Inhibin B measurement is primarily used as an adjunctive test in the diagnosis of ovarian tumors when used in combination with inhibin A and for the monitoring of these patients as well as an adjunctive test in the investigation of infertility.
Originally, inhibin was the name given to a component of serum found to inhibit the secretion of follicular hormone (FSH) from the pituitary gland. In recent years, protein inhibins have been characterized and specific immunological tests have been developed for both inhibin A and inhibin B. These hormones are members of the TGF-β (Transforming Growth Factor beta) family. Structurally, they consist of dimers of two dissimilar protein subunits. Subunit a is common in both inhibins. In women, inhibin B is mainly produced by the small developing ovarian follicles. Testicular Sertoli cells are the main source of inhibin B in men.
In young girls, concentrations of inhibin B increase as adolescence progresses. Therefore, measuring inhibin B can help determine gonad maturation and diagnose early puberty in girls. When women reach reproductive age, inhibin B levels change during the menstrual cycle. Inhibin B is believed to play an important role in regulating FSH levels during the follicular phase of the cycle. Inhibin B levels increase during the follicular phase reaching a peak in ovulation before falling to basal levels during the luteal phase. In postmenopausal women, inhibin B levels fall to low levels.
A woman's fertility decreases with age, and this is largely due to the decrease in the number of follicles in the ovary. It has been suggested that measurement of inhibin B, in combination with other criteria, may be useful in assessing the status of the ovarian reserve. This evaluation is valuable in assessing the likelihood of successful egg retrieval during assisted reproduction or in assessing the potential for normal pregnancy as a woman matures. During the initial phase of menopause, circulating levels of inhibin B in the follicular phase of the menstrual cycle decrease, well before changes in estradiol or inhibin A levels occur. Measurement of inhibin B in the follicular phase may be useful in predicting the onset of menopause.
During in vitro fertilization, it is important to choose the right level of ovarian stimulation. Inadequate stimulation can lead to invalid cycles due to poor response, and high stimulation may run the risk of ovarian hyperstimulation syndrome. Inhibin B measurement has been used to determine ovarian hyperstimulation syndrome and to manage this dangerous condition.
In men, inhibin B is produced by the testicular Sertoli cells and serves as the main regulator of FSH secretion. The levels are relatively high in infants and gradually decrease to lower levels between 6 and 10 years. During childhood, baseline serum inhibin B levels have been used as a direct indicator of testicular presence and function and have been applied to the diagnosis of patients with cryptorchidism or ambiguous genitalia. A classic test for testicular function in adolescent boys with testicular disorders is the measurement of testosterone elevation following administration of human chorionic gonadotropin (hCG). Inhibin B levels have been shown to correlate well with the results of hCG stimulation tests. Inhibin B can also be used as a direct indicator of Sertoli cell function and spermatogenesis in adult males. Serum inhibin B levels have been shown to be correlated with testicular volume and sperm concentration. Very low levels of inhibin B were found in men with little or no sperm production. The combined measurement of Inhibin B and FSH is a better indicator of spermatogenic adequacy than the measurement of each marker alone.
Laboratory test results are the most important parameter for the diagnosis and monitoring of all pathological conditions. 70%-80% of diagnostic decisions are based on laboratory tests. The correct interpretation of laboratory results allows a doctor to distinguish "healthy" from "diseased".
Laboratory test results should not be interpreted from the numerical result of a single analysis. Test results should be interpreted in relation to each individual case and family history, clinical findings, and the results of other laboratory tests and information. Your personal physician should explain the importance of your test results.
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