Inhibin-like immunoreactivity produced by the adrenal gland is circulating in vivo

To clarify the contribution of the inhibin-like immunoreactivity (inhibin-LI) produced by adrenal glands to the total circulating levels of inhibin-LI, we measured serum inhibin-LI in normal and hypogonadal subjects under ACTH-loading or dexamethasone-loading condition. The mean basal concentration of inhibin-LI in the peripheral serum of the hypogonadal cases was 3.6 +/- 1.3 IU/ml (mean +/- SD, n = 5), which corresponded to 19.5 +/- 5.8% of that of normal controls matched for age and sex. The low levels of inhibin-LI in hypogonadal subjects (n = 7) rose significantly (3.6 +/- 1.1 vs 8.1 +/- 1.7 IU/ml, p < 0.001) after the administration of synthetic 1-24ACTH (40 units/day intramuscular injection) for 2 days, while the levels of serum inhibin-LI were not increased in two cases of Addison's disease with hypogonadism after the administration of ACTH. After the oral administration of a low dose of dexamethasone (1 mg) the serum inhibin-LI level in normal subjects (eight males and eight females) decreased significantly (male, 16.2 +/- 3.3 vs 14.5 +/- 4.1 IU/ml; female, 12.9 +/- 6.3 vs 10.8 +/- 5.6 IU/ml; p < 0.01 each) without significant change in the levels of serum gonadotropin (LH and FSH) and those of gonadal steroid (testosterone or estradiol). These results indicate that a small; but significant amount of inhibin-LI is secreted from the adrenal gland and circulating in vivo, and that the proportion of adrenal-derived inhibin-LI is much higher in patients with hypogonadism.