Understanding negative feedback: Changes in high‐molecular‐weight adrenocorticotropic hormone in adrenocorticotropic hormone‐independent Cushing's syndrome

Abstract

AbstractCushing's syndrome is characterized by chronic glucocorticoid oversecretion and diverse clinical manifestations. Distinguishing between adrenocorticotropic hormone (ACTH)‐independent and ACTH‐dependent forms is crucial for determining treatment options. Plasma ACTH levels aid in the differential diagnosis, with undetectable or low levels suggesting ACTH‐independent hypercortisolemia. ACTH is derived from pro‐opiomelanocortin, and its processing involves prohormone convertase 1/3. High‐molecular‐weight ACTH is generally found in ACTH‐producing pituitary tumors and ectopic ACTH syndrome. The mechanism of negative feedback and the process of high‐molecular‐weight ACTH alternation during ACTH‐independent Cushing's syndrome remain unclear. A 40‐year‐old woman with hypertension and multiple fractures developed symptoms suggestive of Cushing's syndrome. Computed tomography revealed a left adrenocortical tumor along with atrophy of the right adrenal gland. ACTH levels were undetectable at the previous clinic, indicating ACTH‐independent Cushing's syndrome. However, subsequent measurements at our hospital revealed non‐suppressed ACTH (18.1 pg/mL), prompting further investigation. Gel exclusion chromatography confirmed the presence of high‐molecular‐weight ACTH. Metyrapone treatment decreased the cortisol levels. In this situation, in which ACTH levels should be elevated, a decrease in high‐molecular‐weight ACTH levels was observed. Histological findings revealed cortisol‐producing adenoma without ACTH expression. This case highlights the importance of assay differences in evaluating ACTH concentrations and introduces a novel finding of circulating high‐molecular‐weight ACTH. The observed decline in high‐molecular‐weight ACTH levels suggests a potential time lag in the negative feedback within the hypothalamic–pituitary–adrenal axis exhibited by glucocorticoids. This temporal aspect of the regulation of ACTH‐related molecules warrants further exploration to enhance our understanding of the hypothalamic–pituitary–adrenal axis feedback mechanism.