Radioimmunoassayable ACTH in dexamethasone nonsuppressible Cushing's syndrome.

Abstract

Radioimmunoassayable serum ACTH has been measured in a patient with Cushing's syndrome due to bilateral adrenal hyperplasia showing failure of dexamethasone suppression of plasma and urinary steroids with up to 32 mg of dexamethasone/day. Plasma ACTH was consistently elevated and showed an abnormal diurnal pattern which altered with increasing doses of dexamethasone. On extremely high doses of dexamethasone, urinary 17-ketogenic steroid excretion doubled over base line at a time when ACTH seemed to lose its diurnal pattern and remain at a relatively high level. The patient's ACTH response to metyrapone was similar to that of a patient with suppressible Cushing's syndrome and adrenal hyperplasia. The intravenous infusion of hydrocortisone produced a dramatic fall in serum ACTH concentration not noted during the administration of dexamethasone. The study suggests a derangement in the feedback receptor for dexamethasone as the cause of this anomalous response of ACTH and adrenal steroids during dexamethasone administration.