Addison's Disease: The Great Pretender
Addison's disease comes and goes as vomiting, tiredness and weight loss, then can cause a sudden collapse. Why it is missed, how it is tested, and the outlook.
Written by thecanineledger
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If your dog collapses, is very weak, has a very slow heartbeat or pale gums, or is vomiting and can’t stand: go to a vet now. In a dog with Addison’s disease, this can be an adrenal crisis, which is a medical emergency.
Addison’s disease is uncommon. A study of 905,543 dogs at UK primary-care practices put the one-year prevalence at 0.06% (Schofield et al., Journal of Small Animal Practice 2021). Most vets in general practice will see only a handful of cases.
That is part of why it is missed. The Merck Veterinary Manual’s owner page warns that “Although signs can be hard to identify while Addison disease is developing, severe consequences, such as shock and evidence of kidney failure, can develop suddenly” (Merck, Disorders of the Adrenal Glands in Dogs).
While Addison’s is rare and its early signs are vague, it is very treatable once found, and a 2025 study found the outlook “excellent”. This page sets out the signs, why it is called the great pretender, and what diagnosis and treatment involve.
1. What it is
The adrenal glands sit near the kidneys and make two key hormones. Merck explains that Addison’s disease “is caused by a deficiency of adrenal gland hormones.”
The salt hormone. Without enough aldosterone, potassium builds up in the blood. Merck: “in severe cases, may cause the heart to slow down or beat irregularly. Some dogs have such a slow heart rate (50 beats per minute or lower) that they can become weak or go into shock.”
The stress hormone. Merck: “Decreased production of glucocorticoids (such as cortisol) results in moderately low blood sugar.”
The cause. Merck: “The cause is usually not known, but an autoimmune condition in which the body destroys some of its own tissue is likely.” Some cases are caused by the drugs used to treat Cushing’s disease, which works the other way.
2. Why it is called the great pretender
The signs are vague. Merck lists “repeated episodes of vomiting and diarrhea, loss of appetite, dehydration, and a gradual loss of body condition. Weight loss is often severe.”
In the UK study, the most common signs in confirmed cases were “lethargy (51/66, 77.3%), anorexia (48/66, 66.7%) and vomiting (48/66, 66.7%)”. All three are signs of many common illnesses.
It can look like kidney failure. A 2026 Romanian hospital study reviewed 34 dogs diagnosed with acute kidney injury. Three of them (8.8%) turned out to have Addison’s disease (Roșca et al., Life 2026). Their blood results at admission were “closely resembling intrinsic AKI”, but returned to normal with fluids and hormone replacement. The authors conclude that “hypoadrenocorticism should be considered in dogs presenting with AKI and electrolyte imbalance.”
It can seem to get better. Merck describes “repeated episodes”. A dog who is ill, recovers with treatment for an upset stomach, and then falls ill again is worth asking about.
Our note: The “ill, better, ill again” pattern is a common description, but we don’t have a source we read that states it. The UK study left out dogs given steroids within 30 days of testing, which suggests steroids can affect the test. Tell your vet about any recent steroids.
→ Vomiting and Diarrhoea: When It Can Wait and When It Can’t
3. Which dogs get it
Merck says it is “most common in young, female, adult dogs”, though “the disease can be seen in any breed, sex, or age”. It may be inherited in “Standard Poodles, West Highland White Terriers, Great Danes, Bearded Collies, Portuguese Water Dogs, and a variety of other breeds.”
The UK study found a very strong breed link: “The standard poodle had 51.38 times the odds” of a confirmed diagnosis compared with crossbreeds. Labradoodles and West Highland White Terriers also had higher odds. The study found no link with sex once other factors were accounted for, which differs from Merck’s description.
Our note: Merck says females are more often affected. The UK primary-care study found “No sex association with hypoadrenocorticism was observed in the multivariable model.” We give both.
→ Choosing a Breed or a Mix: The Health Data Breeders Don’t Lead With
4. Testing
The blood test clue. Merck: a vet “can make a tentative diagnosis based on the history, signs, and certain laboratory abnormalities, such as very low levels of sodium and very high levels of potassium in the blood.” In the UK study, high potassium was found in 47 of 53 tested dogs and low sodium in 46 of 53.
Not every dog has the classic results. Merck notes that “Dogs with atypical Addison disease can have low blood levels of cortisol but normal levels of sodium, potassium, and chloride.” A 2025 US study found these dogs were older at diagnosis, with a median age of 8.3 years against 5.1 years (Sherrod et al., Journal of Small Animal Practice 2025).
The confirming test. Diagnosis “is confirmed by specific evaluation of adrenal function”. The vet measures cortisol, gives a hormone that should make the adrenal glands respond, and measures again. Merck: “Affected dogs have low baseline cortisol levels, and there is little response”.
5. Treatment and outlook
A crisis. Merck: “An adrenal crisis is a medical emergency and requires treatment with intravenous fluids to restore levels of body fluids, salt, and sugar to normal.”
Long-term. “For longterm treatment, replacement hormones can be given by mouth or injection. Lifelong treatment is necessary.” Your vet will monitor blood levels and adjust treatment.
The outlook. In the 2025 US study, dogs with classic Addison’s had a median survival after diagnosis of 5.7 years, and a median age at death of 11.0 years. The authors’ conclusion: “Hypoadrenocorticism carries an excellent prognosis.” Most owners who answered their questionnaire did report side effects of steroid treatment.
Our note: Don’t switch between hormone products without talking to your vet.
→ Emergency Signs in Dogs: When to Go Now, Not Tomorrow
6. What we could not verify
| Commonly said | What we found |
|---|---|
| “Addison’s is a death sentence” | No. A 2025 study found an excellent prognosis with treatment. |
| “Normal salts rule it out” | Not always. Atypical Addison’s can have normal sodium and potassium. |
| “It only affects young females” | Disputed. Merck says young females are most often affected; a large UK study found no sex link. |
| “Your vet will spot it straight away” | Not easily. Early signs are vague, and in one study it accounted for 3 of 34 dogs first diagnosed with kidney injury. |
Sources
- Schofield I, Woolhead V, Johnson A, Brodbelt DC, Church DB, O’Neill DG. Hypoadrenocorticism in dogs under UK primary veterinary care: frequency, clinical approaches and risk factors. J Small Anim Pract 2021: VetCompass, 905,543 dogs
- Sherrod TN, Lashnits E, Lunn KF. Clinical characteristics, treatment, and outcomes of hypoadrenocorticism in dogs. J Small Anim Pract 2025: retrospective cohort, 141 dogs, one US university hospital
- Roșca Ș, Solcan G, Moroz M, et al. Addisonian Crisis Mimicking Acute Kidney Injury in Dogs: A Retrospective Study of 34 Dogs Diagnosed with Acute Kidney Injury in Romania. Life (Basel) 2026. ⚠️ abstract read. One hospital, 34 dogs
- Merck Veterinary Manual: Disorders of the Adrenal Glands in Dogs (pet owner version; Bruyette, DACVIM; full review Jun 2018, last updated May 2026)
Always confirm with your vet before acting on anything on this page.