I recently ran a low-dose dexamethasone suppression test on a 11-year-old male neutered Maltese with clinical features of muscle loss, thinning coat, and a pot-bellied appearance. His serum chemistry panel is completely normal except an alkaline phosphatase activity in the thousands. Below are the results low-dose dexamethasone suppression test, but I am confused as to what to make of them.
Basal cortisol: 5.1 μg/dl (reference range = 1-4 μg/dl)
5-hr post cortisol: <0.7 μg/dl (reference range = <1.5 μg/dl)
8-hr post cortisol: 1.9 μg/dl (reference range = <1.5 μg/dl)
Thanks in advance.
The interpretation would be as follows: Slightly high basal cortisol, with complete suppression of cortisol at 5 hours, followed by escape from cortisol suppression at 8 hours.
The lack of suppression at the 8-hour testing period is diagnostic for hyperadrenocorticism. The fact that the dog suppressed so well at 4 hours tells us that this dog's Cushing's disease cannot be caused by an adrenal tumor (ie, these dogs would not suppress cortisol with either the low- or high-dose dexamethasone suppression test).
The pattern of cortisol suppression at 3-5 hours followed by escape from cortisol suppression at 8 hours, as shown here in this dog, is always diagnostic for pituitary-dependent hyperadrenocorticism.