DIAGNOSIS OF ENDOCRINE DISEASE: The role of the desmopressin test in the diagnosis and follow-up of Cushing's syndrome.

Desmopressin is a vasopressin analogue selective for type 2 vasopressin receptors that mediate renal water retention. In contrast to the native hormone arginine vasopressin, a well-known ACTH secretagogue, desmopressin, exerts minimal or no activity on ACTH excretion. However, in a substantial proportion of patients with ACTH-dependent Cushing's syndrome (CS), desmopressin elicits an ACTH and cortisol response, which contrasts with the minimal responses obtained in healthy subjects. The mechanism underlying this paradoxical response involves upregulation of vasopressin type 3 and/or the aberrant expression of type 2 receptors by neoplastic ACTH-producing cells. This makes desmopressin administration a suitable test enabling the distinction between neoplastic from functional (formerly termed 'pseudo-Cushing syndrome') ACTH-dependent cortisol excess. Several studies have now established an adjunctive role of desmopressin in the initial diagnostic workup of CS. Despite some early data indicating that this test may also have a role in distinguishing between Cushing's disease (CD) and ectopic ACTH secretion, subsequent studies failed to confirm this observation. The ability of the paradoxical response to desmopressin to depict the presence of neoplastic ACTH-secreting cells was also exploited in the follow-up of patients with CD undergoing surgery. Loss of the desmopressin response, performed in the early postoperative period, was a good predictor for a favorable long-term outcome. Moreover, during follow-up, reappearance of desmopressin paradoxical response was an early indicator for recurrence. In conclusion, the desmopressin test is a valid tool in both the diagnosis and follow-up of patients with CD and should be more widely applied in the workup of these patients.