Inhibin A, Ultrasensitive

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.