Addison's disease
Underproduction of cortisol and aldosterone by the adrenal glands. Sometimes called 'the great pretender' because signs are vague. Treatable but life-threatening at presentation.
Addison's disease (hypoadrenocorticism) is the inverse of Cushing's — the adrenal glands don't produce enough cortisol and aldosterone. Most cases in dogs are immune-mediated destruction of the adrenal cortex. Cats are rarely affected.
Signs are vague and intermittent: lethargy, decreased appetite, vomiting, diarrhoea, weakness, weight loss, sometimes shaking. These wax and wane for months. The crisis presentation is dramatic: collapse, severe weakness, low blood pressure, sometimes critically high potassium.
Addison's is sometimes called "the great pretender" because the early intermittent signs are mistaken for many other conditions. The classic clue on routine bloodwork is the sodium-to-potassium ratio — but a portion of dogs (atypical Addison's) don't show this and are missed unless an ACTH stimulation test is run.
Treatment is straightforward once diagnosed: replacement hormones (DOCP injections plus oral prednisolone, or fludrocortisone tablets). With consistent treatment, most dogs live a normal lifespan.
Predisposed breeds include Standard Poodle, Bearded Collie, Portuguese Water Dog, Nova Scotia Duck Tolling Retriever, Great Dane, West Highland White Terrier.