Cushing's syndrome due to an ectopic ACTH-secreting pituitary tumour mimicking occult paraneoplastic ectopic ACTH production

A 32-year-old man presenting with typical features of Gushing's syndrome sh owed baseline cortisol and ACTH values indicating ACTH-dependent disease. D ynamic function tests (dexamethasone, corticotropin releasing hormone (CRH) , desmopressin), were suggestive of paraneoplastic ectopic ACTH production. However, inferior petrosal sinus (IPS) ACTH sampling demonstrated a maximu m baseline central (363 pmol/l)-peripheral (19 pmol/l) ACTH gradient of 19. 1 for the right IFS, conventionally suggestive of Gushing's disease. Howeve r, again, IFS ACTH level did not increase after CRH stimulation. Magnetic r esonance imaging, while showing no evidence of an intrasellar tumour, revea led an 1.5 x 1.0 cm mass in the left sphenoid sinus which was initially int erpreted as most probably being a mucosal polyp. After neurosurgical remova l of the tumour, transient secondary adrenal insufficiency was present, The structure and immunostaining characteristics of the tumour demonstrated an ACTH cell adenoma of the pituitary, Ectopic ACTH-secreting pituitary adeno mas may cause significant difficulties in differential diagnosis, localisat ion and appropriate therapy. Thus, although these tumours are rare, they sh ould be included in the list of possible causes of ACTH-dependent Gushing's syndrome.