Canine Adrenal Testing - Which Test Should I Run?

By Jennifer S. Fryer, DVM

Urine Cortisol:Creatinine Ratio:

· Screening test for Hyperadrenocorticism (Cushing’s or HAC)

· Low Cost, Easy to collect (voided morning urine at home)

· Normal value rules out Hyperadrenocorticism

· Elevated values can indicate stress or Hyperadrenocorticism & adrenal function testing is necessary.

Baseline Cortisol:

· Screening test for Hypoadrenocorticism (Addison’s)

· Values >2 mcg/dl rule out Hypoadrenocorticism (Addison’s)

· Cannot be used to diagnose Hyperadrenocorticism (Cushing’s)

ACTH Stimulation Test:

· Test of choice to diagnose Hypoadrenocorticism (Addison’s)

· Screening test for Hyperadrenocorticism (Cushing’s)

· Used to monitor Trilostane or Lysodren Therapy

· Can be used to differentiate spontaneous vs. iatrogenic HAC

· 60-85% of dogs with HAC will have a positive result on this test.

· 85-90% of dogs without HAC will have a negative result on this test.

· Advantages:

o Can be completed in 1 hour

o No special handling of samples

o Submit for extended Adrenal Panel to document Atypical HAC

· Disadvantages:

o High cost of Cosyntropin

o Low Sensitivity (false negatives are possible)

Low-Dose Dexamethasone Suppression Test:

· Screening test for Hyperadrenocorticism (Cushing’s)

· Helps differentiate pituitary vs. adrenal origin

· 85-95% of HAC dogs have a positive result.

· 70-75% of dogs without HAC have a negative result.

· 40% of dogs with PDH and all adrenal tumor dogs have dexamethasone resistance and will require another differentiating test.

· Advantages:

o Low cost

o Higher sensitivity than ACTH Stim

o No special handling of samples

· Disadvantages:

o All day test requiring three blood samples at 0, 4, 8 hours

o The dog should be kept as minimally stressed as possible during this 8 hour period

High-Dose Dexamethasone Suppression Test:

· Theoretically helps differentiate Hyperadrenocorticism of pituitary vs. adrenal origin.

· Similar results to Low-Dose Dexamethasone Suppression Test at 8 hours.

· Rarely performed.

Endogenous ACTH Measurement:

· Helps differentiate pituitary vs. adrenal HAC

· Single plasma sample required

· Sample handling is difficult & critical to accurate measurement.

· With proper sample handling, this test is very reliable at differentiating pituitary vs. adrenal HAC.

Abdominal Ultrasound:

· Helps differentiate pituitary vs. adrenal HAC.

· May identify adrenal tumor, local invasion or metastasis.

· High cost

· Adrenals can be normally sized in PDH

· Adrenals can be difficult to visualize in some animals

· Ultrasound does not always accurately identify extent of metastasis or local invasion of an adrenal tumor

Computed Tomography (CT Scan):

· Screening test for Pituitary Tumor or Primary Adrenal Tumor and abdominal metastasis &/or local invasion

· Brain CT is not indicated unless a macroadenoma is suspected.

· Very high cost.

· Requires anesthesia.

· Cannot detect 50% of pituitary masses.

· Cannot differentiate between functional and non-functional tumors. Adrenal function tests are still required.

Brain Magnetic Resonance Imagine (MRI):

· Screening test for Pituitary Tumor

· Brain MRI is not necessary unless a macroadenoma is suspected.

· More reliable than CT at detecting small pituitary masses.

· Very high cost.

· Requires anesthesia.

· Not indicated unless a macroadenoma is suspected.

· Cannot differentiate between functional and non-functional tumors. Adrenal function tests are still required.

References

Lennon EM, Boyle TE, Hutchins RG, et al. Use of basal serum or plasma cortisol concentrations to rule out a diagnosis of hypoadrenocorticism in dogs: 123 cases (2000-2005). J Am Vet Med Assoc 2007;231(3):413-6.

Nelson RW, Turnwald GH, Willard MD. Endocrine, Metabolic, and Lipid Disorders. In: Willard MD and Tvedten H, eds. Small Animal Clinical Diagnosis by Laboratory Methods. 4^th edition. St. Louis: Elsevier Saunders, 2004:165-207.

Reusch, CE. Hyperadrenocorticism. In: Ettinger SJ and Feldman EC, eds. Textbook of Veterinary Internal Medicine. 6^th edition. St. Louis: Elsevier Saunders, 2005:1592-1611.