What Is Addison's Disease?
Addison's disease, or hypoadrenocorticism, is the opposite of Cushing's disease. Where Cushing's involves too much cortisol, Addison's involves too little. The condition occurs when the adrenal glands fail to produce sufficient amounts of cortisol and, in most cases, aldosterone, the mineralocorticoid hormone that regulates sodium, potassium, and fluid balance.
The adrenal glands sit just above the kidneys and are responsible for a range of hormones essential to daily physiological function. When they stop working properly, the body's ability to respond to stress, regulate blood pressure, and maintain electrolyte balance is severely compromised.
In the majority of dogs, Addison's disease results from immune-mediated destruction of the adrenal cortex, the outer portion of the adrenal gland. Less commonly, it can be caused by adrenal haemorrhage, fungal infection, or as a consequence of treating Cushing's disease too aggressively. A third form, known as atypical Addison's, involves cortisol deficiency only, with normal electrolytes, and can be even harder to identify.
Why It Mimics So Many Other Conditions
Addison's disease has earned the nickname the great pretender in veterinary medicine. Its clinical signs are vague, wax and wane over time, and overlap extensively with gastrointestinal disease, kidney disease, liver problems, and a host of other conditions. Dogs are frequently treated for other diagnoses before Addison's is eventually considered.
Common presentations include:
- Episodes of vomiting and diarrhoea that appear to resolve, then return
- Lethargy and weakness that fluctuates in severity
- Loss of appetite and gradual weight loss
- Increased thirst and urination
- Shaking or muscle tremors
- Depression and a generally dull demeanour
- Slow heart rate
- Pain or discomfort in the abdominal region
The episodic nature of the signs is one of the diagnostic frustrations. Dogs often appear to improve with supportive care such as intravenous fluids, which temporarily restores electrolyte balance, only to deteriorate again weeks later. This pattern should raise suspicion for Addison's.
Young to middle-aged female dogs appear to be at higher risk, and certain breeds including Standard Poodles, Portuguese Water Dogs, Bearded Collies, and Labrador Retrievers are overrepresented in case studies.
The Addisonian Crisis
The most dramatic and dangerous presentation of Addison's disease is the acute Addisonian crisis, also known as acute adrenal insufficiency. This occurs when the body is placed under significant physiological stress and the adrenal glands are unable to mount an adequate cortisol response.
Signs of a crisis include profound weakness or collapse, vomiting, severe dehydration, dangerously low blood pressure, and an irregular heartbeat. Without prompt treatment, an Addisonian crisis can be fatal. Emergency hospitalisation is required, with intravenous fluids, electrolyte correction, and immediate corticosteroid administration.
Critically elevated potassium is a hallmark finding and can cause life-threatening cardiac arrhythmias. A ratio of sodium to potassium below 27:1 on a standard blood panel is a classic indicator and should prompt immediate investigation.
Diagnosis
The gold standard diagnostic test for Addison's disease is the ACTH stimulation test. A baseline cortisol sample is taken, synthetic ACTH is administered, and cortisol is measured again one hour later. In a healthy dog, cortisol rises significantly. In a dog with Addison's disease, the response is blunted or absent because the adrenal glands are unable to produce the hormone.
Electrolyte disturbances, particularly low sodium and high potassium, are commonly found but are absent in atypical cases, making the ACTH stimulation test even more important for complete assessment. Routine bloodwork may also show low blood glucose, elevated kidney values due to poor perfusion, and mild anaemia.
Long-Term Management
Once diagnosed, Addison's disease is very manageable with appropriate hormone replacement. Most dogs require two types of supplementation.
Mineralocorticoid Replacement
Desoxycorticosterone pivalate (DOCP), administered as a monthly injection, replaces aldosterone and restores electrolyte regulation. An oral alternative, fludrocortisone acetate, provides both mineralocorticoid and some glucocorticoid coverage and is taken daily.
Glucocorticoid Replacement
Prednisolone at a low, physiological dose replaces cortisol. During periods of stress, illness, or surgery, this dose must be increased to prevent an Addisonian crisis. Owners should always inform their vet and any emergency clinic that their dog has Addison's disease, particularly before any procedure or during illness.
With consistent treatment and routine monitoring of electrolytes and kidney values, dogs with Addison's disease typically do extremely well. Many live completely normal lives with no limitations. The prognosis following diagnosis is genuinely good, and the condition, while serious if missed, is one of the more rewarding to manage in veterinary medicine.