Why Are My Dog Or Cat’s Dexamethasone Suppression Test Results Abnormal?
Ron Hines DVM PhD
Causes Of Most Abnormal Blood & Urine Tests
The Dexamethasone Suppression Test, Low and High Dose Methods = LDDST, HDDST
When your veterinarian sees symptoms in your pet (generally a dog but occasionally a cat) that could be due to overactive adrenal glands (Cushing’s disease), your vet might suggest a dexamethasone suppression test. That test requires that your pet stay at the animal hospital for a time. Your veterinarian will take a blood sample prior to the test to determine the pet’s “resting” (baseline) blood cortisol level and then administer a dose of a corticosteroid compound called dexamethasone. Dexamethasone is quite similar to your pet’s natural cortisol in its structure and effects. It is your pet’s two adrenal glands that normally produce it. In normal dogs (or the rare cat that might be a candidate for this test), the low dose of dexamethasone given will suppress blood levels of cortisol in your pet’s blood. That is because it tricks the pituitary gland into thinking cortisol levels need to go down. Your vet will take two more blood samples; one 4 hours and one 8 hours after the dexamethasone injection. In normal dogs, cortisol levels will be lower after the dexamethasone injection. In Cushing’s disease (aka hyperadrenocorticism) no cortisol decrease should occur.
Overactive adrenal glands can be due to a tumor within one or both of the glands. It could be due to a tumor that is producing cortisol, or it could be due to a tumor within the pet’s pituitary gland (= pituitary dependent hyperadrenocorticism aka PDH) that produces the ACTH hormone that stimulates adrenal glands to produce cortisol.
In an attempt to tell whether the tumor is in the pituitary or in the adrenal gland, your veterinarian might suggest a high-dose dexamethasone suppression test. About 80% of Cushing’s disease dogs have the tumor in their pituitary gland and not in their adrenal glands. In that second test, a substantial decrease in blood cortisol after the dexamethasone is administered point to the problem being in your pet’s pituitary gland. However, the high-dose dexamethasone test is not foolproof. For reasons unknown, some cases of PDH fail to suppress cortisol as expected.
Are There Factors That Might Interfere With The Test’s Accuracy?
Icterus (jaundice) or positive blood tests that indicate the presence of liver problems can affect test results. That is because dexamethasone is eliminated (cleared) by your pet’s liver. Dexamethasone can linger in the blood of animals whose livers are unable to process it.
A recent corticosteroid injection or tablets can also make test results inaccurate.
An alternative diagnostic approach is to perform an ACTH stimulation test. That is the test of choice when under-active adrenal glands are suspected (=Addison’s disease).
CBC/ WBC, blood chemistry panel including AP, electrolytes, urinalysis, abdominal ultrasound, plasma ACTH level. When Dexamethasone suppression results do not indicate adrenal problems, tests for diabetes, liver disease or hypothyroidism are appropriate.
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