Inhibin

Definition - What does Inhibin mean?

Inhibin is a protein hormone produced mainly by the female ovaries and male testes and functions to block the release of follicle stimulating hormone (FSH) by providing feedback messages to the anterior pituitary.

In women, the blockage of the release of FSH inhibits follicular development and ovulation; by suppressing FSH, inhibin hampers the release of a mature egg (or ovulation). Raised inhibin levels (or dropping activin levels), in females, therefore, lead to infertility.

In men, inhibin stimulates testosterone production and is thus an enhancer of sterility. FSH also stimulates sperm production in males; inhibin B is released into the bloodstream when the sperm count is too high and inhibits the release of FSH, causing spermatogenesis to slow down. If the sperm count peaks, the Sertoli cells slow down the release of inhibin-B, facilitating a rise in the sperm count.

FertilitySmarts explains Inhibin

Inhibin competes with the hormone activin to block its effects. Inhibin works in a feedback loop with FSH. A rise in FSH stimulates the gonads to throw more inhibin, which in turn blocks FSH release from the pituitary gland. In males, androgens stimulate inhibin production from the testes. In addition to gonads, the pituitary gland, placenta, corpus luteum also make inhibin. There are two forms of inhibin: Inhibin A and B. Females secrete both types of inhibin. Males, however, only seem to synthesize and secrete inhibin-B.

Other than FSH, conditions/factors that lead to increased inhibin levels include:

  • Polycystic ovary syndrome (PCOS): The levels of FSH plummet due to inhibin produced from the cells lining the ovarian follicles, while luteinizing hormone (LH) levels peak
  • Ovarian tumors called granulosa cell tumors (inhibin serves as a marker for these tumors)
  • Down's syndrome: Inhibin levels are elevated; hence, measurement of inhibin A is included in the prenatal quad screen

Factors that reduce inhibin levels include:

  • GnRH (gonadotropin-releasing hormone): A hormone released from the hypothalamus that induces the release of gonadotropins (FSH and LH) from the pituitary but suppresses the production of inhibin
  • The period before menopause: A few years prior to menopause, the FSH levels rise partly due to reduced inhibin production from the ovarian follicles.

Serum inhibin-B levels (in addition to FSH) are used as a marker for spermatogenesis, decreased sperm count (azoospermia) and male infertility: Higher levels of inhibin B indicate successful chances of achieving pregnancy through testicular sperm extraction (TESE).

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