SPECIFIC TESTS
TO
CONFIRM CUSHING’S DISEASE



    THE LOW DOSE DEXAMETHASONE SUPPRESSION TEST


    Dexamethasone is a cortisone-type hormone which is used therapeutically for numerous conditions. The dog’s pituitary gland will perceive the presence of dexamethasone and shut off its stimulatory message to the adrenal glands. In the normal animal, this means that a drop in blood cortisol level will be seen 8 hours after a tiny dose of dexamethasone is given.

    If a pituitary tumor is present, the pituitary is not about to shut off its stimulatory message and ignores the presence of the dexamethasone. No drop in cortisol level is seen at the end of eight hours.

    THIS TEST IS CURRENTLY CONSIDERED THE MOST ACCURATE IN THE CONFIRMATION OF CUSHING’S DISEASE.


    TO RUN THIS TEST


    Ideally this test is run in the morning. A baseline cortisol level is measured, a low dose of dexamethasone is given intravenously, and blood samples are checked again in 8 hours. Sometimes a 4 hour sample is also drawn as the pattern of suppression over the entire 8 hours may help classify the type of Cushing’s disease. The pet will require at least 8 hours in the hospital.

    THE ACTH STIM TEST


    Central to the concept of Cushing’s disease is the over-production of cortisol. It follows then that the adrenal glands of the Cushing’s patient would possess large amounts of stored hormone due to their chronic stimulation.

    We have been talking about the stimulatory message sent from the pituitary gland to the adrenal glands. This message consists of a hormone called Adrenocorticotrophic hormone or ACTH. In this test, a dose of ACTH is given to the patient. If a larger than expected, rise in cortisol levels is measured in 2 hours we may diagnose Cushing’s syndrome.

    TO RUN THIS TEST


    Ideally, the patient is fasted overnight and the test is performed in the morning between 8 A.M. and 10 A.M. A blood sample is drawn, a dose of ACTH is given, and two hours later a second blood sample is drawn.

    WHEN WOULD WE RUN THIS TEST?


    Given that the Low Dose Dexamethasone Suppression test is more accurate, when might we run this test instead? It turns out that this is the only test that can be used if the iatrogenic form of Cushing’s disease is suspected. This test is also crucial in monitoring patients with Cushing’s disease, depending on which medications are used.

    THE URINE CORTISOL/CREATININE RATIO


    This is a screening test for Cushing’s disease; a positive test here does NOT confirm Cushing’s syndrome but a negative test here DOES rule it out. In this test a single urine sample is collected and the relative amounts of cortisol and creatinine (creatinine is a protein metabolyte that is excreted in urine constantly). If there is a high ratio (a relatively high amount of cortisol being excreted), further testing is in order.



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