Hyperadrenocorticism is a relatively commonly diagnosed condition in dogs. The low dose dexamethasone suppression test and ACTH stimulation tests are used to diagnose this condition, but are of limited use in differentiating between pituitary-dependent hyperadrenocorticism (PDH) and adrenal-dependent hyperadrenocorticism (ADH).
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
In a study of 109 dogs with confirmed hyperadrenocorticism, there was no overlap in ACTH concentrations between those dogs with pituitary-dependent hyperadrenocorticism (PDH) and those with adrenal-dependent hyperadrenocorticism (ADH). All dogs with ADH had ACTH levels below the low detection limit of the assay (<5 pg/ml). Dogs with PDH had a median ACTH of 30 pg/ml (range 6 - 1250 pg/ml). 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
- Preferably fast the animal overnight prior to collection.
- The animal should not be stressed or excited.
- Collect a minimum of 1 mL of blood into a plastic EDTA tube.
- Immediately place the sample in the refrigerator; this is important as ACTH deteriorates rapidly at room temperature. Do not centrifuge the sample.
- If possible, keep the sample cool during transit to the laboratory; this is particularly important during the summer months.
- Samples collected and stored correctly are stable for 24 - 48 hours.
- Samples not analyzed within 24 - 48 hours should be submitted to the lab frozen (should not defrost during transit).
- The test is run Monday to Saturday with results available within 24 hours.
REF: Rodriguez Pineiro et al. J Vet Int Med 2009, 23:850-855.
DCI-06 Endogenous ACTH (V2.0, 14-09-2004)