Cushing’s Disease is also called “Hyperadrenocorticism”, which means an overactive adrenal gland. The adrenal gland sits just on top of the kidneys, and is typically smaller than a peanut. Despite being such a small gland, it has several important roles in the body and is divided into two parts. The outer part is called the adrenal cortex. The adrenal cortex produces sex hormones, helps control blood levels of sodium and potassium and also secrets glucocorticoids or steroids in the body that help maintain normal cardiovascular and gastrointestinal function among many other things. The inside, or medulla, of the adrenal gland produces epinephrine (also called adrenaline) and norepinephrine (or noradrenaline) which are important hormones released during times of stress when the fight or flight response is initiated.
Cushing’s disease occurs when there is an excessive amount of hormones being secreted by the adrenal gland. In particular, Cushing’s disease involves excessive amounts of glucocorticoids, the steroids that in healthy pups help maintain normal heart and digestive health. High levels of glucocorticoids can cause symptoms such as panting, increased drinking and urination, poor hair coat, a pot-bellied appearance as well as muscle wasting. Cushing’s can also make pets more susceptible to infections.
There are two main causes of Cushing’s disease. Either there is a tumor associated with the adrenal gland itself or a small tumor in a part of the brain called the pituitary gland. Pituitary gland tumors cause roughly 75% of cases of Cushing’s disease. Cushing’s caused by pituitary gland tumors is also known as Secondary or Pituitary-Dependant Hyperadrenocorticism. These tumors tend to be very small, but they secrete an “activating hormone” that causes the adrenal glands to become overactive and pump excessive amounts of hormones throughout the body. In rare cases, the tumor may cause neurological symptoms as well.
Cushing’s disease can be difficult to definitively diagnose, and often veterinarians will use multiple tests to determine if Cushing’s is present. Typically, vets will begin with a routine blood test. Depending on blood test results, imaging of the adrenal glands may be performed by an experienced ultrasonographer. Additional blood tests may be necessary. One test in particular called a dexamethasone suppression test requires an injection of steroids and several blood samples over the course of eight hours. In rare cases, specialized urine tests and MRI imaging of the brain may be necessary to determine the diagnosis of Cushing’s.
The two most commonly prescribed pet medicationsfor Cushing’s disease are Mitotane (Lysodren) and Trilostane (Vetoryl). Mitotane is actually toxic to the adrenal glands, essentially working by killing the portion of the gland that secretes excessive steroids. When using this medication, there is a risk that your dog can develop Hypoadrenocorticism, or under-active adrenal gland, also known as Addison’s disease. Trilostane is a newer drug that interrupts normal production of the steroids so they are not active. Trilostane is often reported to have fewer side effects, however this has not yet been conclusively proven. Use of either drug requires close monitoring by a veterinarian. Maintaining lower hormone levels helps control Cushing’s disease and prevents life-threatening complications.