Inhibin is an endocrine hormone produced in the ovaries and the testes. This hormone has several functions in the body, with inhibin levels in women being linked to the menstrual cycle and playing a role in fetal development. Inhibin and Activin are two closely related protein complexes that have opposing biological effects.
Inhibin is released into the bloodstream to control and inhibit the secretion of follicle stimulating hormone by the pituitary gland. It is an important part of an endocrine feedback loop. There are two functional forms of inhibin, A and B. Inhibin A has two protein subunits, an a subunit and a ßA subunit, whereas inhibin B has an a subunit and a ßB subunit. In both forms, the two subunits, of almost the same size, are held together by covalent linkages. On the other hand, Activin enhances FSH biosynthesis and secretion, and participates in the regulation of the menstrual cycle. Many other functions have been found to be exerted by activin, including roles in cell proliferation, differentiation, apoptosis, metabolism, homeostasis, immune response, wound repair, and endocrine function.