An i.v. infusion
dexamethasone (Dex) test was used to investigate the
ACTH feedback response in 9 normal subjects, 12 obese patients, and 11 patients with
Cushing's syndrome. Dex
phosphate was infused iv for 4 h, starting at 1100 h (1 mg/h). Plasma concentrations of
beta-lipotropin (
beta LPH) and
cortisol were measured every 20 min between 0900 and 1600 h, then every 2 h until midnight and at 0900 h the next day. In normal subjects and obese patients, plasma
beta LPH and
cortisol concentrations fell rapidly to less than 40 ng/liter and 3 micrograms/dl, respectively, at the end of Dex infusion. Subsequent values remained low through 0900 h the next day. In 7 patients with
Cushing's disease, basal plasma
beta LPH and
cortisol concentrations declined by greater than 50% during the Dex infusion. In these patients, rapid escape from suppression occurred between 1600 and 2400 h; by 0900 h the following day,
beta LPH and
cortisol levels were higher than 100 ng/liter and 10 micrograms/dl, respectively. In 3 patients with adrenal
tumors,
beta LPH concentrations were low, and
cortisol concentrations did not decline during the Dex infusion. In 1 patient with ectopic
ACTH secretion,
beta LPH concentrations were high and were not suppressed by the Dex infusion. We conclude that the iv infusion Dex suppression test can distinguish patients with
Cushing's syndrome from normal or obese subjects and can aid in the etiological diagnosis of
Cushing's syndrome.