Effect of functional adrenocortical tumors on plasma cortisol and corticotropin concentrations in dogs.

Plasma endogenous adrenocorticotropic hormone (ACTH) concentrations as well as basal and post-ACTH-stimulation plasma cortisol concentrations were measured in 6 dogs ultimately shown to have hyperfunctioning adrenocortical tumors. The basal and post-ACTH-stimulation plasma cortisol concentrations were found to fluctuate in 4 dogs in which repeat studies were done. Three dogs had at least 1 normal and 1 abnormally high plasma cortisol concentration after exogenous ACTH administration. A 4th dog had 1 normal and 1 abnormally high resting cortisol concentration. One dog tested twice 3 days apart had similar test results each time and a 6th dog was tested once. All of the dots had at least 1 plasma endogenous ACTH concentration greater than 2 standard deviations below the normal mean. All endogenous ACTH concentrations were less than those previously reported for dogs with pituitary-dependent hyperadrenocorticism. The results indicated that the diagnosis of adrenocortical tumor cannot reliably be made on the basis of the plasma cortisol response to exogenous ACTH; however, the use of plasma endogenous ACTH determination can be an important diagnostic aid.