Inhibin A, Ultrasensitive

Assess the function of the antral follicles the dominant follicle/corpus luteum of the ovaries in women.

Results from this ultrasensitive inhibin A test are not interchangeable with values obtained using the inhibin A test that is available as part of LabCorp's prenatal screening profiles.

Historically, inhibin was the name given to a component of serum that was found to inhibit secretion of follicle-stimulating hormone (FSH) by the pituitary.1,2 In recent years, the inhibin proteins have been characterized and specific immunoassays have been developed for both inhibin A and inhibin B.1 These hormones are members of the transforming growth factor-B superfamily.1,2 Structurally, they consist of dimers of two dissimilar protein subunits. The α subunit is common to both inhibins. The α subunit of inhibin B is covalently linked to a β-B subunit by disulfide bridges. In women, inhibin A is made largely by the dominant follicle/corpus luteum.1 Normal men do not produce measurable levels of inhibin A.1

In young girls, the concentrations of inhibin A increase as puberty progresses.3,4 Therefore, the measurement of inhibin A could aid in determining gonadal maturity and diagnosing precocious puberty in girls. Once women reach reproductive age, inhibin A levels change with the menstrual cycle. Levels rise through the follicular phase to a maximum in the luteal phase with an intermediate peak at ovulation. In the early perimenopausal phase of the menopausal transition, the circulating follicular phase levels of inhibin decline.5 In postmenopausal women, inhibin A levels fall to <5 pg/mL.

During pregnancy, the fetoplacental unit produces relatively large amounts of inhibin A. Assessment of inhibin A concentration relative to gestational age of fetus has been applied extensively to prenatal screening for Down syndrome and in predicting preëclampsia.1 This ultrasensitive inhibin A test was not designed for these applications and instead can be used to accurately measure the lower inhibin A levels that are found in nonpregnant individuals.

1. Groome NP, Evans LW. Does measurement of inhibin have a clinical role? Ann Clin Biochem. 2000 Jul; 37(Pt 4):419-431. PubMed 10902857

2. Risbridger GP, Schmitt JF, Robertson DM. Activins and inhibins in endocrine and other tumors. Endocr Rev. 2001 Dec; 22(6):836-858. PubMed 11739336

3. Sehested A, Juul AA, Andersson AM, et al. Serum inhibin A and inhibin B in healthy prepubertal, pubertal, and adolescent girls and adult women: Relation to age, stage of puberty, menstrual cycle, follicle-stimulating hormone, luteinizing hormone, and estradiol levels. J Clin Endocrinol Metab. 2000 Apr; 85(4):1634-1640. PubMed 10770209

4. Crofton PM, Evans AE, Groome NP, Taylor MR, Holland CV, Kelnar CJ. Dimeric inhibins in girls from birth to adulthood: Relationship with age, pubertal stage, FSH, and oestradiol. Clin Endocrinol (Oxf). 2002 Feb; 56(2):223-230. PubMed 11874414

5. Santoro N, Adel T, Skurnick JH. Decreased inhibin tone and increased activin A secretion characterize reproductive aging in women. Fertil Steril. 1999 Apr; 71(4):658-662. PubMed 10202875