Addison's and Cushing's Diseases
Addison’s disease is also called the great pretender. This disease can present with vague symptoms that are common to many diseases. These include lethargy, diarrhea, and weakness. Addison’s disease results from inadequate production (by the adrenal glands) of steroids. This inadequate production is secondary to an infection or immune-mediated destruction of the adrenal glands. Glucocorticoids are responsible for controlling the immune system, regulating carbohydrate metabolism, and enabling the body to respond to stress. Mineralocorticoids are responsible for maintaining blood pressure and blood volume by regulating the amount of electrolytes and water excreted and reabsorbed by the kidneys.
Addison’s disease is fatal if untreated because of the lack of mineralocorticoids. Aldosterone is the main steroid hormone that is missing in this disease. Aldosterone affects the part of the kidney (the distal tubule) where sodium and potassium are normally exchanged. With an aldosterone deficiency, the distal tubules retain excess sodium and water and excrete excess potassium. This leads to an electrolyte imbalance that is quickly lethal, causing heart failure.
Addison’s disease is diagnosed by a blood test called an ACTH stimulation test. A blood sample is drawn and then an injection of adrenocorticotropic hormone (ACTH) is administered. Blood is drawn again, usually 1 or 2 hours later. The cortisol is measured in the blood. If the cortisol is very low, even after the ACTH is administered, then a diagnosis of Addison’s disease is made.
Once a diagnosis has been made, treatment must be started. Replacement of the mineralocorticoids is essential for life. The most effective way to accomplish this is by an injectable medication called DOCP. An oral medication called Florinef is sometimes used, but doesn’t work for all dogs. In addition, oral glucocorticoids, usually in the form of prednisone, must be given.
Close monitoring of blood values and medical condition are important for managing this disease.
Cushing’s disease is essentially the opposite of Addison’s. It is caused by an excess production of glucocorticoids by the adrenal glands. There are two causes of this excessive production: the first is a tumor in the adrenal gland, the second is a micro-tumor on the pituitary gland. Adrenal tumors grow within the part of the gland that produces cortisol and excess cortisol is secreted into the body. Pituitary micro-tumors send a signal to the adrenal glands to produce more cortisol, in this case both glands are affected.
Symptoms of Cushing’s disease that are most noticeable to you are increased thirst and increased urination. These symptoms can also be indications of kidney disease, diabetes, and liver disease, so veterinary care is necessary to get a diagnosis. Other symptoms include recurrent bladder infections, hair loss along the trunk of the body, distended abdomen, and thinning skin.
Cushing’s disease is diagnosed by a blood test, just like Addison’s disease. A low-dose dexamethasone suppression test (LDDST) is usually performed to make the diagnosis. Sometimes an additional test may be necessary to determine whether the disease is being caused by an adrenal tumor or pituitary micro-tumors. Once a diagnosis is made, treatment can be pursued.
In cases of adrenal tumors, the treatment of choice is surgical removal of the tumor. These are usually only on one adrenal gland, but can be very invasive and often involve major blood vessels. Sometimes they are inoperable.
Cushing’s disease caused by pituitary micro-tumors is treated with medication that destroys the adrenal glands’ ability to produce cortisol. Sometimes the adrenal glands are completely destroyed and the patient must be treated for Addison’s disease. Addison’s is actually easier to manage than Cushing’s disease, so this may be considered desirable by some veterinarians.
The adrenal glands are important for survival. Dysfunction of these important glands causes significant illness. Understanding these diseases will help you be alert for the symptoms so that you can seek out veterinary care for your pet.
Please note that this information does not replace onsite, professional, veterinary care. It is solely for educational purposes. Your pet's medical condition should be evaluated by a veterinarian before any medical decisions are implemented. If there is a potentially life-threatening emergency involving your pet, take your pet to a veterinarian or veterinary facility immediately.
Veterinary Technical Services Department