Hormone Test
Inhibin B Test
A comprehensive guide to the Inhibin B test: its role in regulating FSH, what it reveals about ovarian reserve in women, and how it predicts spermatogenesis quality in men.
Test Overview
Inhibin B is a glycoprotein hormone secreted by granulosa cells in developing ovarian follicles (women) and by Sertoli cells in the testes (men). It suppresses FSH production through direct pituitary feedback, making its level an indirect measure of the active follicle or sperm-producing cell population.
Key Timing
For women: Day 3 of the menstrual cycle, alongside FSH and estradiol. For men: any time of day; there is no cycle-dependent variation.
1. Purpose and uses of the Inhibin B test
In women
- Assess ovarian reserve when AMH and FSH results are borderline or inconsistent.
- Predict ovarian response to IVF stimulation, particularly in women with elevated FSH and uncertain reserve.
- Evaluate Premature Ovarian Insufficiency (POI) or early menopause.
- Provide supporting data in the investigation of irregular or absent menstruation.
In men
- Evaluate Sertoli cell function as a proxy for active sperm production when semen analysis results are unclear.
- Distinguish obstructive azoospermia (normal inhibin B, ducts blocked) from non-obstructive azoospermia (low inhibin B, production failure).
- Predict the likelihood of finding sperm during surgical retrieval (TESA or TESE).
- Monitor recovery of spermatogenesis after chemotherapy or testicular injury.
2. Procedure and preparation
Inhibin B is measured through a standard blood draw. For women, the sample must be collected on Day 3 of the menstrual cycle. The hormone fluctuates across the cycle and is at its most diagnostically useful during the early follicular phase when the follicle cohort is just beginning to activate.
No specific fasting is required. Medications that suppress ovarian function, including hormonal contraceptives, can lower inhibin B and should be noted when results are interpreted. Men can test at any time with no special preparation.
3. How Inhibin B works
In women: the FSH feedback loop
Granulosa cells lining the antral follicles produce inhibin B in proportion to the number of active follicles. As inhibin B rises during the early follicular phase, it signals the pituitary gland to reduce FSH output. This negative feedback prevents too many follicles from maturing simultaneously.
When ovarian reserve is low, fewer follicles are available to produce inhibin B. Pituitary FSH suppression weakens, causing FSH to rise. This is the biological reason why elevated Day 3 FSH is a sign of diminished reserve: the ovaries are no longer producing enough inhibin B to hold FSH in check.
In men: Sertoli cell activity
Sertoli cells in the seminiferous tubules produce inhibin B in direct proportion to their number and functional activity. Because Sertoli cells physically support developing sperm cells throughout spermatogenesis, inhibin B level in men reflects how actively sperm are being produced rather than simply whether a sperm pathway is open.
4. Normal reference ranges
Reference ranges vary between assays and laboratories. The following values represent general population benchmarks cited in reproductive endocrinology literature.
| Patient group | Normal range | Low threshold | Interpretation |
|---|---|---|---|
| Women, Day 3 | 45 to 100 pg/mL | Below 45 pg/mL | Possible diminished reserve |
| Women, postmenopausal | Below 10 pg/mL | N/A | Expected; ovarian activity ceased |
| Men, adult | 80 to 300 pg/mL | Below 80 pg/mL | Spermatogenic impairment likely |
Always use the specific reference range provided by the laboratory that processed the sample. Immunoassay methods differ in their calibration, and values are not interchangeable between platforms.
5. Interpreting results
Low Inhibin B in women
A low Day 3 inhibin B, particularly when paired with a high FSH and low AMH, indicates a small active follicle cohort. This combination predicts a poor response to IVF stimulation and fewer eggs at retrieval. Inhibin B adds the most diagnostic value when AMH and FSH results appear inconsistent with each other.
A woman with a low inhibin B but normal AMH may have a functioning reserve but reduced granulosa cell activity in early-cycle follicles. Clinicians use all three markers together rather than treating any single result as definitive.
Low Inhibin B in men
In a man presenting with azoospermia or severe oligospermia, low inhibin B points to a production failure rather than a blockage. Higher inhibin B in men with no sperm in the ejaculate suggests active Sertoli cells are still present, which increases the probability that sperm can be found surgically in the testes.
An inhibin B level above 80 pg/mL in an azoospermic man is associated with a higher success rate for testicular sperm extraction (TESE). A result below 40 pg/mL significantly reduces that likelihood.
6. Inhibin B vs. AMH: when each adds value
| Factor | Inhibin B | AMH |
|---|---|---|
| Cycle timing required | Yes: Day 3 only | No: any day |
| Cycle-to-cycle stability | More variable | More stable |
| Use in male infertility | Yes: Sertoli cell marker | Very limited |
| IVF response prediction | Moderate predictive value | Stronger predictor |
| Routine clinical use | Second-line; supports borderline AMH/FSH | First-line standard |
AMH has become the primary ovarian reserve marker in most fertility clinics. Inhibin B remains useful in specific scenarios: borderline results, male azoospermia workups, and cases where AMH and FSH point in opposite directions.