Addison's Disease in Dogs Test: ACTH Stim and Na:K Ratio

An Addisonian crisis is a medical emergency. If your dog has collapsed, cannot stand, is profoundly weak, is vomiting repeatedly, feels cold, has pale gums or seems to be in shock, go to an emergency veterinary hospital immediately rather than waiting for an appointment or for test results. Call your regular veterinarian the same day for milder but repeating signs: unexplained lethargy, appetite loss, vomiting, diarrhea, trembling, weight loss or increased drinking that keeps coming back after seeming to resolve. Tell whoever answers the phone that the episodes are recurring, because that pattern is the clue that often prompts adrenal testing.

Yes. Addison's disease in dogs, known clinically as hypoadrenocorticism, is confirmed with an ACTH stimulation test, which measures cortisol in blood samples taken before and after an injection of synthetic ACTH. Routine blood work almost always comes first and supplies the clue that starts the investigation: high potassium, low sodium, and a sodium to potassium ratio that has slipped below the expected range. Electrolytes alone, though, never close the case. No single blood value diagnoses Addison's disease, because the ACTH stimulation test is the only test that confirms it, and every other result in the workup simply raises or lowers the suspicion that leads you there.

Why this disease is usually caught late

Addison's has been called the great pretender for good reason. The adrenal glands stop producing enough cortisol, and in most affected dogs, enough aldosterone as well. The result is a dog who is intermittently off: lethargic for a few days, uninterested in food, vomiting, passing loose stool, shaky, losing weight, drinking more, then apparently fine again for weeks.

Many of these dogs receive fluids and anti-nausea medication at an emergency visit, improve overnight, and go home with a diagnosis of dietary indiscretion. Fluids temporarily correct the volume and electrolyte problem, so the improvement is real. It just is not an answer. The relapse pattern is the signal.

Other dogs present the opposite way, collapsing in an Addisonian crisis with a dangerously slow heart rate, weakness and shock. That is a genuine emergency and needs immediate veterinary care.

Risk skews toward young to middle-aged dogs, more often female, and several breeds are overrepresented, including standard poodles, Portuguese water dogs, bearded collies, Nova Scotia duck tolling retrievers and West Highland white terriers. Any dog of any breed can develop it.

If your dog has had two or three unexplained bouts of vomiting, weakness or appetite loss that resolved with supportive care and then came back, say that out loud to your veterinarian and ask directly whether adrenal testing is warranted. Pattern recognition from the owner is very often what gets the test ordered.

What the blood work shows, and why the sodium to potassium ratio matters

A standard workup begins with a complete blood count, a serum chemistry panel including electrolytes, and a urinalysis. In a dog with the typical form, the results often read like this:

• Potassium high, sodium low. This is mineralocorticoid deficiency showing up directly on the panel. Chloride is frequently low as well.
• Elevated BUN and creatinine with urine that is not well concentrated. This pattern mimics kidney failure and is one of the most common reasons an Addison's dog is misdiagnosed.
• Low blood glucose in some dogs, occasionally low enough to cause weakness or trembling.
• Elevated calcium in a subset of cases.
• Mild non-regenerative anemia, sometimes hidden by dehydration until fluids are given.
• No stress leukogram. This one is subtle and extremely useful. A genuinely sick, stressed dog should show high neutrophils with low lymphocytes and few or no eosinophils, because cortisol drives that pattern. A sick dog whose lymphocytes and eosinophils look normal or elevated is a dog whose cortisol may be missing.

Veterinary references commonly describe a normal sodium to potassium ratio as roughly 27:1 to 40:1. A ratio below about 27:1 raises suspicion, and a ratio below roughly 20:1 is strongly suggestive of hypoadrenocorticism. Your laboratory's own reference interval is the one that counts, so read the ratio against the range printed on your report rather than against a number found online.

Two cautions change how that ratio should be interpreted.

First, it is not specific to Addison's. Whipworm infection, severe gastrointestinal disease, kidney disease, urinary tract rupture or obstruction, fluid accumulation in a body cavity and certain cardiac conditions can all produce a similar electrolyte picture. So can laboratory artifact: hemolysis in the sample, delayed separation of serum from cells, EDTA contamination, and the naturally high red cell potassium found in Akitas and some other Japanese breeds. A fecal examination is inexpensive and rules out one of the most common mimics.

Second, a normal electrolyte panel does not rule Addison's out. A meaningful minority of dogs have the atypical form, in which cortisol production has failed but aldosterone output has not, so sodium and potassium look unremarkable while the dog keeps getting sick. Those are the dogs most often missed for months.

The ACTH stimulation test, step by step

The ACTH stimulation test asks one question: can these adrenal glands produce cortisol when they are told to?

A baseline blood sample is drawn and cortisol is measured. Synthetic ACTH, the pituitary hormone that normally commands cortisol release, is then given by injection. A second sample is drawn at the interval your veterinarian's protocol specifies, and cortisol is measured again.

A healthy dog shows a clear rise between the two samples. A dog with Addison's shows a flat line, with both the pre and post values sitting at or near the bottom of the assay's detection limit. That flat, low result is the diagnosis.

Several practical points matter here.

Recent steroid exposure can interfere with the test. Some steroid medications cross-react with cortisol assays and can distort the result, while others generally do not, which is one reason a veterinarian may select a particular option when a collapsing dog needs treatment started before testing is complete. Never stop a prescribed medication on your own in order to prepare for a test. Instead, give your veterinarian a complete list of every medication, topical product, and ear or eye preparation your dog has received recently, so the test can be timed and interpreted correctly.

The test can still be run on an unstable dog. A patient in crisis is stabilised with intravenous fluids and supportive care first, and the diagnostic sequence is fitted around that, never the reverse.

