Hypoadrenocorticism (Addison's disease) is a relatively uncommon but serious endocrine disorder in dogs caused by insufficient production of glucocorticoids and/or mineralocorticoids from the adrenal cortex. With appropriate diagnosis and lifelong management, affected dogs can achieve excellent quality of life.
Pathophysiology
The adrenal cortex produces two essential hormone classes: glucocorticoids (cortisol — stress response, blood glucose, immune function) and mineralocorticoids (aldosterone — sodium/potassium balance, blood pressure). Primary hypoadrenocorticism involves destruction of all adrenal cortical layers — most commonly immune-mediated. Secondary hypoadrenocorticism results from insufficient ACTH from the pituitary, affecting only glucocorticoid production.
Clinical Presentation
Addison's disease is famously the "great pretender" — clinical signs are vague and waxing-waning, often mimicking other conditions:
- Episodic weakness, lethargy, and vomiting — often triggered by stress
- Weight loss and poor appetite
- Polyuria and polydipsia
- Shaking or muscle tremors
- Addisonian crisis: Acute collapse, severe bradycardia, shock, and death if untreated — triggered by severe physiological stress in inadequately supplemented dogs
Diagnosis
The ACTH stimulation test is definitive — baseline and post-ACTH cortisol measurement; inadequate cortisol response confirms hypoadrenocorticism. Electrolytes show the classic sodium:potassium ratio <27:1 in typical (mineralocorticoid-deficient) cases. Hyperkalaemia (elevated potassium) can cause bradycardia and cardiac arrhythmias — a life-threatening feature of Addisonian crisis.
Treatment
Acute crisis: IV fluid resuscitation (normal saline or Hartmann's) corrects dehydration and dilutes hyperkalaemia; IV dexamethasone provides rapid glucocorticoid replacement; cardiac monitoring essential.
Long-term maintenance:
- Mineralocorticoid replacement: Monthly intramuscular desoxycorticosterone pivalate (DOCP/Zycortal) injections, or daily oral fludrocortisone
- Glucocorticoid replacement: Daily oral prednisolone at physiological doses (0.1–0.2 mg/kg); increase 5-10× during any stressful event, illness, or surgery (stress dosing)
Welfare and Quality of Life
Dogs on appropriate treatment have excellent quality of life comparable to normal dogs. The key welfare risk is inadequate stress dosing during illness or stressful events — owners must be clearly educated about when to increase prednisolone doses and when to seek emergency care. Medical alert tags identifying the dog as an Addisonian patient are strongly recommended. Annual electrolyte monitoring and ACTH stimulation testing guides dose adjustment over time.
Breed Predisposition
Standard Poodles, Great Danes, Portuguese Water Dogs, West Highland White Terriers, and Bearded Collies are predisposed. Young to middle-aged females are disproportionately affected. Genetic research is identifying contributing loci in predisposed breeds, potentially enabling genetic screening to guide breeding decisions and reduce incidence.