Pituitary oncocytoma presenting as Cushing's disease

A 19-year-old girl presented with classical features of Cushing's syndrome. Endocrinal evaluation was consistent with pituitary source of ACTH; but imaging showed normal pituitary. Bilateral inferior petrosal sinus sampling confirmed the diagnosis. A successful remission was achieved after adenomectomy by transphenoidal route. Histopathological examination was consistent with pituitary oncocytoma and immunohistochemistry was positive for synaptophysin, chromogranin, neuron specific enolase, S-100, ACTH, prolactin, and GH.