Inhibin is a hormone that occurs in both men and women. Playing an integral role in the reproductive system, it may also have a non-reproductive involvement in the aging process. In women, the hormone is secreted in the ovaries, and acts to prevent the synthesis and secretion of follicle-stimulating hormone (FSH). FSH actually stimulates the secretion of inhibin, so the latter hormone is part of a feedback mechanism to prevent an overabundance of FSH. This feedback cycle, in turn, has a cascade effect that contributes to the cyclical nature of female fertility.
Both males and females produce FSH and gonadotropin-releasing hormone (GnRH), and inhibin prevents this release in both genders. In women, the hormone produced in the gonads, placenta, and pituitary gland accomplishes this goal. In males, testicular Sertoli cells secrete the needed inhibin. Males also use this in the testes as a means of regulating sperm production. The presence of androgen increases inhibin levels, which in turn seems to promote spermatogenesis.
Inhibin performs the opposite role of the hormone activin in the menstrual cycle. Activin promotes FSH secretion, which in turn helps to ensure the development of ovarian follicles. This hormone may compete with activin for similar receptors, or bind to its own receptor, to serve as a counterweight to activin. During different phases of ovulation, one of two forms may be active. During ovulation, and the mid-follicular phase, inhibin B is produced, and during the mid-luteal phase, more inhibin A is secreted.
The opposite effects of activin and inhibin are shown in their effects on bone development in aging. While activin promotes bone growth and differentiation, inhibin suppresses these effects, even in the presence of activin. Changes in the production of both hormones may result in bone turnover, even before the onset of menopause. The decline in inhibin in both genders over time could increase the risk of certain bone-related conditions.
While its exact role in pregnancy is not yet known, it also has a role as an indicator for both pregnancy and male fertility. Inhibin A levels could serve as a predictor of Down's syndrome in the fetus, fetal growth restriction, and miscarriage, even before clinical symptoms appear. In males, higher levels of inhibin B has been significantly associated with fertility. Tests for these conditions have been recommended as predictors because it has a shorter half life than the hormones normally measured to detect them.