How does the ACTH (Adrenocorticotropic Hormone) stimulation test work?
ACTH is a hormone naturally produced by the pituitary gland (a gland located in the brain). The purpose of ACTH is to tell the adrenal glands when and how much cortisol to produce. As the blood level of cortisol increases, it causes the pituitary gland to decrease its production of ACTH, which results in a lowering of the cortisol level. The ACTH simulation test mimics this natural stimulatory pathway and demonstrates the capacity of the pets adrenal glands to produce cortisol in response to an injection of synthetic ACTH.
This test requires two blood samples. The first blood sample is taken, then an injection of synthetic ACTH is given, and the second blood sample is taken 2 hours following the injection. When the levels of cortisol in the two samples are compared, they may show a normal response, an exaggerated response, or very little response.
What do we expect to see if the patient has Cushing’s disease?
With pituitary origin Cushings disease, we normally see a markedly increased cortisol level following the synthetic ACTH injection. This exaggerated response occurs because the adrenal glands have been over-stimulated with naturally occurring ACTH secreted by the diseased pituitary (pituitary origin Cushings disease). These adrenal glands are therefore highly responsive to stimulation by the synthetic ACTH. With adrenal origin Cushings disease, an exaggerated cortisol level after the synthetic ACTH injection may also be seen.
Since the ACTH stimulation test may be positive in both types of Cushings disease, the test confirms that your pet has Cushings disease, but not the type or cause of Cushings disease. Some dogs with Cushings disease of either pituitary or adrenal origin do not respond to the ACTH stimulation test and additional diagnostic testing is required to confirm a diagnosis.
Dogs that are showing signs of Cushings disease due to the effects of corticosteroid containing medications (called iatrogenic Cushings syndrome) will show either a very mild response or no response to synthetic ACTH injection. This type of muted response, in conjunction with a history of medication use, is confirmatory for iatrogenic Cushings syndrome.
Another use for the ACTH stimulation test is to monitor your dogs response to therapy for Cushings disease. Periodic ACTH stimulation tests during the initiation and maintenance phases of treatment will be required. As your dog responds to treatment, the increases in cortisol will become less exaggerated in response to injection of synthetic ACTH.
As the cortisol concentration in the blood fluctuates greatly throughout the day in both healthy dogs and dogs with Cushing’s, diagnosis cannot be confirmed by just one measurement of cortisol. The two tests most commonly used to confirm diagnosis are the low-dose dexamethasone suppression test and the ACTH stimulation test. In most cases your pet will stay at the vets while the tests are undertaken.
However, it is important to remember that neither of these tests are perfect and your vet may have to perform both tests in order to gain a diagnosis. Equally, it may be that for your pet, repetition of tests over time is required. We recommend you speak to your vet in more detail about the diagnostic procedure for your individual dog. https://www.youtube.com/embed/ktO8vYXOhTU