Cost and laboratory turnaround vary widely by clinic, by region and by whether the sample is processed in house or sent out, so ask for an estimate when you book rather than relying on a figure from elsewhere.

The rest of the diagnostic picture, test by test

Most workups use several of the following, chosen according to how sick the dog is and what the first panel showed.

Test	What it measures	What it tells you	Where it fits
Electrolyte panel and Na:K ratio	Sodium, potassium, chloride	A pattern consistent with mineralocorticoid deficiency, but not specific to it	Usually the first clue
Resting (basal) cortisol	A single cortisol value	A result comfortably above the laboratory's cutoff makes Addison's very unlikely; a low result does not confirm it	Screening and rule-out
ACTH stimulation test	Adrenal cortisol reserve	Confirms or excludes hypoadrenocorticism	Confirmatory, the definitive test
Endogenous ACTH	Pituitary ACTH output	Separates primary adrenal failure from secondary pituitary causes	After diagnosis; needs careful sample handling
Aldosterone, pre and post ACTH	Mineralocorticoid output	Helps characterise typical versus atypical disease	Selected cases
Abdominal ultrasound	Adrenal gland size and shape	Thin, small adrenals support the suspicion but do not prove it	Supportive imaging
ECG	Effect of high potassium on the heart	Slow heart rate, tall T waves, flattened or absent P waves	Emergency assessment
Radiographs	Heart size, esophagus	Small heart silhouette from low circulating volume, occasionally megaesophagus	Supportive imaging
Fecal examination	Whipworm eggs	Identifies a parasite that can mimic the electrolyte picture	Differential diagnosis

A resting cortisol is often used as the gatekeeper because it is cheaper and needs only one sample. It is a good rule-out and a poor rule-in. If it comes back low, the ACTH stimulation test is still required before anyone can say the word Addison's.

What to watch at home, and what to tell your veterinarian

Because the signs come and go, the most useful thing an owner can do while a workup is underway is keep a plain written record. Results vary from dog to dog, and no two histories look the same, so specifics beat impressions.

Worth tracking day by day:

• Energy and attitude. Note days your dog seems flat, reluctant to walk, or slow to get up, and days that look normal.
• Appetite and water. Meals refused or half eaten, and any clear change in how much your dog is drinking and urinating.
• Vomiting and stool. How often, what it looked like, and whether blood was present.
• Weight. Weigh on the same scale when you can, and note the dates.
• Trembling, wobbliness or hind end weakness, particularly after exercise.
• Triggers. Episodes that follow stress such as boarding, travel, a move, a thunderstorm or a vet visit are a pattern worth mentioning.
• Gum colour and hydration. Pale, grey or tacky gums are a reason to be seen, not a wait and see.

Bring that log to the appointment along with copies of any previous blood work, discharge notes from earlier emergency visits, and a written list of everything your dog receives: prescribed medications, flea and tick products, topical treatments, ear and eye preparations, chews and anything given by another household member. Say clearly whether your dog improved quickly with fluids in the past, because that response is itself a piece of information.

Signs that turn this into an emergency. Go straight to an emergency veterinary hospital if your dog collapses, cannot stand or walk, is unresponsive or disoriented, is vomiting repeatedly and cannot keep water down, has a very slow or irregular heartbeat, feels cold at the ears and paws, has pale or grey gums, seems to be breathing oddly, seizures, or passes blood. Dogs in an Addisonian crisis can decline quickly, and stabilisation happens first, with diagnostics fitted around it. If you are unsure, call the nearest emergency clinic and describe what you are seeing rather than waiting to see whether the episode passes on its own.

Key Takeaways

• Yes, Addison's disease in dogs can be tested for, and the ACTH stimulation test is the only test that confirms the diagnosis.
• Collapse, repeated vomiting, profound weakness, pale gums or a very slow heart rate are emergencies. Seek immediate veterinary care rather than waiting for test results.
• Routine blood work usually gives the first clue: high potassium, low sodium, elevated kidney values with dilute urine, sometimes low glucose or high calcium.
• A normal sodium to potassium ratio is commonly cited as roughly 27:1 to 40:1; below about 27:1 raises suspicion and below roughly 20:1 is strongly suggestive, always read against your own laboratory's range.
• That ratio is not specific. Whipworms, kidney disease, urinary obstruction, cavity effusions and sample handling artifacts can all produce it.
• Normal electrolytes do not rule Addison's out, because atypical cases lose cortisol while aldosterone keeps sodium and potassium looking fine.
• A sick dog without a stress leukogram, meaning normal or high lymphocytes and eosinophils, is an underrated red flag.
• Recent steroid medication can distort cortisol measurement. Disclose everything your dog receives, and never discontinue a prescribed drug on your own.
• Confirmed Addison's is usually managed with lifelong treatment that your veterinarian prescribes and adjusts using recheck blood work.

Working with your veterinary team

Your veterinarian owns the diagnosis and the treatment plan. They order the ACTH stimulation test, read the electrolyte pattern in the context of your dog's history, prescribe and adjust treatment, and decide when to recheck. If the diagnosis is confirmed, Addison's is generally a lifelong condition that your vet manages with prescribed medication and periodic monitoring, and doses are reviewed over time rather than set once.

Ask what each test is looking for, what the results would change, when you should expect to hear back, and exactly which signs should send you to an emergency hospital in the meantime. Write the answers down. Nothing in this article diagnoses, treats, cures or prevents any condition; every decision about your dog's care belongs with the veterinarian examining your dog.

Related guides: Senior Dog Bloodwork : What a Senior Blood Panel Shows and How to Test Dog Kidney Function? (Bloodwork Explained).