Use of low- and high-dose dexamethasone tests for distinguishing pituitary-dependent from adrenal tumor hyperadrenocorticism in dogs

Abstract

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, i.v., and 0.1 mg/kg, i.v.; respectively).

Results: In response to the low-dose test, all but 3 dogs had an 8-hours post-dexamethasone plasma cortisol concentration that was consistent with a diagnosis of hyperadrenocorticism, that is, > or = 1.4 micrograms/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 micrograms/dl, or an 8-hours 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 micrograms/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.