The hemogram may show anemia, absolute lymphocytosis and eosinophilia, but a normal hemogram and chemistry profile does not rule out hypoadrenocorticism. Dogs with Addison's disease can have low or low-normal glucose concentration, hypoalbuminemia, low serum cholesterol concentration, hypercalcemia, low sodium and high potassium concentrations from aldosterone deficiency, azotemia, and low urine specific gravity. About 10% of dogs with Addison's disease have normal serum electrolyte concentrations (so-called “atypical” Addison's disease).

The test of choice for diagnosing hypoadrenocorticism is the ACTH Stimulation Test. Affected dogs have little or no rise in cortisol levels in response to exogenous ACTH. It is currently not known whether the majority of dogs with atypical Addison's disease have primary or secondary hypoadrenocorticism, or whether they will progress to develop mineralocorticoid deficiency as well. Measuring aldosterone levels in affected dogs may help clarify their type of hypoadrenocorticism, but whether this test is predictive of developing mineralocorticoid deficiency is not known. The most reliable way to distinguish primary from secondary disease is to measure the endogenous ACTH concentration. In primary hypoadrenocorticism, endogenous ACTH concentrations are high, whereas they are low or very low in the secondary form of this disease.

Immune-mediated adrenal disease may be accompanied by other immune disorders including hypothyroidism, diabetes mellitis, hypoparathyroidism, inflammatory bowel disease, glomerulonephritis, and chronic hepatitis. When the combination of Schmidt's syndrome is present, correcting the thyroid dysfunction can improve control of the Addison's disease.

Regardless of the primary or secondary nature of Addison's disease, treatment requires replacement doses of prednisone (0.1-0.2 mg/kg/day) to correct their glucocorticoid deficiency. During times of stress, such as grooming, boarding, illness or surgery, extra prednisone may be necessary. For those with mineralocorticoid deficiency as well, fluid therapy is often needed to reestablish normal electrolyte balance, and mineralocorticoid replacement with daily flucortisone acetate (Fluorinef®), or injections of desoxycorticosterone pivalate (Percorten®) given every 3-4 weeks is needed. Dietary supplementation with salt is rarely necessary.