Using the Desmopressin Stimulation Test to Assess for Residual Tumor in Cushing Disease With Cyclic Hypercortisolism.

Cushing disease is caused by excess ACTH secretion by a pituitary adenoma leading to hypercortisolism. Cyclic Cushing syndrome, in which periods of cortisol excess are interspersed by periods of normal or low values, poses a challenge to diagnostic testing and postoperative monitoring. We present a 26-year-old woman with cyclic Cushing syndrome who achieved apparent biochemical remission after transsphenoidal resection of an ACTH-producing pituitary tumor, confirmed on pathology. Despite initial clinical improvement, she later experienced recurring symptoms. Biochemical evidence of hypercortisolism was documented, but 1 month later morning serum cortisol was undetectable. A desmopressin stimulation test (DesST) produced a rise in ACTH and cortisol, indicating likely residual tumor tissue. After repeat surgery, pathology again confirmed an ACTH-secreting tumor. Postoperatively, ACTH and cortisol levels were again low, but a repeat DesST was now negative, suggesting successful resection of the residual tumor, and she remains in remission 3 years later. This case describes the unique utility of the DesST to detect a pituitary corticotroph tumor in cyclic Cushing disease during periods of low disease activity. It also highlights the potential role of the DesST in postoperative monitoring.