messengers within the body, traveling through the bloodstream to initiate specific actions in target organs and tissues. Gonadotropins are a specialized class of these chemical signals, named for their primary targets: the gonads (the ovaries in females and the testes in males). These hormones regulate the growth, development, and function of the reproductive system throughout a person’s life. They are central players in the endocrine system, ensuring the proper production of sex hormones and the maturation of gametes, which controls fertility.
Identifying the Gonadotropins and Their Production Sites
The class of gonadotropins includes three hormones: Follicle-Stimulating Hormone (FSH), Luteinizing Hormone (LH), and Human Chorionic Gonadotropin (hCG). FSH and LH are produced and secreted by specialized cells, called gonadotrophs, located in the anterior pituitary gland at the base of the brain. These two hormones are structurally classified as glycoprotein hormones, meaning they are composed of a protein component linked to carbohydrate chains.
Each of these hormones is a heterodimer, consisting of two separate peptide chains: a common alpha subunit and a unique beta subunit. While the alpha subunit is nearly identical across FSH and LH, the distinct beta subunit gives each hormone its specific function and ability to bind to its appropriate receptor on target cells. Unlike FSH and LH, Human Chorionic Gonadotropin (hCG) is primarily produced by the syncytiotrophoblast cells of the developing placenta following implantation. This placental origin leads to hCG being referred to as the “pregnancy hormone.”
Orchestrating Reproductive Cycles
The actions of FSH and LH are coordinated through the hypothalamic-pituitary-gonadal axis. The hypothalamus releases Gonadotropin-Releasing Hormone (GnRH) in a pulsatile manner, which travels to the anterior pituitary to stimulate the release of both FSH and LH. The frequency of these GnRH pulses influences which gonadotropin is favored for secretion; a low pulse frequency promotes FSH release, while a high pulse frequency stimulates LH release.
In females, FSH initiates the growth and maturation of ovarian follicles, which are the fluid-filled sacs that contain the eggs. As these follicles develop, they begin to produce estrogen, facilitated by the combined action of FSH on the granulosa cells and LH on the theca cells within the ovary. Low levels of estrogen initially create a negative feedback loop, suppressing the further release of FSH and LH from the pituitary.
As the follicular phase progresses, a dominant follicle emerges. The resulting sustained, high concentration of estrogen switches the feedback mechanism from negative to positive. This positive feedback triggers a rapid surge of LH from the pituitary gland, known as the LH surge. The LH surge signals the mature follicle to rupture, releasing the egg in the process called ovulation. Following ovulation, LH stimulates the remaining follicular cells to transform into the corpus luteum, which secretes progesterone and estrogen to prepare the uterine lining for implantation.
In males, FSH and LH work together to ensure healthy sperm production and sex hormone regulation. LH targets the Leydig cells in the testes, stimulating them to synthesize and secrete testosterone. Testosterone is an androgen necessary for the development of male secondary sexual characteristics and the maintenance of the reproductive tract. Simultaneously, FSH acts on the Sertoli cells, which support the developing sperm cells within the seminiferous tubules. FSH enhances the production of androgen-binding protein by the Sertoli cells, which concentrates testosterone locally to sustain the process of spermatogenesis.
Clinical Applications and Diagnostic Uses
Measuring and manipulating gonadotropin levels is fundamental to modern reproductive medicine and diagnostics. Blood tests measuring circulating levels of FSH and LH are routinely used to evaluate reproductive function in both men and women. High levels of FSH and LH often indicate primary gonadal failure, such as primary ovarian insufficiency in women or testicular failure in men. This occurs because the gonads are not producing enough sex hormones to suppress the pituitary through negative feedback.
Conversely, low levels of FSH and LH can suggest a problem with the pituitary gland or the hypothalamus, leading to hypogonadotropic hypogonadism. Medical professionals utilize synthetic versions, known as exogenous gonadotropins, for fertility treatments like in vitro fertilization (IVF). Administering FSH stimulates the development of multiple ovarian follicles, increasing the number of mature eggs that can be retrieved. Once follicles are mature, a dose of hCG is given to trigger final egg maturation and ovulation, mimicking the natural LH surge. In men with infertility, gonadotropins can stimulate testosterone production and the initiation of sperm development. Beyond fertility, the presence of hCG in a woman’s blood or urine is the most common biochemical marker for pregnancy, as its levels become detectable shortly after implantation.