Dogs with Addison’s disease may be affected with any one of three types of the condition: primary addison’s disease (discussed here), atypical Addison’s disease, and secondary Addison’s disease. It is better named “hypoadrenocorticism”, as it is a condition characterized by decreased production of hormones from the adrenal glands.
Normal Anatomy and Function of the Adrenal Glands in the Dog
Dogs, as well as most mammals, have two adrenal glands. They are called adrenal glands because they lie just in front of each kidney. The outer layer, the adrenal cortex, produces steroid hormones of two types: glucocorticoids and mineralocorticoids.
This video gives more information about the normal structure and function of the adrenal glands.
Glucocorticoids are hormones responsible, among many other functions throughout the body, for maintaining normal blood sugar (glucose) during periods of stress. Glucocorticoids are known to increase appetite and contribute to an overall feeling of well-being, but also reduce healing and immune function at high levels.
Mineralocorticoids are also produced by the adrenal cortex and, as the name implies, are responsible for controlling the “minerals” or electrolytes in the blood. Electrolytes affect where fluid travels in the body, hydration, and electrical activity in the heart and brain.
The inner layer of the adrenal gland is called the cortex and produces epinephrine and norepinephrine. When released, these two hormones cause a broad range of responses throughout the body, which are collectively referred to as “Fight of Flight Syndrome.” Epinephrine and norepinephrine increase heart rate and cardiac output to allow both humans and our pets to deal with perceived life-threatening situations. The adrenal medulla is not affected by either Addison’s Disease or Cushing’s Disease in the dog.
Cause of Primary Addison’s Disease in the Dog
The cause of primary Addison’s disease in the dog is thought to be destruction of all or part of the adrenal cortex by the dog’s own immune system. It is not clearly understood why the immune system targets its own tissue, and the cortex specifically, in some dogs and not others. At some level of destruction unique to each patient, hormone production is no longer adequate to maintain health. This is especially the case during periods of stress or illness, when increased amounts of glucocorticoids and mineralocorticoids are necessary.
Secondary Addison’s disease in dogs develops when the medications used to treat Cushing’s disease overshoot and drive cortical hormone levels too low. Low cortical hormone levels is also called hypoadrenocorticism.
Clinical Signs (Symptoms) of Addison’s Disease in the Dog
Addison’s disease in the dog typically affects young or middle-aged dogs of any breed, though larger breeds (such as Labrador Retrievers, Portuguese Water Dogs and Standard Poodles) are more susceptible. Addison’s disease is exceedingly rare in the cat.
Addison’s disease is associated with a broad range of signs, mimicking a number of other diseases. Signs may be mild and non-specific or severe and life-threatening. Affected dogs may become progressively or suddenly lethargic, lose their appetite (anorexia), or drink increased amounts of water. When the disease reaches a certain threshold, vomiting and diarrhea can occur. When electrolyte abnormalities become severe, profound weakness and a shock-like state (Addisonian crisis) can develop. If untreated, these signs can be life-threatening.
Diagnosis of Addison’s Disease in the Dog
The early, subtle signs of Addison’s disease in the dog make diagnosis difficult. However, when the syndrome is more advanced and the dog is severely ill, routinely-performed blood tests (minimum data base) will often be diagnostic.
The classic blood test abnormalities of Addison’s disease in the dog are increased serum potassium (hyperkalemia) and decreased sodium (hyponatremia)/chloride (hypochloremia) levels. These result from lack of mineralocorticoid hormones, which are responsible for electrolyte balance. High levels of potassium are toxic to cells throughout the body, especially the heart. Poor heart function causes blood pressure to drop, which becomes even more dangerous if the heart rate drops due to further deterioration. The result is a profound shock-like state known as an “Addisonian crisis”. At this stage, the blood sugar level may be low (hypoglycemia). Also, dehydration and low blood pressure may result in concentration of the blood and abnormal kidney function, both of which are revealed by routine blood tests. Changes in white blood cell counts (increased lymphocytes and eosinophils) are also commonly observed.
Measuring the amount of cortisol in the blood is the definitive diagnosis of Addison’s disease in the dog. A single level of cortisol (basal cortisol) that is very low may be all that is needed to make the diagnosis. If that test is ambiguous (mild disease), it may be necessary to take a repeat measurement of blood cortisol after stimulating the adrenal glands with a drug called ACTH (ACTH stimulation test). Absence of the normal sharp increase in cortisol level following ACTH stimulation is diagnostic of a glucocorticoid deficiency.
For more information about Addison’s disease in dogs, you can visit the University of Washington’s veterinary school website .