Addison’s disease in dogs is most commonly caused by the immune system attacking and destroying the adrenal glands, the small organs near the kidneys that produce essential hormones. This immune-mediated destruction accounts for the majority of cases, though medications, genetics, and pituitary gland problems can also trigger the condition. Understanding the cause matters because it shapes how your dog will be treated and what to expect long term.
Immune System Destruction of the Adrenal Glands
In most dogs with Addison’s disease, the body’s own immune system gradually destroys the outer layer of the adrenal glands, called the adrenal cortex. This is the part responsible for producing two critical types of hormones: cortisol, which helps the body respond to stress and regulate metabolism, and aldosterone, which controls the balance of sodium and potassium in the blood.
The destruction is slow and progressive. Dogs typically don’t show symptoms until roughly 85 to 90 percent of the adrenal cortex has been damaged. By the time your dog seems sick, the glands have already lost most of their ability to function. This is why Addison’s often seems to appear suddenly even though the underlying process has been building for months.
Why the immune system turns on the adrenal glands isn’t fully understood, but it follows the same pattern seen in other autoimmune diseases. White blood cells infiltrate the adrenal tissue and slowly break it down. Once that tissue is gone, it doesn’t regenerate, which is why most dogs with immune-mediated Addison’s need lifelong hormone replacement.
Breeds With a Genetic Predisposition
Addison’s disease can develop in any breed, and mixed-breed dogs are actually diagnosed most frequently overall. But certain breeds are significantly overrepresented, pointing to a strong genetic component. According to Cornell University’s College of Veterinary Medicine, breeds at higher risk include:
- Standard Poodle
- West Highland White Terrier
- Bearded Collie
- Great Dane
- Portuguese Water Dog
- Labrador Retriever
- Rottweiler
- Wheaten Terrier
- Nova Scotia Duck Tolling Retriever
The Standard Poodle has been studied most closely. Researchers at the University of Minnesota’s Canine Genetics Laboratory have been investigating whether variants in MHC genes (the genes that help the immune system distinguish the body’s own cells from invaders) are linked to Addison’s in this breed. MHC genes are the most diverse genes in animals and are associated with many autoimmune diseases across species, making them a likely piece of the puzzle. A dog doesn’t need to be purebred to carry risk genes, but having a parent or sibling with Addison’s substantially raises the odds.
Medications That Suppress or Damage the Adrenal Glands
Two common scenarios can cause drug-induced Addison’s disease. The first involves treatment for the opposite condition, Cushing’s disease, where the adrenal glands produce too much cortisol. Drugs used to treat Cushing’s work by suppressing adrenal hormone production, and sometimes they suppress it too much. In most dogs, stopping the medication allows the adrenals to recover. But in some cases, the drug causes permanent damage to adrenal tissue, leaving the dog with Addison’s disease for life. Research published in the Journal of Veterinary Internal Medicine suggests that the adrenal destruction may actually be driven by the body’s own hormonal response to the drug rather than the drug itself.
The second scenario involves long-term steroid use for other conditions like allergies, inflammatory bowel disease, or immune disorders. When a dog receives synthetic steroids for weeks or months, the adrenal glands gradually shut down their own production because the body is already getting what it needs from the medication. If the steroids are stopped abruptly instead of tapered slowly, the adrenal glands may not be able to restart production quickly enough. The risk increases with duration: short courses under a month rarely cause problems, while use lasting months or longer carries a meaningful risk of adrenal suppression. This form is often temporary if managed carefully, but an abrupt withdrawal can trigger a life-threatening adrenal crisis.
Secondary Addison’s: A Pituitary Problem
Not all Addison’s disease starts in the adrenal glands. In secondary hypoadrenocorticism, the problem originates in the pituitary gland at the base of the brain. The pituitary sends a signaling hormone called ACTH to the adrenal glands, telling them to produce cortisol. If the pituitary is damaged by a tumor, trauma, or inflammation, it stops sending that signal, and the adrenal glands slowly shrink from disuse.
The key difference with secondary Addison’s is that aldosterone production usually remains intact because it’s regulated by a separate system involving sodium and potassium levels in the blood rather than signals from the pituitary. This means dogs with secondary Addison’s typically have normal electrolyte levels but still suffer from cortisol deficiency. They experience many of the same symptoms (lethargy, poor appetite, vomiting, weight loss) but may not develop the dangerous electrolyte crashes that make primary Addison’s so acutely life-threatening.
Why Electrolyte Imbalances Happen
In primary Addison’s disease, the destruction of the adrenal cortex eliminates the production of aldosterone. This hormone’s job is to tell the kidneys to hold onto sodium and excrete potassium. Without it, dogs lose too much sodium in their urine and retain too much potassium in their blood. The result is a dangerous shift in the sodium-to-potassium ratio.
Veterinarians use this ratio as a screening tool. A sodium-to-potassium ratio at or below 27 is highly specific for primary Addison’s disease, with specificity ranging from 87 to 99 percent depending on the study. When a dog’s bloodwork shows this ratio, an adrenal function test is the next step. The definitive test measures cortisol levels before and after an injection of synthetic ACTH. In a healthy dog, the injection triggers a spike in cortisol. In a dog with Addison’s, cortisol stays flat, typically below 2 mcg/dL, because the adrenal glands can’t respond. A resting cortisol level above 2 mcg/dL effectively rules out the diagnosis.
Atypical Addison’s Disease
Some dogs develop what’s called atypical Addison’s, where cortisol production fails but aldosterone production continues normally. These dogs have all the vague symptoms of cortisol deficiency, like intermittent vomiting, diarrhea, lethargy, and weight loss, but their electrolyte levels look completely normal on routine bloodwork. This makes atypical Addison’s notoriously difficult to catch. It’s often misdiagnosed as gastrointestinal disease or a behavioral issue for months before the real cause is identified.
Atypical Addison’s can occur when the immune destruction targets only certain zones of the adrenal cortex, sparing the outermost layer that produces aldosterone. It also occurs in secondary Addison’s, where the pituitary problem affects cortisol but not aldosterone. Some dogs initially diagnosed with the atypical form eventually progress to full primary Addison’s as adrenal destruction continues, so monitoring electrolytes over time remains important even after diagnosis.
What Makes Addison’s Hard to Catch Early
The symptoms of Addison’s disease are maddeningly nonspecific. A dog that’s tired, off their food, occasionally vomiting, or losing weight could have dozens of conditions. Many dogs with Addison’s have a waxing and waning pattern where they seem sick for a few days, then bounce back, then get worse again. Stressful events like boarding, travel, or surgery can push a dog with borderline adrenal function into a full crisis because stress demands a cortisol surge the glands can no longer deliver.
This crisis, called an Addisonian crisis, is often the moment the disease is finally discovered. The dog collapses, becomes severely dehydrated, and develops a dangerously slow heart rate from high potassium levels. It’s a medical emergency, but once stabilized and diagnosed, most dogs with Addison’s disease respond well to hormone replacement and go on to live normal lifespans.