The results of a low dose dexamethasone suppression test may suggest ADH vs PDH, but do not confirm it.

In ADH, the adrenal tumour(s) secrete excess cortisol which inhibits the release of endogenous ACTH. In PDH, the pituitary lesion produces excess ACTH which stimulates the adrenal glands to produce excess cortisol. The measurement of endogenous ACTH has been found to be useful in discriminating between the two conditions. An elevated ACTH is evidence of pituitary-dependent hyperadrenocorticism, but it is important to note that a normal ACTH does not rule out pituitary hyperadrenocorticism as some dogs with pituitary disease do not have elevated endogenous ACTH (this is assumed to be due to the pulsatile nature of ACTH secretion, even by pituitary tumours).

ACTH Levels (Scott-Moncrieff et al)
(Note: lower limit of detection is 10 pg/mL)

**Scott-Moncrieff reported that approximately 55% of cases (6/11) confirmed as having PDH had elevated endogenous ACTH (this is similar to the proportion reported for other methods). In a preliminary study at VETPATH, 75% of dogs (9/12) with LDDST results consistent with hyperadrenocorticism (PDH or ADH) had elevated ACTH levels indicative of PDH.

Sample Requirements

1. Preferably fast the animal overnight prior to collection.
2. The animal should not be stressed or excited.
3. Collect a minimum of 1 mL of blood into a plastic EDTA tube.
4. Immediately place the sample in the refrigerator; this is important as ACTH deteriorates rapidly at room temperature.Do not centrifuge the sample.
5. If possible, keep the sample cool during transit to the laboratory; this is particularly important during the summer months.
6. Samples collected and stored correctly are stable for at least 48 hours.
7. The test is run Monday to Friday with the results available the same day.

REF: Scott-Moncrieff JK, Koshko MA et al. (2003). Validation of a chemiluminescent enzyme immunometric assay for plasma adrenocorticotropic hormone in the dog.Vet Clin Path 23:180-187.

DCI-06 Endogenous ACTH (V2.0, 14-09-2004)