Pituitary Tumors

By Debbie Stoewen DVM, MSW, RSW, PhD; Christopher Pinard, DVM

What is the pituitary gland?

The pituitary gland is a tiny organ, smaller than a pea, located at the base of the brain. It is responsible for producing specialized chemicals (or hormones) that influence many other organ systems in the body.

These hormones can affect the adrenal glands (responsible for producing stress hormones), the thyroid gland (responsible for producing thyroid hormone), and the parathyroid glands (responsible for regulating calcium), as well as the pancreas and other tissues in the body. The pituitary gland is often referred to as the ‘master gland’ because of its importance in controlling other parts of the endocrine (hormone) system.

What is pituitary tumor? 

A pituitary tumor develops as the result of abnormal, replication or growth of the cells that make up the pituitary gland. Tumors may develop from the functional glandular tissue (i.e., the tissue that produces hormones) – these are called functional pituitary tumors – or from the nonfunctional parts of the pituitary gland. About 10% of pituitary tumors in dogs are nonfunctional.

What causes this cancer?

The reason why a particular pet may develop this, or any tumor or cancer, is not always straightforward. Very few tumors and cancers have a single known cause. Most seem to be caused by a complex mix of risk factors.

Although there are no known dietary or environmental causes for pituitary tumors, certain breeds appear to be predisposed, including Miniature Poodles, Boxer Dogs, Boston Terriers, and Dachshunds. While no breed predisposition has been identified in cats, as in dogs, tumors tend to develop in those that are middle-aged to older.

What are the signs of a pituitary tumor?

The signs of a pituitary tumor depend on the region of the pituitary gland that is affected. If the tumor develops from the functional glandular tissue, the signs will be associated with the effects of these hormones.

In dogs, the most common pituitary tumor affects the region that produces adrenocorticotrophic hormone (ACTH). Excess ACTH causes the adrenal glands to make too much cortisol (a naturally produced steroid) leading to a condition called hyperadrenocorticism or Cushing’s disease. The clinical signs of Cushing’s disease include increased appetite and thirst, increased urination (sometimes leading to accidents in the house), lethargy, weakness, panting, heat intolerance, hair loss, obesity, and a pot-bellied appearance.

"The signs of a pituitary tumor depend on the region of the pituitary gland that is affected."

In cats, the most common pituitary tumor affects the region that produces somatotropin or growth hormone (GH). An increase in GH causes a condition called acromegaly. Because acromegaly causes insulin-resistant diabetes mellitus, signs of diabetes such as increased thirst, urination, and appetite are seen. In addition, excess GH causes excess growth of the legs, paws, chin, skull, and other parts of the body, and weight gain, causing the cat’s physical appearance to change.

If the tumor develops from a nonfunctional part of the pituitary gland, the size of the mass may cause neurological signs such as lethargy, decreased appetite, behavioral changes, blindness, seizures, and gait abnormalities.

How is this type of cancer diagnosed? 

If your dog has signs of Cushing’s disease, your veterinarian will run routine and specialized blood and urine tests. If the test results are inconclusive, retesting may be needed 3 to 6 months later. Once the disease is confirmed, however, additional blood tests are usually done to determine whether the cause is a tumor of the pituitary gland or of the adrenal gland. As well, the adrenal glands can be evaluated using abdominal x-rays or ultrasound, and the pituitary gland with more sophisticated methods of diagnostic imaging, such as computed tomography (CT) or magnetic resonance imaging (MRI).

If your cat has signs of acromegaly, although blood and urine tests will be helpful, the most definitive diagnostic test is a CT scan or MRI of the pituitary region.

How does this cancer typically progress?

As pituitary tumors are nearly always benign, they only grow locally. They do not metastasize (spread to other areas of the body). If the tumor is functional and your pet is not treated, the associated condition will continue or worsen, and your pet’s health will deteriorate. If the tumor is nonfunctional and your pet is not treated, the neurological signs will continue or worsen, or become more difficult to control, as the tumor grows.

What are the treatments for this type of tumor?

The choice of treatment depends on whether the pituitary tumor is functional or nonfunctional.

Many functional tumors can be managed with medication. Mitotane and trilostane are two medications that have been shown to be effective in dogs with Cushing’s disease. Radiation therapy is another option; new techniques are very effective and have few side effects. Medications may still be necessary for a few months after radiation therapy. Surgical techniques to remove pituitary tumors in dogs are being studied, but surgery is not a widely available option. Your veterinarian can recommend the best treatment for your dog based on your dog’s condition.

Managing the clinical signs of diabetes by increasing the dosage of insulin is the most conservative and most common method for managing cats with acromegaly. Radiation probably offers the greatest chance of successful treatment, improving both insulin resistance and neurological signs. The rate of tumor shrinkage, however, is slow (more than 3 years) and there is risk of damaging the pituitary gland and nearby brain tissue. Surgery to remove the pituitary gland (called a transsphenoidal hypophysectomy) is possible, and in some veterinary medical centers it has become the primary treatment, but it has significant risks and requires lifelong hormone supplementation to compensate for the loss of pituitary function.

The most common treatment for nonfunctional pituitary tumors is radiation therapy. The goal is to specifically target the enlarged pituitary and reduce the size for as long as possible. Radiation therapy in veterinary medicine has progressed with advances in technology that allow veterinary oncologists to specifically target a tissue with minimal effects to the surrounding tissue.

Is there anything else I should know?

The outlook for dogs with Cushing’s disease is good, with an average life expectancy of 2 years with medication. At least 10% of dogs that are treated live 4 more years. Dogs treated with radiation may survive 2–5 years.

"In all cases, the long-term outlook tends to improve with early diagnosis and treatment."

The outlook for cats with untreated acromegaly is fair to good in the short-term, using various medications to treat the signs. Because this does not address the cause of the condition, however, the long-term outlook is relatively poor. Most cats will die of congestive heart failure, chronic kidney failure, or signs related to the growing pituitary tumor.

In all cases, the long-term outlook tends to improve with early diagnosis and treatment.

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