Cushing’s Disease (Hyperadrenocorticism) in Dogs

Cushing’s disease is the condition in which there is too much cortisol in the bloodstream. The adrenal gland, in the kidney, is where cortisol is produced. The pituitary gland, in the brain, is where the Adrenocorticotropic hormone (ACTH) is produced. ACTH regulates production of cortisol. If the production of either ACTH or cortisol is disturbed, regulation of cortisol in the bloodstream fails. This can lead to too little cortisol in the bloodstream (known as hypoadrenocorticism or Addison’s disease), or too much.

Cushing’s disease results when the levels of cortisol in the bloodstream are chronically elevated. Cortisol is produced by the adrenal gland when the body encounters stress. In dogs with Cushing’s disease, cortisol levels are high all of the time.

Cushing’s disease is a serious health concern that occurs mainly in older dogs. Symptoms take significant time to develop and progress gradually over weeks, months, or even years. Cushing’s rarely presents as an emergency, but prompt veterinary care is crucial to mitigate complications like kidney damage, diabetes, anemia, blood clots, hypertension, and osteoporosis.

Iatrogenic Cushing’s syndrome occurs when a dog is given steroid-containing medication at inappropriately high doses or for too long. Dogs with iatrogenic Cushing’s are expected to fully recover and require no therapy other than gradual discontinuation of the medication.

Most symptoms are progressive and worsen over time if the condition is untreated.

The most common form of Cushing’s disease, called ACTH-dependent hyperadrenocorticism, is almost always caused by a tumor on the pituitary gland. These tumors cause the pituitary to secrete an excess of ACTH which in turn triggers the adrenal gland to produce an excess of cortisol.

The less common form of Cushing’s disease, called ACTH-independent hyperadrenocorticism, is caused by a tumor on the adrenal gland(s). The tumor leads directly to overproduction of cortisol by the adrenal gland. This form of Cushing’s disease is also known as adrenal-dependent hyperadrenocorticism (ADH) or functional adrenal tumors (FAT).

A third form of Cushing’s disease, called Iatrogenic hyperadrenocorticism, is caused by excessive or prolonged administration of corticosteroids. Administered steroids produce the same symptoms as those manufactured by the body.

In addition, skin changes such as blackheads and scaly patches may be present.

Cushing’s disease is difficult to diagnose because the symptoms are common in other conditions and routine screening blood tests do not always suggest the diagnosis. Multiple types of blood tests are usually necessary to make the initial diagnosis of Cushing’s and categorize it.

Diagnostic tools include:

• Physical exam
• Routine blood work
• Urinalysis
• Adrenal function tests (specialized blood work)
• Ultrasound
• MRI

Treatment depends on the underlying cause. In the case of ACTH dependent forms, drug therapy using trilostane or mitotane is common.

In ACTH independent forms, surgery is the only cure. In cases where the adrenal tumor is malignant (cancerous), chemotherapy is necessary.

In iatrogenic forms, gradual discontinuation of corticosteroids is required. This often results in the reemergence of the disease that was originally being treated.

In all cases, it is necessary to undertake regular monitoring by a veterinarian to ensure appropriate levels of cortisol are in the bloodstream. Damage caused to other systems cannot be reversed, but the progression can be stopped.

With early diagnosis and treatment, the prognosis for iatrogenic Cushing’s is good and no additional treatment, complications, or recurrences are expected as long as the dog is not given inappropriate doses of steroid medications.

Prognosis is complicated for the other types of Cushing’s. Since the symptoms of non-iatrogenic Cushing’s do not appear until late life, life expectancy is 1 to 2 years, but impacted dogs usually die of other causes. Diligent observation and long-term management of Cushing’s disease are usually sufficient for good health for the remaining years of life.

No preventative measures have been determined. There is some evidence that Cushing’s disease has a genetic component.

Cushing’s disease is one of the most common endocrine disorders in dogs. ACTH-dependent hyperadrenocorticism is more common than the ACTH-independent forms. Others, such as ectopic ACTH, are much rarer.

For ACTH dependent forms (pituitary tumors): drugs such as trilostane or mitotane.

For ACTH independent forms: surgery and chemotherapy, or drugs such as trilostane.

For Iatrogenic: the gradual discontinuation of corticosteroids.

Monitoring for the remainder of the life.