Yes, inhibin is a hormone. Specifically, it’s a glycoprotein hormone produced by the reproductive organs that plays a central role in regulating fertility. Its primary job is to suppress the release of follicle-stimulating hormone (FSH) from the pituitary gland, a small structure at the base of the brain that controls reproductive function. Beyond that core role, inhibin has become a valuable clinical tool, used in prenatal screening, fertility assessment, and cancer monitoring.
What Inhibin Does in the Body
Inhibin works through a negative feedback loop between the reproductive organs and the brain. Here’s how it plays out: the pituitary gland releases FSH, which signals the ovaries or testes to support egg or sperm development. In response, certain cells in those organs produce inhibin, which travels back through the bloodstream and tells the pituitary to dial FSH production back down. When researchers block inhibin using antibodies, FSH levels rise sharply, confirming that inhibin acts as a brake on FSH release. Importantly, inhibin is selective. It suppresses FSH but does not affect luteinizing hormone (LH), the other major reproductive hormone released by the pituitary.
This feedback loop is essential for keeping reproductive cycles on track. In women, it helps regulate the development of ovarian follicles each month. In men, it helps maintain steady sperm production over time.
Where Inhibin Is Produced
In women, inhibin is primarily made by granulosa cells, which are the support cells surrounding each developing egg inside the ovary. In men, it comes mainly from Sertoli cells, the cells inside the testes that nurture developing sperm. Leydig cells in the testes also contribute a smaller amount. The relationship between these cells and the pituitary is reciprocal: FSH stimulates the granulosa and Sertoli cells to produce inhibin, and inhibin in turn suppresses further FSH release.
Two Forms: Inhibin A and Inhibin B
Inhibin comes in two closely related forms, inhibin A and inhibin B. Both are protein dimers, meaning they’re made of two subunits linked together. They share the same alpha subunit but differ in their beta subunit. Both suppress FSH, but they behave quite differently depending on sex and timing.
In women, both forms are present, but their levels shift throughout the menstrual cycle. Inhibin A and inhibin B show different patterns during follicle development, suggesting they come from follicles at different stages of maturity. In men, the picture is simpler: inhibin B is the dominant form circulating in the blood, while inhibin A is largely undetectable. This makes inhibin B the more clinically useful form for evaluating male reproductive health.
Inhibin B as a Fertility Marker in Men
Because Sertoli cells produce inhibin B in direct proportion to how well spermatogenesis is functioning, a blood test for inhibin B can serve as a window into testicular health. A study of 349 Danish men found a significant positive correlation between inhibin B levels and sperm concentration. There was also a strong inverse correlation between inhibin B and FSH: when inhibin B drops, FSH rises as the pituitary tries to compensate for reduced testicular output.
The clinical utility is striking. In that same study, when inhibin B fell below 80 pg/mL and FSH rose above 10 IU/L, the combination predicted sperm counts below 20 million per milliliter with 100% accuracy. That makes inhibin B a practical, non-invasive marker for identifying impaired sperm production, useful alongside a standard semen analysis.
Inhibin A in Prenatal Screening
Inhibin A has a completely separate clinical application in pregnancy. It’s one of four blood markers measured in the second-trimester “quad screen,” a routine test used to assess the risk of Down syndrome (trisomy 21). The other three markers are human chorionic gonadotropin (hCG), alpha-fetoprotein (AFP), and unconjugated estriol.
In pregnancies affected by trisomy 21, inhibin A levels roughly double compared to unaffected pregnancies, rising to about two multiples of the median value. AFP and unconjugated estriol, by contrast, drop. Inhibin A was chosen over related proteins like activin A because its measurement is simpler and more specific. It’s been part of standard prenatal screening protocols since the 1990s.
Inhibin as a Tumor Marker
Granulosa cell tumors of the ovary, a relatively uncommon type of ovarian cancer, actively secrete inhibin. Every granulosa cell tumor studied in key research has been shown to produce functional inhibin, making it a reliable blood marker for this specific cancer type. Clinicians use inhibin levels to track how well treatment is working and to detect recurrence early. For granulosa cell tumors, inhibin is considered a better marker than estradiol.
Some other types of ovarian cancer also produce inhibin, though less consistently. Modest elevations have been found in patients with endometrioid, mucinous, undifferentiated, and clear cell ovarian cancers. In postmenopausal women, where inhibin levels are normally very low, an unexpected elevation can be an important diagnostic signal.
What Happens When Inhibin Levels Change
Because inhibin is part of a tightly regulated feedback loop, changes in its levels have predictable consequences. After menopause, the ovaries stop producing significant amounts of inhibin, which removes the brake on FSH. This is why FSH levels climb steeply in postmenopausal women. The same pattern occurs after any form of gonadal damage: inhibin drops, and FSH rises in response.
In men, declining inhibin B can signal problems with the Sertoli cells or with spermatogenesis more broadly. Conditions that damage the testes, whether from infection, toxin exposure, or genetic causes, typically show up as low inhibin B paired with elevated FSH. This hormonal signature helps clinicians distinguish between problems originating in the testes and those caused by pituitary dysfunction, where both inhibin and FSH would be low.