Addison's disease in dogs: the 'great imitator' you don't want to miss

Addison's disease (hypoadrenocorticism) is the opposite of Cushing's — the adrenal glands produce too little cortisol and, in the typical form, too little aldosterone. It is called the 'great imitator' because the signs are vague and come and go: lethargy, poor appetite, vomiting, diarrhoea, weight loss, and sometimes shaking.

What makes Addison's dangerous is the Addisonian crisis. Under stress — illness, travel, surgery — a dog that cannot raise its cortisol can collapse with weak pulses, a slow heart rate, low blood sugar and severe electrolyte disturbances. This is a true emergency that needs immediate intravenous treatment.

Young to middle-aged female dogs are over-represented, and certain breeds (Standard Poodles, Portuguese Water Dogs, Nova Scotia Duck Tolling Retrievers, Leonbergers) have a higher baseline risk, but any dog can develop it.

The clue is often the pattern: a dog with intermittent, non-specific illness that improves with fluids and resolves, then recurs. Bloodwork may show low sodium and high potassium — a classic combination that should prompt an ACTH stimulation test, the definitive diagnosis.

Treatment replaces what the body cannot make. Daily oral prednisolone replaces cortisol, and a monthly injection of desoxycorticosterone pivalate (or daily oral fludrocortisone) replaces aldosterone. Most dogs with Addison's live a full, normal life once stable — the owner's job is to never skip medication and to increase the prednisolone dose during stress (illness, surgery, travel).

If your dog has recurring mystery illness that always seems to settle with supportive care, ask your vet whether an electrolyte panel and an ACTH stimulation test are worth running. Addison's is uncommon, but the cost of missing it is high.