i. Histopathologic confirmation of an ACTH-staining adenoma in patients who have had prior pituitary surgery. OR ii. MRI confirmation of pituitary adenoma > 6 mm OR iii. Bilateral inferior petrosal sinus sampling (BIPSS) with either CRH or DDAVP stimulation for patients with a tumor ≤ 6mm. The criteria for a confirmatory BIPSS test are any of the following: Pre-dose central to peripheral ACTH gradient > 2; Post-dose central to peripheral ACTH gradient > 3 after either CRH or DDAVP stimulation