Adrenocorticotrophin-independent macronodular adrenal hyperplasia in a patient with lysine vasopressin responsiveness but insensitivity to gastric inhibitory polypeptide

The aetiology of ACTH-independent macronodular adrenal hyperplasia (AIMAH) is uncertain. We examined a 55 year old man with Cushing's syndrome due to AIMAH, whose cortisol levels increased after stimulation with lysine-8-vasopressin (LVP) in vitro as well as in vivo. Abdominal MRI revealed nodular enlargement of both adrenal glands. No adenoma was evident on pituitary MRI. 131I-adosterol scintigraphy exhibited marked uptake into both adrenal glands. Although baseline plasma cortisol levels were within normal limits, urinary free cortisol excretion was 3-fold higher than the upper limit of the normal range. Plasma ACTH levels were undetectable. Oral dexamethasone failed to suppress plasma cortisol levels irrespective of dose, and administration of corticotrophin releasing hormone failed to increase plasma ACTH and cortisol levels. LVP injection failed to increase plasma ACTH levels, but elicited an increase in plasma cortisol levels. The direct stimulatory effect of LVP on cortisol secretion was confirmed in vitro in cultured adrenocortical cells from macronodules obtained at surgery. Food intake, gastric inhibitory polypeptide (GIP), or octreotide administration, which were reported to regulate cortisol release in patients with AIMAH, failed to affect plasma cortisol levels. In conclusion, plasma cortisol responsiveness to LVP, GIP, and octreotide is heterogeneous in patients with AIMAH.