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 " 47Normal Ranges for Inhibin AView LargeTABLE 16 " 47Normal Ranges for Inhibin A Age/Phase Inhibin A (Dimer) (pg/mL) Normal cycling females Early follicular phase ( ’ 14 to ’ 10) 1.8 " 17.3 Mid-follicular phase ( ’ 9 to ’ 4) 3.5 " 31.7 Late follicular phase ( ’ 3 to ’ 1) 9.8 " 90.3 Midcycle (day 0) 16.9 " 91.8 Early luteal (1 " 3) 16.1 " 97.5 Mid-luteal (4 " 11) 3.9 " 87.7 Late luteal (12 " 14) 2.7 " 47.1 IVF-peak levels 354.2 " 1,690.0 PCOS " ovulatory 5.7 " 16.0 Postmenopausal <7.9 Normal males <2.1
TABLE 16 " 48Normal Range for Inhibin B by Sex and AgeView LargeTABLE 16 " 48Normal Range for Inhibin B by Sex and Age Male Female <3 y: no range established <3 y: no range established 3 " 9 y: <162 pg/mL 3 " 9 y: <30 pg/mL 10 " 13 y: 42 " 339 pg/mL 10 " 13 y: < 93 pg/mL 14 " 17 y: 68 " 300 pg/mL 14 " 17 y: <140 pg/mL ≥18 y: <305 pg/mL Premenopausal: <255 pg/mL Postmenopausal: <30 pg/mL
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
Limitations
- Inhibin levels fluctuate during menstrual cycle.