Distinguishing dogs with functioning adrenocortical tumors from dogs with pituitary-dependent hyperadrenocorticism

SUMMARY Fifty-seven dogs with hyperadrenocorticism were studied. In each dog the histologic diagnosis was an adrenocortical tumor or pituitary-dependent hyperadrenocorticism. The plasma concentration of endogenous ACTH was determined at least once in each dog. Dogs with an ACTH concentration <20 pg/ml ultimately were shown to have an adrenocortical adenoma or carcinoma. All dogs with an ACTH concentration >40 pg/ml ultimately were shown to have pituitary-dependent hyperadrenocorticism. Retrospectively, ACTH concentrations were correct in localizing the disorder in 54 of 57 dogs. Three dogs had endogenous ACTH concentrations of 20 to 40 pg/ml, which were considered nondiagnostic. Three dexamethasone suppression testing protocols were reviewed. Thirteen dogs were given 0.1 mg of dexamethasone/kg of body weight, iv. Plasma cortisol concentrations were obtained before and 8 hours after injection. Complete suppression was described as a cortisol value <50% of base line after dexamethasone administration. Three dogs with adrenocortical tumors did not have complete suppression, whereas 9 of 10 dogs with pituitary-dependent hyperadrenocorticism did have complete suppression. Two additional dexamethasone suppression protocols were found to be less reliable. Administration of 1 mg of dexamethasone/kg of body weight, iv, with comparison of base-line and 8-hour plasma cortisol values, was diagnostic in 6 of 9 dogs. Administration of 0.2 mg of dexamethasone/kg of body weight, im, tid, with comparison of the plasma cortisol concentration before starting the test and 8 hours after the final dose was given, was diagnostic in 11 of 14 dogs.