Gonadotropic hormones

Gonadotropins are protein hormones secreted by gonadotrope cells of the pituitary gland of vertebrates.

Gonadotropin is sometimes abbreviated Gn. The British spelling is gonadotrophin.

Types
The two principle gonadotropins are luteinizing hormone (LH) and follicle stimulating hormone (FSH). Both hormones consist of two peptide chains, an alpha chain and a beta chain, linked by disulfide bonds. LH and FSH share nearly identical alpha chains, while the beta chain provides specificity for receptor interactions.

A third human gonadotropin is human chorionic gonadotropin (hCG), produced by the placenta during pregnancy.

Mechanism
Gonadotropin receptors are embedded in the surface of the target cell membranes and coupled to the G-protein system. Signals triggered by binding to the receptor are relayed within the cells by the cyclic AMP second messenger system.

Gonadotropins are released under the control of gonadotropin-releasing hormone (GnRH) from the arcuate nucleus and preoptic area of the hypothalamus. The gonads &mdash; testes and ovaries &mdash; are the primary target organs for LH and FSH. The gonadotropins affect multiple cell types and elicit multiple responses from the target organs. As a simplified generalization, LH stimulates the Leydig cells of the testes and the theca cells of the ovaries to produce testosterone (and indirectly estradiol), while FSH stimulates the spermatogenic tissue of the testes and the granulosa cells of ovarian follicles.

Diseases
Gonadotropin deficiency due to pituitary disease results in hypogonadism. Failure or loss of the gonads usually results in elevated levels of LH and FSH in the blood.