The plasma cortisol response to an intramuscular injection of 10 units of lysine vasopressin was evaluated in 19 normal subjects and in 35 patients with various disturbances of endocrine function and was compared with the response to standard adrenocorticotrophic hormone (ACTH) and metyrapone tests. A rise in plasma cortisol of 6 µg/100 ml or greater occurred in all 19 normal subjects and in 22 of the 35 patients studied. In general, the results of metyrapone and vasopressin testing were in agreement, but discordant results were obtained in 9 of the 35 patients. Two of these nine patients had a normal vasopressin but subnormal metyrapone response, and, conversely, seven patients had a subnormal vasopressin and normal metyrapone response. These data suggest that vasopressin may yield evidence of impaired ACTH release not otherwise shown by metyrapone. Furthermore, these data—particularly the diminished vasopressin responses in two patients with hypothalamic disease—raise serious doubt concerning the theory that hypothalamic and pituitary disease may be distinguished on the basis of vasopressin and metyrapone testing.
Six of seven patients with Cushing's syndrome with adrenocortical hyperplasia, including one with a pituitary tumor, responded normally to vasopressin, whereas one patient with adrenocortical carcinoma failed to respond, thus supporting the contention that vasopressin may be useful in distinguishing Cushing's syndrome on a pituitary basis from that secondary to other causes. However, further studies are indicated in such patients, particularly in those with functioning adrenocortical adenoma and ectopic ACTH-producing tumor.