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Addison's Disease in Dogs

2002: Hypoadrenocorticism (Addison’s Disease) in Dogs  by Janice Selinger, DVM

Hypoadrenocorticism is a condition which results when the outer layer, or cortex, of the adrenal glands fails in its secretion of the mineralocorticoid and glucocorticoid hormones. It is often called Addison's

disease after the man who first described this syndrome in humans in 1855. It is not a very common disease in dogs, but unfortunately Tollers have an increased incidence of this condition. Middle-aged animals and particularly females are most commonly affected.

The adrenal glands, which lie near the kidneys, have several important functions. Different areas secrete different hormones, with several different actions. The mineralocorticoids, of which the main one is called aldosterone, are important in the regulation of electrolytes in the body. Their main action is to conserve sodium, chiefly by promoting reabsorption by the kidneys. Water follows the sodium, so they indirectly affect the water balance of the body. They are also responsible for excreting potassium.

The main glucocorticoid hormone is cortisol. Cortisol is essential in an animal's response to physical and mental stress. Cortisol levels rise when an animal is stressed. This hormone has many metabolic effects.

Primary adrenal insufficiency results from destruction of the adrenal cortex. The most common cause of this is immune-mediated destruction of the adrenal gland. Like other immune-mediated diseases, Addison's is thought to have a familial pattern. It is not common in vizslas, but some lines of standard poodles have been devastated by this disease. Other rarer conditions (e.g. tumor) resulting in destruction of the adrenal gland can also cause primary hypoadrenocorticism.

Hypoadrenocorticism can also be induced. If an animal is being given corticosteroid drugs such as prednisone, its body stops producing its own corticosteroids. If the drug administration is abruptly stopped, the dog can't start making it again fast enough, and can go into an Addisonian crisis. For this reason, dosages of corticosteroids must always be tapered off.

Lack of cortisol affects most tissues in the body. Tolerance to stress is reduced. The animal may develop vomiting, diarrhea, loss of appetite, and weight loss. The mental state may be abnormal: lethargic or depressed. Glucose production is decreased.

Without aldosterone, a dog cannot conserve sodium and is unable to get rid of potassium. Water and chloride are lost along with the sodium. The animal may become dehydrated or weak. The increased potassium results in a slow heart rate and muscle weakness.

Not surprisingly, the signs of Addison's disease tend to be a combination of the results of both lack of cortisol and lack of aldosterone (although occasionally only cortisol is deficient). In a study of 100 dogs with hypoadrenocorticism, the following were the most common findings:

Loss of appetite 77%
Vomiting 68%
Lethargy/depressio 64%
Weakness 38%
Weight loss 23%
Diarrhea 22%
Shaking/shivering 21%
Increased urination 15%
Waxing/waning signs 10%
Abdominal pain 9%

Owners may only notice progressive, intermittent, vague problems, or the dog may go into acute adrenal crisis and have severely slowed heart rate, shock, dehydration, and weak pulses.

Since the signs of Addison's disease are not very specific, the diagnosis must be confirmed with laboratory tests. Blood tests will usually reveal low levels of sodium and high levels of potassium. The definitive test is the ACTH stimulation test. Administration of ACTH stimulates the release of cortisol in normal animals. If the cortisol level is low before the animal is given ACTH, and stays low afterwards, this confirms that the dog is Addisonian.

Fortunately, most dogs with hypoadrenocorticism respond very well to treatment with mineralocorticoid drugs. If they are severely ill, they may need therapy such as intravenous fluids to stabilize their condition. The blood sodium and potassium levels are monitored and the drug dosage adjusted accordingly over 3 to 4 weeks, and then checked every 3-4 months. The two most commonly used are fludrocortisone acetate ("Florinef,") given orally daily, and desoxycorticosterone pivalate ("DOCP"), a long-acting injectable given once every 25 days. While most dogs are well controlled on Florinef, the majority have more normal electrolyte balances and feel better when treated with DOCP. As well as mineralocorticoid replacement, about 50% of dogs will also need glucocorticoids, particularly in times of stress. Usually prednisone is used, and the dose can be increased when the dog is stressed.

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