What Is Addison's Disease?
Addison's disease, formally known as hypoadrenocorticism, is a condition in which the adrenal glands fail to produce adequate amounts of essential hormones. The adrenal glands — small structures that sit just in front of each kidney — normally produce two critical categories of hormones: mineralocorticoids (primarily aldosterone), which regulate sodium and potassium levels and control blood pressure, and glucocorticoids (primarily cortisol), which help the body respond to stress and regulate inflammation, blood sugar, and many other physiological processes.
When the adrenal glands are damaged or dysfunctional, production of both hormone types falls, leading to a cascade of imbalances that affect virtually every body system. This is why Addison's disease has earned the nickname "the great pretender" — its symptoms are so varied and non-specific that it is frequently mistaken for gastrointestinal disease, kidney failure, neurological conditions, or simple lethargy before the correct diagnosis is made.
Which Dogs Are Most Affected?
Addison's disease can occur in any dog, but it is significantly more common in females, and typically presents in young to middle-aged dogs. Breeds with a recognised predisposition include Standard Poodles, Bearded Collies, Portuguese Water Dogs, Nova Scotia Duck Tolling Retrievers, and West Highland White Terriers. In many cases, the disease is immune-mediated, meaning the immune system mistakenly destroys the adrenal cortex, though other causes such as trauma, haemorrhage, or the abrupt withdrawal of long-term steroid medication can also be responsible.
The Classic Symptoms
Dogs with Addison's disease most commonly present with a waxing and waning history of vague illness. Episodes of lethargy, weakness, vomiting, diarrhoea, reduced appetite, weight loss, shaking, and abdominal pain often come and go, sometimes resolving on their own before returning weeks or months later. Many dogs are treated repeatedly for gastroenteritis before Addison's is considered.
A particularly classic but easily missed feature is the "waxing and waning" pattern of illness, where the dog appears to recover briefly — often after receiving intravenous fluids or supportive care — only to deteriorate again later. This improvement with IV fluids is an important clue, as fluids temporarily correct the electrolyte imbalances that underlie many of the symptoms.
The Addisonian Crisis
An acute Addisonian crisis represents the most severe and dangerous manifestation of the disease. When the body is subjected to a stressor — illness, surgery, travel, or even an emotional upset — the healthy animal responds by increasing cortisol output from the adrenal glands. A dog with Addison's disease cannot do this. The result is a sudden and profound collapse of physiological function.
Dogs in an Addisonian crisis typically present in a state of collapse or extreme weakness, with a very slow heart rate (bradycardia), a weak or absent pulse, dangerously low blood pressure, and severe dehydration. The characteristic electrolyte abnormalities are hyponatraemia (low sodium) and hyperkalaemia (high potassium), and these can be life-threatening on their own — hyperkalaemia in particular disrupts the electrical conduction of the heart and can cause fatal arrhythmias.
A sodium-to-potassium ratio of less than 27 on a blood chemistry panel is a strong indicator of hypoadrenocorticism and should immediately prompt suspicion of Addison's disease. This ratio is often calculated as a routine part of blood work interpretation, and a low result in a dog with compatible clinical signs warrants urgent investigation and empirical treatment.
Diagnosis: The ACTH Stimulation Test
The definitive test for Addison's disease is the ACTH stimulation test. Synthetic ACTH (adrenocorticotropic hormone) is injected, and cortisol levels are measured before and one hour after the injection. In a healthy dog, cortisol rises significantly in response to ACTH stimulation. In a dog with Addison's disease, cortisol levels remain flat — both before and after the injection — because the adrenal glands are unable to respond. A blunted or absent cortisol response is diagnostic of hypoadrenocorticism.
Blood electrolytes, a full blood count, and a biochemistry panel are also essential. In addition to the sodium-to-potassium ratio abnormality, dogs with Addison's often show a mild anaemia, elevated urea and creatinine (from poor perfusion and dehydration), and a lack of the stress leucogram (a high white blood cell count with neutrophilia) that would be expected in a seriously ill dog. This absence of a stress response is itself a diagnostic clue.
Treatment
Managing the Acute Crisis
A dog in an Addisonian crisis requires immediate intensive care. Treatment involves aggressive intravenous fluid therapy with normal saline to restore blood volume, correct dehydration, and dilute excess potassium. Supplemental corticosteroids — such as dexamethasone or hydrocortisone — are given intravenously to replace the missing glucocorticoids. Once the dog is stabilised, longer-term hormone replacement can be introduced.
Long-Term Hormone Replacement
The mainstay of long-term management is desoxycorticosterone pivalate (DOCP), sold under the brand names Percorten-V or Zycortal. This is a long-acting synthetic mineralocorticoid given as a subcutaneous or intramuscular injection, typically once every 25 to 28 days. It replaces the aldosterone that the adrenal glands can no longer produce, maintaining sodium and potassium balance between doses.
In addition, most dogs with Addison's also require oral prednisolone at a low daily dose to replace the missing glucocorticoid component. The dose required is usually very small — often just 0.1 to 0.2 mg per kilogram per day — and most dogs tolerate it well without the side effects commonly seen at higher immunosuppressive doses.
Treatment is lifelong. Dogs that are managed correctly can live entirely normal, active, and happy lives, with no reduction in life expectancy.
Stress Dosing
Owners must be aware of the concept of stress dosing. When a dog with Addison's disease faces a significant physical stressor — such as an illness, injury, surgical procedure, or even a particularly stressful event such as fireworks — the oral prednisolone dose should be temporarily doubled or tripled. This mimics the cortisol surge that a healthy dog's adrenal glands would produce in response to stress. Failure to stress dose can precipitate an Addisonian crisis. Owners should discuss the specific protocol with their vet and carry a stress dose supply at all times.
Monitoring and Follow-Up
Electrolytes should be checked one to two weeks after each DOCP injection when the dog is first diagnosed, and then periodically once the dose is established and stable. The injection interval or dose may need adjustment over time. Most dogs settle into a predictable routine, and owners often become adept at recognising subtle signs that the next injection is due — such as increasing thirst, subtle weakness, or a slight reduction in energy in the days before the scheduled dose.