ClinQuiz: LDDS - Answer 1B
1B—Incorrect. Although this dog has abnormally high cortisol concentrations at all time points, ADH cannot be definitively diagnosed based on these results. Dogs with adrenal tumors never exhibit cortisol concentration suppression. Because these dogs’ endogenous ACTH concentrations are already suppressed, administering dexamethasone will not cause additional ACTH suppression and, subsequently, cortisol concentration suppression. Furthermore, up to 30% of dogs with PDH also fail to show suppression during an LDDS test. Dogs with PDH that fail to suppress may have disease in the pars intermedia, which is not responsive to dexamethasone, or they may have large pituitary tumors (macroadenomas). Thus, these results do not help you distinguish between ADH and PDH, and an endogenous plasma ACTH concentration should be measured.