Hypercortisolism due to an autonomously functioning pituitary adenoma: limitations in diagnostic test procedures.

We report a patient with pituitary-dependent Cushing's syndrome in whom standard biochemical testing procedures failed to define the pituitary as the source of autonomous hormone secretion. Anomalous findings included: 1) wide variations of urinary steroid excretion during a 4-day dexamethasone suppression test, 2) a rise of the serum cortisol after the single dose (8 mg) overnight dexamethasone suppression test, 3) a diminished serum 11-deoxycortisol response to the metyrapone test, and 4) an absent plasma ACTH response to CRH. While the results of these studies were most consistent with an ectopic source of ACTH production, magnetic resonance imaging suggested that Cushing's syndrome was due to a pituitary adenoma. The adenoma was removed by transphenoidal surgery. However, serum and urinary steroid levels remained measurable for 2 weeks because of the perioperative use of im cortisone acetate. Subsequent im administration of 200 mg cortisone acetate again led to prolonged serum and urinary steroid responses to this compound, thus emphasizing the pitfalls that may accompany the perioperative use of cortisone acetate. This patient demonstrates the importance of performing multiple tests, including magnetic resonance image scanning, in evaluation Cushing's syndrome, since standard biochemical testing may not indicate pituitary ACTH hypersecretion as the cause.