Inhibins A and B, Serum

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Subject: Inhibins A and B, Serum

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Definition

  • Polypeptide hormones that belong to transforming growth factor family; these are secreted by granulosa cells of the ovary and Sertoli cells of the testis. Inhibit pituitary production of FSH. Secreted by the placenta during pregnancy. Inhibins are heterodimers and hence have two forms: alpha-beta A (inhibin A) and alpha-beta B (inhibin B).

  • Normal ranges: see Tables 16.47 and 16.48.

 
TABLE 16–47
Normal Ranges for Inhibin A
 
TABLE 16–48
Normal Range for Inhibin B by Sex and Age

Use

  • Females:

    • Inhibin A is mostly produced by the corpus luteum.

      • Undetectable before puberty.

      • Very low levels in postmenopausal state due to absent follicular secretions.

      • During pregnancy is secreted by the placenta. Inhibin A peaks at 8–10 weeks, declines until 20 weeks, and then increases gradually to term.

      • Also used as an effective marker for Down syndrome pregnancies.

    • Inhibin B is produced by granulosa cells of small developing antral follicles.

      • Rises to peak in early puberty; constant level thereafter.

      • Gradually declines after age 40. In early menopause, follicular-phase inhibin B declines, whereas inhibin A and estradiol are still within normal range.

      • May indicate low ovarian reserve in perimenopausal women and transition to menopause; useful for assisted reproduction. Measured on days 3–5 of menstrual cycle.

    • After menopause, inhibins A and B fall to very low levels.

    • May be useful to screen for preeclampsia.

    • As an aid in the diagnosis of patients with granulose cell tumors of the ovary, when used in combination with inhibin B.

  • Males:

    • Inhibin B is predominant in males and supports spermatogenesis by negative feedback of FSH.

    • Inhibin A is not significant in males (normal values <480 pg/mL); values remain fairly constant.

    • May be decreased in male infertility.

Interpretation

Increased In

  • Inhibin A: elevated during normal pregnancy

  • Preeclampsia, Down syndrome, and some cancers

    • Seventy percent of patients with granulose cell tumors

    • Twenty percent of patients with epithelial ovarian tumors

Decreased In

  • Ovarian aging

Limitations

  • Inhibin levels fluctuate during menstrual cycle.

 
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