Objective
To evaluate low- and high-dose dexamethasone suppression tests for differentiating pituitary dependent hyperadrenocorticism (PDH) from adrenal tumor hyperadrenocorticism (ATH) in dogs.
Design
Prospective study.
Animals
181 dogs with PDH and 35 dogs with ATH.
Procedure
Plasma Cortisol concentrations from dogs with naturally developing hyperadrenocorticism were evaluated before, and 4 and 8 hours after administration of standard low- and high-doses of dexamethasone (0.01 mg/kg of body weight, IV, and 0.1 mg/kg, IV; respectively).
Results
In response to the low-dose test, all but 3 dogs had an 8-hour post-dexamethasone plasma Cortisol concentration that was consistent with a diagnosis of hyperadrenocorticism, that is, ≥ 1.4 μg/dl. Criteria used to distinguish PDH from ATH in response to low-dose dexamethasone included a 4-hour post-dexamethasone plasma Cortisol concentration < 50% of the basal value or < 1.4 μg/dl, or an 8-hour post-dexamethasone plasma Cortisol concentration < 50% of the basal concentration. Criteria used to distinguish PDH from ATH in response to high-dose dexamethasone included 4- or 8-hour post-dexamethasone plasma Cortisol concentrations < 50% of the basal concentration or < 1.4 μg/dl. In response to the low-dose test, 111 dogs met criteria for suppression (each had PDH). In response to the high-dose test, 137 dogs met criteria for suppression (2 had ATH, 135 had PDH). Twenty-six dogs with PDH (12%) had indications of adrenal suppression in response to high-dose but not low-dose testing.
Clinical Implications
Low-dose dexamethasone test has value as a discrimination test to distinguish dogs with PDH from those with ATH. The high-dose test need only be considered in dogs with hyperadrenocorticism that do not have adrenal suppression in response to the low-dose test. (J Am Vet Med Assoc 1996;209:772–775)
