Addison's disease refers to the syndrome that results from failure of the adrenal glands to produce the hormones that they normally make. The adrenal glands are two small structures located alongside each kidney. The main hormones produced by the adrenal gland are steroids. There are two major classes of these steroids: mineralocorticoids and glucocorticoids. Aldosterone, the main hormone in the mineralocorticoids class, plays a major role in regulating sodium, potassium, and water balance. Cortisol, the main hormone in the glucocorticoids class, acts on almost every major tissue in the body, helping to regulate glucose production and metabolism, influencing fat and protein breakdown, stimulating red blood cell formation, helping to regulate blood pressure, counteracting stress, and suppressing inflammation.
Despite their different control mechanisms, both classes of steroids usually are affected by primary adrenal gland failure in Addison's disease. However, some animals will have symptoms primarily related to mineralocorticoid deficiency, while others will experience problems primarily from glucocorticoid deficiency. Although sex hormones such as estrogens and androgens also are produced by the adrenal glands, signs due to deficiencies of these hormones do not occur in dogs with Addison's disease.
Destruction of 85 to 90 percent of the steroid-producing cells in the adrenal gland appears necessary for signs to develop secondary to deficiencies of mineralocorticoids and glucocorticoids. This destruction is most commonly due to immune system-mediated destruction of the adrenal glands. Less frequently, infections, inflammation, cancer, drug therapy, or abnormalities in blood supply to the adrenal gland can contribute to the development of Addison's disease. Secondary adrenal gland failure due to problems that affect the hypothalamus or pituitary gland may also occur, resulting in the signs seen with Addison's disease.
Symptoms of Addison's disease may follow an intermittent course, often coming and going over a long period of time before the illness is suspected. Occasionally, Addison's disease can be diagnosed in dogs with relatively mild symptoms. However, it is common for dogs not to be diagnosed until a life-threatening crisis due to Addison's disease develops. Severe signs of illness including shock and collapse characterize these crises. Usually, the animal can be stabilized successfully if it receives immediate treatment with fluid resuscitation and medications to improve electrolyte and acid-base system abnormalities and to replace deficient glucocorticoids. Once the initial crisis passes, maintenance treatment with either oral or injectable mineralocorticoids, and for many dogs, oral glucocorticoids will be necessary for life. Despite the serious nature of Addison's disease, the vast majority of dogs can be well controlled with medication. However, supplementing some dogs with glucocorticoid insufficiency will be necessary during any stressful period.
Atypical Addison's disease refers to primary or secondary adrenal gland failure in dogs that do not exhibit the classic symptoms or electrolyte abnormalities usually seen in Addison's disease. Animals diagnosed with this condition may have more subtle changes on blood tests and other diagnostic procedures. Although these dogs will not have the classic findings with Addison's disease, they will exhibit abnormally low responses to ACTH on the ACTH stimulation test, and generally they will respond to treatment with glucocorticoids alone, since the sodium and potassium regulation will remain normal.
