Primary Addison’s disease in the dog is characterized by decreased or complete loss of production of both glucocorticoid and mineralocorticoid hormones of the adrenal glands. Atypical Addison’s disease in the dog, however, is cause by decreased production of only glucocorticoid hormones from the adrenal glands.
This video gives more information about the normal structure and function of the adrenal glands.
Cause of Atypical Addison’s Disease in the Dog
Similarly to primary Addison’s disease, the cause of atypical Addison’s disease in the dog is suspected to be an immune-mediated. This means that the dog’s own immune system damages or destroys the adrenal cortex. Therefore, the essential hormones normally produced by the adrenal cortex fall below the levels necessary to maintain health.
Clinical Signs of Atypical Addison’s Disease in the Dog
Signs of atypical Addison’s disease in the dog are often much more subtle than with primary Addison’s disease. You may note mild lethargy, vomiting, and/or decreased appetite in your pet intermittently over weeks or months. These clinical signs may be brought on by periods of stress, such as boarding, travel, illness, anesthesia, or surgical procedures.
Diagnosis of Atypical Addison’s Disease in the Dog
The diagnosis of atypical Addison’s disease is very difficult in dogs because the disease is very uncommon and signs are usually subtle. Suspicions may be raised by clues on routine blood tests, including decreased blood protein (albumin), blood glucose, and/or cholesterol. Occasionally, a complete blood count (CBC) may reveal increased lymphocytes and eosinophils.
If more common causes of your pet’s clinical signs have been ruled out, the definitive diagnosis of atypical Addison’s disease in the dog requires an ACTH stimulation test. Normally, ACTH is released from the pituitary gland in the brain and stimulates the adrenal glands to produce cortisol. If immune-mediated or other destruction of the cortex has occurred, it will not respond to that signal and blood cortisol levels will remain low and static. ACTH stimulation test compares blood cortisol levels before and after stimulation with synthetic ACTH. Whereas a normal dog would produce a sharp increase in cortisol following stimulation, a dog with atypical Addison’s disease will show little or no change between the two samples.
Treatment of Atypical Addison’s Disease in the Dog
Replacement of normal physiologic levels of cortisol in the form of prednisone is necessary and easily achieved. Treatment is aimed at providing enough prednisone for the dog to have a normal appetite and to have a “feeling of well being”. Excess prednisone causes excessive thirst, urination, appetite and weight gain. Over time, some dogs will lose the ability to produce adequate mineralocorticoid hormones, or, in other words, convert to primary hypoadrenocorticism. Electrolytes alterations typical of Addison’s disease in the dog will then develop, and lifelong replacement of mineralocorticoid hormones will then be necessary.