Inhibin B: the testicle's own report on sperm production
Inhibin B is produced by the Sertoli cells of the testicle, the cells that nurture developing sperm, and its level in the blood rises and falls with the activity of spermatogenesis. It is the most direct hormonal window into what the testicles are doing, and the signal that tells the pituitary how much FSH to release. It is a specialised blood test, ordered by a urologist or reproductive endocrinologist rather than in routine screening, and read alongside FSH.
What your number means
| Very low | under 30 pg/mL | Sertoli cell function is severely reduced. With a high FSH, this is the pattern of primary testicular failure. In non-obstructive azoospermia, very low values lower the odds that sperm will be found at micro-TESE, though they do not rule it out. |
|---|---|---|
| Low | 30–80 pg/mL | Spermatogenesis is impaired. Usually accompanies oligospermia or non-obstructive azoospermia and an FSH at or above the upper limit. In obstructive azoospermia, inhibin B is typically normal despite no sperm in the ejaculate. |
| Normal | above 80 pg/mL | Sertoli cells are active and sperm production is likely to be at least partly intact. A normal inhibin B with azoospermia points toward obstruction rather than a production problem. |
What it measures
Inhibin B is a glycoprotein hormone secreted by Sertoli cells in the seminiferous tubules. In adult men its production depends on both FSH stimulation and the presence of developing germ cells, so the blood level tracks the amount of active sperm-producing tissue. When spermatogenesis is healthy, inhibin B is high; when the tubules are damaged or empty, it falls.
Its main job is feedback. Inhibin B travels to the pituitary and specifically suppresses FSH release. That is why the two hormones move in opposite directions: falling inhibin B lets FSH rise, and a raised FSH is in part a consequence of low inhibin B. Measuring both gives a clearer picture than either alone.
It is drawn on a morning blood sample; fasting is not required. Inhibin B is not part of a standard chemistry panel and is usually sent to a reference laboratory, which is one reason it is reserved for specialist evaluation rather than initial screening.
Why it matters for sperm production
Among blood tests, inhibin B correlates most closely with sperm concentration and with testicular volume, because it comes from the same cells that support sperm development. In men with oligospermia, low inhibin B confirms that the problem is reduced production inside the testicle rather than something happening after the sperm are made.
Its most useful role is in azoospermia. In obstructive azoospermia, sperm are being produced normally, so inhibin B is normal and FSH is normal; the blockage simply stops sperm from reaching the ejaculate. In non-obstructive azoospermia, production has failed, so inhibin B is low and FSH is high. The combination separates the two categories more reliably than FSH alone.
Inhibin B has also been studied as a predictor of whether sperm will be found at surgical retrieval (micro-TESE) in non-obstructive azoospermia. Lower values are associated with lower retrieval rates, and very low values with the lowest. But the overlap between men who do and do not have sperm found is large, and the predictive value is moderate rather than definitive. Guidelines do not recommend refusing retrieval on the basis of inhibin B or FSH alone, and men with very low values still have sperm found in a meaningful minority of cases.
What raises or lowers it
Inhibin B falls with any damage to the seminiferous tubules: Klinefelter syndrome and Y-chromosome microdeletions, a history of undescended testicles, chemotherapy or radiation, mumps orchitis, testicular torsion or trauma, and severe varicocele. It declines gradually with age as Sertoli cell number and function decrease.
It also falls when FSH stimulation is absent, because Sertoli cells need FSH to produce it. Men on testosterone therapy or anabolic steroids, whose FSH is suppressed, show low inhibin B along with a low sperm count, and it recovers as FSH returns after stopping. Pituitary disorders and hypogonadotropic hypogonadism lower it by the same mechanism.
Inhibin B is not raised by lifestyle changes or supplements in any meaningful way; it reflects the underlying state of the tubules. It rises when spermatogenesis recovers, for example after stopping testosterone, after gonadotropin therapy in hypogonadotropic men, or in some men after varicocele repair.
What your doctor may do about it
Inhibin B is an interpretive test, not a treatment target. A reproductive urologist uses it with FSH, testicular volume and a confirmed semen analysis to decide which path an azoospermic man is on. Normal inhibin B and FSH point to obstruction, and the next steps are imaging, genetic testing for cystic fibrosis gene variants, and a discussion of surgical repair or straightforward sperm retrieval, which succeeds in nearly all obstructive cases.
Low inhibin B and high FSH point to non-obstructive azoospermia. A specialist may order a karyotype and Y-chromosome microdeletion testing, since some deletions predict that no sperm will be found, and will discuss micro-TESE. Inhibin B contributes to that conversation about likelihood, but the decision to attempt retrieval rests on the whole picture and the patient's preferences, not on a single hormone value.
Where inhibin B is low because FSH is suppressed, treating the cause, stopping testosterone or starting gonadotropins for hypogonadotropic hypogonadism, restores FSH, and inhibin B and sperm production follow. This is the one situation in which a low inhibin B is expected to improve.
When to retest
Inhibin B is a stable marker and is not usually repeated unless the underlying situation has changed. If the first result is borderline or does not match the FSH and semen analysis, a repeat on a separate morning, drawn with FSH, clarifies the picture.
After stopping testosterone or steroids, or during gonadotropin therapy, a specialist may recheck inhibin B alongside FSH and a semen analysis every 3 to 6 months to track recovery, since one cycle of sperm production takes about 74 days.
Inhibin B is never the first test. A semen analysis through Hera is the starting point for any man wondering about his fertility; inhibin B is added by a specialist once azoospermia or severe oligospermia is confirmed.
Related diagnoses
Common questions
What is a normal inhibin B level in men?
Adult men typically have inhibin B above about 80 pg/mL, with many fertile men well above 100 pg/mL, though the exact range depends on the laboratory and assay. Values under 80 pg/mL suggest reduced sperm production, and values under about 30 pg/mL indicate severely impaired Sertoli cell function.
Can sperm still be found if inhibin B is very low?
Yes, in a meaningful minority of men. Low inhibin B lowers the probability of finding sperm at micro-TESE in non-obstructive azoospermia, but the overlap between men who do and do not have sperm retrieved is large. Guidelines do not recommend declining retrieval on the basis of inhibin B or FSH alone. A reproductive urologist weighs it with the other findings and your priorities.
Why is inhibin B measured with FSH?
Because inhibin B is the signal that suppresses FSH. Healthy Sertoli cells produce inhibin B and keep FSH low; when they fail, inhibin B falls and FSH rises. Looking at both confirms whether a high FSH really reflects testicular failure and helps distinguish obstructive azoospermia (both normal) from non-obstructive (inhibin B low, FSH high).
Does inhibin B change with testosterone use?
Yes. Testosterone therapy and anabolic steroids suppress FSH, and Sertoli cells need FSH to make inhibin B, so the level falls along with sperm production. After stopping, FSH returns first, then inhibin B and the sperm count recover over the following months. Tell any doctor evaluating your inhibin B about current or past testosterone or steroid use.