Inhibin B in Men and Women

Inhibin B as a Biomarker of Spermatogenesis in Men and Ovarian Reserve in Women

What is Inhibin B?

Inhibin B is a glycoprotein produced in the testes (by Sertoli cells) in men and in the ovaries (by granulosa cells) in women. It consists of two subunits connected by a disulfide bond, an alpha subunit and a beta subunit. Its primary role is to inhibit FSH (Follicle-Stimulating Hormone) through negative feedback on its secretion by the pituitary gland.

Inhibin B in Men

After birth, Inhibin B levels rise rapidly, peaking between 4 and 12 months of age, and then drop to remain low until puberty. During puberty, Inhibin B levels increase until around the age of 20–30, when they reach a second peak. After that, the concentration of Inhibin B gradually declines with age.

Why is Inhibin B Measured in Men?

Since Inhibin B is a direct product of the Sertoli cells in the seminiferous tubules of the testes, its production reflects the presence of healthy germ cells in the testes. Inhibin B is considered a marker of spermatogenesis in men (the process of sperm production in the testes). It is often measured alongside other tests, such as sperm analysis and hormone levels like testosterone or FSH. Together, these tests provide a comprehensive picture of a man’s reproductive health. Normal Inhibin B levels generally indicate healthy spermatogenesis, while low levels may be associated with reproductive issues. Low Inhibin B levels in men may be seen in cases such as:

• Arrested spermatogenesis (failure of sperm maturation) or hypospermatogenesis (reduced sperm production in the testes)
• Men with Del Castillo syndrome (Sertoli cell-only syndrome), where extremely low levels of Inhibin B are observed.

In addition to assessing spermatogenesis in men, this test is also used for:

• Diagnosing cryptorchidism (a common genital issue in males where one or both testicles fail to descend into the scrotum)
• Diagnosing precocious puberty
• Predicting the outcome of testicular sperm extraction in men with non-obstructive azoospermia
• Assessing potential spermatogenesis damage in men following radiotherapy or chemotherapy

It is important to note that the results of this test should be interpreted in consideration of other factors, such as the patient’s age, health status, and any underlying conditions. Therefore, interpreting the results is the physician’s responsibility, aligned with the clinical picture.

Inhibin B in Women

In women, serum inhibin B levels fluctuate throughout the menstrual cycle. Inhibin B concentrations rise early in the follicular phase, peaking as FSH levels begin to decline, and then decrease during the follicular phase. There is a brief peak in inhibin B levels two days after the luteinizing hormone (LH) surge. Inhibin B levels remain low throughout the luteal phase. The timing of the inhibin B rise suggests its role in regulating folliculogenesis through negative feedback on FSH production. In menopause, with the decline in ovarian follicle numbers, serum inhibin B decreases to very low or undetectable levels.

Why is Inhibin B measured in women?

Various studies have shown that decreased inhibin B secretion is associated with elevated FSH levels, reduced oocyte quality, and decreased fertility potential. A woman is born with a finite number of follicles containing oocytes, and this number gradually decreases with age. The remaining number of follicles in the ovary at any time is referred to as the ovarian reserve. For this reason, ovarian reserve assessment is often performed in cases of female infertility. Tests commonly used include FSH and Inhibin B on the second or third day of the cycle (early follicular phase) and AMH (Anti-Müllerian Hormone). The amount of inhibin B measured in serum (on day 2 or 3 of the cycle) directly reflects the number of follicles in the ovary. The higher the inhibin B, the more ovarian follicles are present. Therefore, inhibin B is considered a predictor of ovarian reserve.
However, the later appearance of inhibin B during the transition to menopause suggests that it might be a better indicator of ovarian activity rather than ovarian reserve due to its direct association with growing follicles. In addition to evaluating ovarian reserve, inhibin B testing is also used in diagnosing various types of ovarian tumors in women.

Inhibin B Blood Test

This test is performed using serum (venous blood) with no special preparation required from the patient. Sample collection is done in the morning, as with other hormone tests.

Reference Values for Inhibin B in Blood

Recommended values for healthy individuals depend on the type of test and the test manufacturer. At BioDiagnostica Laboratory, the concentration of Inhibin B in the blood is determined using the ELISA technique with the test provided by Beckman Coulter, USA.