Why Are My Dog Or Cat’s Dexamethasone Suppression Test Results Abnormal?

Dexamethasone Suppression Test

Ron Hines DVM PhD

To see what normal blood and urine values are, go here

For an explanation of causes of most abnormal blood and urine tests, go here

To see how tests are grouped, go here

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. The vet will take a blood sample prior to the test to determine its resting (baseline) blood cortisol  level and then administer a dose of a corticosteroid compound called dexamethasone. That compound is quite similar to cortisol it its structure and effects. In normal dogs, the low dose of dexamethasone given will suppress blood levels of cortisol in your pet’s blood (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 = HC) no cortisol decrease will occur.

Overactive adrenal glands can be due to a tumor within one or both adrenal glands that produces cortisol or a tumor within the pet’s pituitary gland (pituitary dependent hyperadrenocorticism) 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 vet may attempt a high dose dexamethasone suppression test (about 80% of Cushing’s dogs have the tumor in their pituitary gland). In that test, a substantial decrease in blood cortisol after the dexamethasone is administered point to the problem being in the pet’s pituitary gland. The test, however, is not foolproof as some cases of PDH fail to suppress cortisol in this test.

Are There Factors That Might Interfere With The Test’s Accuracy?

Yes,

Icterus (jaundice) or blood tests that indicate liver problems can affect the test results. That is because dexamethasone is eliminated (cleared) by the pet’s liver.

A recent corticosteroid injection or tablets can also make the test results inaccurate.

An alternative diagnostic approach is to perform an ACTH stimulation test. That is the test of choice when underactive adrenal glands are suspected (Addison’s disease).

Complementary Tests:

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 and hypothyroidism are appropriate.

DxMe

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