What is Inhibin B?
Inhibin B is a hormone produced in the gonads and affects the regulation of the body's reproductive system. In men, inhibin B is produced primarily by the Sertoli cells in the testicles, while in women the hormone is mainly produced by granulosa cells in the ovarian follicles. An important function of inhibin B is to inhibit the release of FSH, follicle-stimulating hormone, from the pituitary gland. Inhibin B is thus part of the body's feedback system between the pituitary gland and the gonads and can provide information about how the testicles and ovaries function.
What does an analysis of S-Inhibin B show?
S-Inhibin B measures the concentration of inhibin B in serum. The significance of the analysis differs between men and women because the hormone is produced in different parts of the gonads and reflects different parts of the reproductive function.
In men, inhibin B is primarily used as a marker for Sertoli cell function and sperm production. In women, the hormone can provide information about the activity of ovarian follicles and is used in certain contexts as a complement in assessing ovarian function.
Inhibin-B in men
In men, inhibin B is produced by the Sertoli cells in the testicles. These cells have a central function for spermatogenesis, i.e. the formation and development of sperm.
FSH from the pituitary gland stimulates the Sertoli cells, which in turn release inhibin B. Inhibin B then inhibits FSH release through negative feedback. The concentration of inhibin B can therefore provide information about how active and functioning the sperm-producing part of the testicle is.
In healthcare, the analysis can be used for, among other things, the following:
- investigation of male infertility.
- assessment of Sertoli cell function.
- assessment of spermatogenesis.
- investigation of gonadal function.
- assessment of testicular function together with FSH and other sex hormones.
A low concentration of inhibin B in a man may be consistent with impaired Sertoli cell function or impaired sperm production, but the analysis cannot replace a semen analysis when investigating fertility.
Inhibin B in women
In women, inhibin B is produced primarily by granulosa cells in growing follicles in the ovaries. The concentration varies during the menstrual cycle and is therefore dependent on both age and cycle phase.
Inhibin B can provide information about follicular activity and has previously been used as a marker of ovarian reserve. In today's fertility investigations, AMH is often used as a more established marker of ovarian reserve, but inhibin B can still provide supplementary information in certain clinical situations.
Karolinska lists inhibin B as a possible supplementary analysis together with AMH, FSH, LH and estradiol when assessing ovarian function.
How are Inhibin B and FSH related?
Inhibin B and FSH are part of a feedback system between the gonads and the pituitary gland. FSH stimulates cells in the testes and ovaries, while inhibin B acts as a brake signal back to the pituitary gland and reduces the release of FSH.
This means that in some situations, low inhibin B can occur together with elevated FSH when the function of the gonads is impaired. The combination of inhibin B and FSH can therefore provide more information than each hormone assessed separately.
What can low Inhibin B mean?
The significance of a low value depends on gender, age and the clinical question.
In men, low inhibin B can be seen, among other things, in impaired Sertoli cell function and reduced spermatogenesis. A low value can therefore occur in men with impaired sperm production or other effects on testicular function.
In women, inhibin B normally decreases with decreasing follicular activity and increasing age. Low levels can be seen in impaired ovarian function and after menopause.
What can high Inhibin B mean?
Elevated levels need to be assessed based on gender, age and clinical context. In women, inhibin B can rise during strong stimulation of the ovaries, for example in connection with fertility treatment.
Inhibin B can also be used in specialist care as a tumor marker for certain granulosa cell tumors in the ovaries. The analysis is then used both diagnostically and for follow-up after treatment. However, elevated inhibin B in a person without a known tumor does not in itself mean that a tumor is present.
Inhibin B in male fertility
Inhibin B is particularly interesting in assessing male reproductive function because the hormone has a close connection to Sertoli cells and spermatogenesis. Karolinska describes S-Inhibin B as a marker for both Sertoli cell function and sperm production in infertility investigations.
However, in a fertility investigation, the result needs to be assessed together with, for example, FSH, LH, testosterone and, above all, sperm analysis. A normal inhibin B does not guarantee normal fertility, and a low value alone cannot determine the cause of reduced fertility.
Which samples can be assessed together with Inhibin B?
Depending on the question, S-Inhibin B can be supplemented with, for example:
In male infertility investigation, semen analysis is a central examination. In women, AMH and ultrasound-based assessment of follicles can provide additional information about ovarian reserve.
Reference range for Inhibin B
The reference range for S-Inhibin B varies clearly with gender and age and is also dependent on the analysis method. In women, the value is also affected by the menstrual cycle and menopausal status.
Karolinska specifies for its current analysis, among other things, a reference range of 25–325 ng/L for men, while the limits for women and children differ depending on age and reproductive status. The reference range specified on the current laboratory result should always be used when interpreting.
Analysis method
S-Inhibin B is analyzed in serum using an immunochemical method. Karolinska uses an ELISA-based analysis and the result is stated in ng/L. The analysis is a special analysis and is not performed as frequently as regular hormone tests.
Important things to know about Inhibin
Inhibin B is a specialized hormone marker that is primarily used for specific questions about gonadal function and fertility. A high or low value is not diagnostic in itself and always needs to be assessed in relation to gender, age, possible menstrual cycle, other hormones and the clinical situation.
In men, the analysis is primarily a marker for Sertoli cell function and spermatogenesis. In women, it can provide information about follicular activity and is used in certain specialist medical contexts, but AMH is often a more established marker when the question specifically concerns ovarian reserve.
