gonadotropin

gonadotropin, any of several hormones occurring in vertebrates that are secreted from the anterior pituitary gland and that act on the gonads (i.e., the ovaries or testes).

Gonadotrophs, cells that constitute about 10 percent of the pituitary gland, secrete two primary gonadotropins: luteinizing hormone (LH) and follicle-stimulating hormone (FSH). The amount and rate of secretion of these hormones vary widely at different ages and at different times during the menstrual cycle in women. Secretion of LH and FSH is low in both males and females prior to puberty. Following puberty, more LH than FSH is secreted. During the menstrual cycle there is a dramatic increase in the serum concentrations of both hormones at the time of ovulation, and the secretion of both hormones increases 10- to 15-fold in postmenopausal women. Another type of gonadotropin found in women is human chorionic gonadotropin (HCG), which is produced by the placenta during pregnancy. The detection of HCG forms the basis of pregnancy tests.

In men, FSH stimulates the development of spermatozoa, in large part by acting on special cells in the testes called Sertoli cells. LH stimulates the secretion of androgen (male) hormones by specialized cells in the testes called Leydig cells. In women, FSH stimulates the synthesis of estrogens and the maturation of cells lining the spherical egg-containing structures known as Graafian follicles. In menstruating women, there is a preovulatory surge in serum FSH and LH concentrations. The preovulatory surge of LH is essential for rupture of the Graafian follicle (ovulation), after which the egg enters the fallopian tube and travels to the uterus. The empty Graafian follicle becomes filled with progesterone-producing cells, transforming it into a corpus luteum. LH stimulates the production of progesterone by the corpus luteum (see reproductive system, human). Inhibin, a hormone secreted by the Graafian follicles of the ovaries and by the Sertoli cells of the testes, inhibits the secretion of FSH from the pituitary gonadotrophs.

Patients with diseases involving the anterior pituitary gland often have gonadotropin deficiency. Thus, the disappearance of menstrual periods may be the first sign of a pituitary tumour or other pituitary disease in women. In men the most common symptoms of gonadotropin deficiency are loss of libido and erectile dysfunction. Isolated deficiencies of both LH and FSH do occur but only rarely. In men isolated LH deficiency (“fertile eunuch”) is characterized by symptoms and signs of androgen deficiency; however, there is sufficient secretion of FSH to permit the maturation of spermatozoa. Some pituitary tumours produce an excess of LH or FSH, whereas other pituitary tumours produce the hormonally inactive alpha chain subunit of the glycoprotein hormones.