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 Cushing's syndrome showed baseline cortisol and ACTH values indicating ACTH-dependent disease. Dynamic function tests (dexamethasone, corticotropin releasing hormone (CRH), desmopressin), were suggestive of paraneoplastic ectopic ACTH production. However, inferior petrosal sinus (IPS) ACTH sampling demonstrated a maximum baseline central (363 pmol/l)-peripheral (19 pmol/l) ACTH gradient of 19.1 for the right IPS, conventionally suggestive of Cushing's disease. However, again, IPS ACTH level did not increase after CRH stimulation. Magnetic resonance imaging, while showing no evidence of an intrasellar tumour, revealed an 1.5 x 1.0 cm mass in the left sphenoid sinus which was initially interpreted as most probably being a mucosal polyp. After neurosurgical removal 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 adenomas may cause significant difficulties in differential diagnosis, localisation and appropriate therapy. Thus, although these tumours are rare, they should be included in the list of possible causes of ACTH-dependent Cushing's syndrome.