Testing for Addison’s disease in dogs starts with blood work and is confirmed with a specific hormone stimulation test. The process typically involves an initial screening, followed by the gold standard ACTH stimulation test, where your vet measures how your dog’s adrenal glands respond to a synthetic hormone injection. The whole diagnostic workup can often be completed in a single vet visit, though results may take a day or two depending on the lab.
Initial Blood Work and Electrolyte Screening
When a vet suspects Addison’s disease, the first step is usually a standard blood panel. The classic red flag is an abnormal ratio of sodium to potassium in the blood. In a healthy dog, sodium levels are much higher than potassium levels. When the adrenal glands stop producing enough of the hormone that regulates salt and water balance, sodium drops and potassium rises. This electrolyte shift is one of the earliest clues, and it’s often what prompts further testing.
However, not every dog with Addison’s shows this electrolyte pattern. A subset of dogs have what’s called “atypical” Addison’s, where sodium and potassium levels look completely normal at the time of diagnosis. These dogs are only deficient in cortisol, the stress hormone, making them harder to catch on routine blood work alone. This is why electrolyte screening can point toward Addison’s but can’t rule it out on its own.
Resting Cortisol as a Quick Screen
A simple baseline cortisol measurement, drawn from a single blood sample, is a useful screening tool. If your dog’s resting cortisol level comes back above 2 μg/dL, Addison’s disease is extremely unlikely, and your vet can move on to investigating other causes. This test has a sensitivity of 99.4% to 100%, meaning it catches virtually every dog that actually has the disease.
The catch is that specificity is low, between 63% and 78%. That means a low resting cortisol doesn’t confirm Addison’s. Many dogs without the disease also have low readings, especially if they’re stressed or sick from something else. So a low result simply tells your vet that the next step, the ACTH stimulation test, is warranted.
The ACTH Stimulation Test
This is the gold standard for diagnosing Addison’s disease in dogs. The test measures whether your dog’s adrenal glands can produce cortisol when they receive a direct signal to do so.
Here’s what happens: your vet draws a baseline blood sample, then injects a synthetic version of the hormone that normally tells the adrenal glands to release cortisol. One hour after the injection, a second blood sample is drawn. In a healthy dog, cortisol levels rise significantly after the injection. In a dog with Addison’s, the adrenal glands are too damaged (or too shrunken) to respond, and cortisol stays flat.
A post-stimulation cortisol level below 2 μg/dL (55 nmol/L) confirms the diagnosis. If the adrenal glands produce a normal cortisol response, Addison’s is ruled out. Your dog will need to stay at the clinic for about an hour to two hours for the test, but it’s straightforward and doesn’t require sedation. Lab fees for the ACTH stimulation test itself run around $44 at university labs, though the total cost of the visit will be higher once you factor in the exam, blood draws, and any additional panels your vet runs.
Steroid Washout Before Testing
If your dog has recently been on prednisone, dexamethasone, or any other steroid medication, those drugs will interfere with cortisol measurements and can produce misleading results. Your vet will need to taper the steroids and then wait a minimum of three weeks before running the ACTH stimulation test. If your dog is in crisis and needs emergency stabilization with steroids before testing is possible, your vet may use a specific steroid that doesn’t cross-react with the cortisol assay, or plan to test once your dog is stable and off medication long enough.
Distinguishing Primary From Secondary Addison’s
The ACTH stimulation test confirms that the adrenal glands aren’t working, but it doesn’t explain why. In most dogs, the cause is primary: the adrenal glands themselves are destroyed, usually by the dog’s own immune system. Less commonly, the problem is secondary: the pituitary gland in the brain isn’t sending the signal that tells the adrenals to work.
To tell the two apart, your vet can measure endogenous ACTH, the natural version of the hormone the pituitary gland produces. In primary Addison’s, the pituitary is desperately trying to wake up the adrenals, so ACTH levels are sky-high. In secondary Addison’s, the pituitary itself is the problem, so ACTH levels are abnormally low. One study found that a cortisol-to-ACTH ratio of 0.01 or below had 100% sensitivity and 99% specificity for diagnosing the disease.
This test requires careful sample handling because the hormone breaks down quickly outside the body, and the blood sample must be collected before any steroid treatment is given. Not every clinic runs this test routinely, but it’s valuable when the clinical picture is unclear or when atypical Addison’s is suspected.
Atypical Addison’s Is Easy to Miss
Dogs with atypical Addison’s disease have normal electrolytes but deficient cortisol. Because their sodium and potassium look fine, the initial blood work doesn’t raise the usual alarm bells. These dogs often show vague symptoms like chronic vomiting, diarrhea, weight loss, and lethargy that wax and wane over weeks or months. They’re frequently misdiagnosed with inflammatory bowel disease or other gastrointestinal conditions before someone thinks to check cortisol.
The ACTH stimulation test still works for diagnosing atypical cases, but the diagnostic cutoff may not be as clear-cut. Some dogs with atypical Addison’s produce post-stimulation cortisol levels that fall below the normal reference range but remain above the standard 2 μg/dL threshold used for classic cases. These borderline results put dogs in a gray zone where further investigation is needed. Your vet may recommend monitoring over time, measuring endogenous ACTH levels, or ruling out other conditions before committing to a diagnosis.
Conditions That Mimic Addison’s
Before your vet can be confident in a diagnosis, they’ll want to rule out a few conditions that can look remarkably similar on blood work. The most notable is a whipworm infection. These intestinal parasites can cause the same sodium-potassium imbalance seen in classic Addison’s disease, a phenomenon sometimes called “pseudo-Addison’s.” The worms damage the intestinal wall severely enough to disrupt electrolyte balance, creating a blood panel that’s nearly indistinguishable from the real thing.
The key difference is that a dog with pseudo-Addison’s will have a normal ACTH stimulation test, because the adrenal glands are perfectly healthy. Deworming leads to a complete recovery. Standard fecal smear tests miss whipworms more than 90% of the time, so your vet may use a centrifugation-based fecal test or even abdominal ultrasound to check. Other conditions that can produce similar electrolyte shifts include kidney disease, gastrointestinal fluid losses, and certain heart conditions, all of which should be considered before landing on an Addison’s diagnosis.
What to Expect on Test Day
If your vet wants to run an ACTH stimulation test, plan for your dog to be at the clinic for about one to two hours. Your dog may need to fast beforehand, since fat in the blood from a recent meal can interfere with the cortisol measurement. The test involves two blood draws and one injection, all of which are quick and minimally uncomfortable.
Results typically come back within one to two days if sent to an outside lab, though some clinics with in-house analyzers can have preliminary numbers the same day. If the test confirms Addison’s, treatment usually begins immediately with hormone replacement, and most dogs respond dramatically within days. If results are borderline or the clinical picture doesn’t quite fit, your vet may recommend additional testing, including the endogenous ACTH measurement or imaging of the adrenal glands via ultrasound, to clarify what’s going on.