Disorders of the Adrenal Glands in Dogs
The adrenal glands are located just in front of the kidneys. The adrenal gland has 2 parts—the cortex and the medulla.
The adrenal cortex is subdivided into 3 layers, and each layer produces a different set of steroid hormones. The outer layer produces the mineralocorticoids, which help to control the body’s balance of sodium and potassium salts. The middle layer produces glucocorticoids, which are involved in metabolizing nutrients as well as in reducing inflammation. The inner layer produces sex hormones such as estrogen and progesterone.
The adrenal medulla plays an important role in response to stress or low blood sugar (glucose). It releases epinephrine (sometimes also called adrenaline) and norepinephrine, both of which increase heart output, blood pressure, and blood glucose, and slow digestion.
Cushing disease, also referred to as hyperadrenocorticism, is a common endocrine disease in adult and aged dogs. The signs result primarily from chronic excess of the hormone cortisol. Increased cortisol levels in dogs may result from one of several mechanisms. The most common cause (85% to 90% of cases) is a tumor in the pituitary gland. The pituitary tumor produces a hormone that triggers excessive development of the adrenal gland. Less common (10% to 15% of cases) is a tumor in the adrenal glands themselves. Longterm use of corticosteroid drugs (for example, to decrease inflammation or treat an immune disorder) can also cause signs of Cushing disease. (Veterinary.heading on page Disorders of the Pituitary Gland in Dogs : Cushing Disease)
Addison disease, also called hypoadrenocorticism, is caused by a deficiency of adrenal gland hormones. It is most common in young to middle-aged dogs. The cause is usually not known, but an auto-immune condition in which the body destroys some of its own tissue is likely. The adrenal gland can also be destroyed by other conditions, including cancer in other parts of the body. Secretion of aldosterone, the main mineralocorticoid hormone, is reduced, which affects the levels of potassium, sodium, and chloride in the blood. Potassium gradually builds up in the blood and, in severe cases, may cause the heart to slow down or beat irregularly. Some dogs have such a slow heart rate (50 beats per minute or lower) that they can become weak or go into shock.
Signs of Addison disease include repeated episodes of vomiting and diarrhea, loss of appetite, dehydration, and a gradual loss of body condition. Weight loss is often severe. Although signs can be hard to identify while Addison’s disease is developing, severe consequences, such as shock and evidence of kidney failure, can develop suddenly.
A veterinarian can make a tentative diagnosis based on the history, signs, and certain laboratory abnormalities, such as very low levels of sodium and very high levels of potassium in the blood. The diagnosis is confirmed by specific evaluation of adrenal function. This is done by measuring the level of cortisol in the blood, treating the dog with adrenocorticotropin (a hormone that stimulates the adrenal gland in healthy animals), and then measuring the level of cortisol in the blood a second time. Affected dogs have low baseline cortisol levels, and there is little response to administration of adrenocorticotropin.
An adrenal crisis is a medical emergency and requires treatment with intravenous fluids to restore levels of body fluids, salt, and sugar to normal. Hormone replacement treatment can often be started while the pet is being stabilized. Laboratory values are monitored regularly to assess response to treatment and adjust doses if needed. For longterm treatment, replacement hormones can be given by mouth or injection. Additional sodium may need to be added to the diet.
Pheochromocytomas are tumors of the adrenal medulla that are able to secrete epinephrine (adrenaline), norepinephrine, or both. Often, there are no signs, and the tumor is found by chance during treatment for other conditions. When signs are present, they may include increased thirst and urination, increased heart rate, restlessness, and a distended abdomen. Diagnosis is often made based on signs and ultrasonography. Treatment involves surgery (if feasible) and management of high blood pressure. Other tumors in the adrenal glands, such as neuroblastomas and ganglioneuromas, can develop from nerve cells.