Small Animal Dermatology, 3rd edition

CHAPTER 26 HYPERADRENOCORTICISM, CANINE 399

Can potentially demonstrate both increased cortisol production and decreased

sensitivity to glucocorticoid feedback

Sensitivity 75–100%; specificity 20–25% in random sampling of hospital patients

Sensitivity 99%; specificity 77% in patients exhibiting physical and biochemical

changes consistent with HAC.

Tests to Differentiate PDH from ADH

 PDH and ADH treatment options and prognosis differ; several tests are used to dis-

tinguish the exact etiology: HDDST, HDDST with UCCR, endogenous ACTH, and

abdominal ultrasound.

 HDDST:
75% of dogs with PDH will demonstrate suppression of cortisol levels; 25% of

dogs with PDH do not suppress with HDDST (likely due to a large pituitary

tumor or tumor developing from the pars intermedia)

Nearly 100% of dogs with ADH will not demonstrate suppression of cortisol

levels

PDH: high dose of corticosteroid (dexamethasone) results in a decrease in ACTH

release from the pituitary and a resultant decrease in plasma cortisol

ADH: tumor secretes cortisol autonomously, suppressing ACTH production;

dexamethasone has no effect on plasma cortisol because ACTH is already sup-

pressed

Testing methodology: same as for LDDST except the dose of dexamethasone is

higher at 0.1 mg/kg given intravenously; the 4-h sample is omitted

Testing interpretation: an 8-h cortisol of less than 1.0–1.4μg/dL is consistent

with PDH.

 Endogenous ACTH:
Canine ACTH (cACTH) is secreted episodically from the pituitary gland
Concentrations of cACTH do not differ from normal dogs and PDH patients (not

a viable screening test for HAC)

Most accurate stand-alone test for differentiating PDH from ADH (normal or

elevated concentrations of cACTH=PDH; suppressed levels=ADH)

Very accurate but highly susceptible to sample mishandling causing the ACTH

levels to be falsely low and giving the false impression that the dog has ADH; use

of a preservative in EDTA tubes called aprotinin may result in fewer handling

errors (follow reference laboratory guidelines)

PDH cases have normal to high levels of endogenous ACTH

ADH cases have low to undetectable levels of endogenous cACTH because the

adrenal tumor produces high levels of cortisol, suppressing cACTH production

by the pituitary gland.

 Abdominal ultrasound:
Evaluate adrenal size (normal width should be less than 7 mm), symmetry, and

invasion of adjacent structures

Some adrenal tumors are mineralized

Often used in place of HDDST or endogenous ACTH testing.