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.