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Inhibin A

Inhibin is a hormone secreted by granulosa cells in the ovarian follicles and by Sertoli cells in the testis. Its physiological role is to suppress secretion of follicle stimulating hormone (FSH) by the pituitary.

Inhibin exists as two different isoforms, inhibin A and inhibin B. Both isoforms are comprised of 2 subunits, the alpha and beta subunits. The alpha subunit is the same for both inhibin A and B, but the beta subunit differs. Inhibin A contains a beta-A subunit and inhibin B contains a beta-B subunit. Plasma levels of inhibin A and B fluctuate during the menstrual cycle. At menopause, serum inhibin A and B decrease to very low or undetectable levels.

Ninety five percent of ovarian cancers are derived from epithelial cells. The remaining 5% are germ cell and sex cord-stromal tumors. Inhibin A is elevated up to 6 times the upper limit of the reference range in approximately 70% of granulosa cell tumors. Inhibin A is also elevated in about 20% of patients with epithelial tumors of the mucinous type. Inhibin is a better tumor marker for these two types of ovarian cancer than cancer antigen 125 (CA 125).

 Inhibin A is most useful as a tumor marker in postmenopausal women, because plasma levels are normally very low. Inhibin A levels are much harder to interpret in premenopausal women, because plasma levels are higher and fluctuate throughout the menstrual cycle. Because of its low sensitivity, a normal inhibin A level does not rule-out a mucinous or granulosa ovarian cell tumor. Inhibin B might be elevated in these cases.

Inhibin A level decreases shortly after surgery for ovarian cancer. Patients in remission have normal levels of inhibin A. Elevations of inhibin A after treatment are suggestive of residual or recurrent disease. Inhibin A may become elevated prior to onset of clinical symptoms. Inhibin A levels are increased in normal pregnancy, preeclampsia and Down syndrome.

Specimen requirement is blood collected into a red top tube. Reference range using a Beckman Coulter Unicel chemiluminescent assay is <2.1 pg/mL for males, <97.5 pg/mL for premenopausal females and <6.9 pg/mL for postmenopausal females. 

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