Twelve-hour cycles of adrenocorticotropin and cortisol secretion in Cushing's disease.

A 50-yr-old woman presented with a 7-yr history of Cushing's syndrome. She had high urinary free cortisol levels (711 micrograms/day) and a positive response to metyrapone and suppression with 8 mg dexamethasone, suggesting pituitary ACTH-dependent adrenocortical hyperfunction. Plasma cortisol and ACTH levels showed marked temporal variation. Samples obtained at 2-h intervals revealed that plasma ACTH and cortisol levels surged during periods lasting approximately 12 h and fell to levels below normal during the remainder of the 24-h cycle. When urinary free cortisol was measured in urines collected in 12-h aliquots, the corresponding surge in free cortisol excretion was also evident. These cycles were not affected by a 2.5-month course of cyproheptadine. Pituitary irradiation and mitotane therapy resulted in remission of Cushing's syndrome, with disappearance of the peaks of ACTH and cortisol secretion. This case represents an unusual example of the abnormal circadian rhythm of ACTH secretion on Cushing's disease.