Pituitary Macroadenoma in Dogs

By Krista Williams, BSc, DVM, CCRP; Catherine Barnette, DVM.

What is a pituitary macroadenoma?

Pituitary macroadenomas are large, non-cancerous (benign) tumors of the pituitary gland (a pea-sized organ at the base of the brain responsible for producing hormones that influence many organ systems in the body). These tumors are typically greater than ½ inch in diameter, though there is some subjective variation in the definition of macroadenoma depending on the animal’s skull size. Pituitary macroadenomas are more common in dogs than in cats.

These tumors may be functional (hormone-secreting) or non-functional (do not secrete hormones), and their effects depend on whether the tumor is producing hormones. The effects of non-functional pituitary macroadenomas are directly related to the physical pressure that the growing tumor places on surrounding brain structures, while the effects of functional macroadenomas are dependent on the type of hormones produced by the tumor.

"In most cases, pituitary macroadenomas are functional tumors."

In most cases, pituitary macroadenomas are functional tumors. The most common functional pituitary macroadenomas in dogs release a hormone known as adrenocorticotrophic hormone (ACTH). This hormone triggers the adrenal glands to release cortisol, leading to a condition called Cushing’s disease. Cushing’s is a common metabolic disease in dogs, especially in Boxers, Pugs, Bulldogs, Boston Terriers, and Dachshunds. (See handout “Cushing’s Disease in Dogs” for more information.)

What are the clinical signs of a pituitary macroadenoma?

In dogs with a functional pituitary macroadenoma causing Cushing’s disease, signs include increased appetite, increased thirst and urination, alopecia (hair loss), dry skin, blackheads, and a pendulous (sagging) belly. Dogs with Cushing’s disease also have a weakened immune system, so recurrent or chronic infections may be observed.

"Dogs with Cushing’s disease also have a weakened immune system, so recurrent or chronic infections may be observed."

A dog with a non-functional pituitary macroadenoma may show signs related to the tumor’s compression of nearby brain structures. Several optic (eye) structures are located in the area of the pituitary gland; therefore, visual defects are often seen with pituitary macroadenomas. Additionally, these tumors can interfere with the production of hormones that help to concentrate urine. If this happens, a dog can develop increased thirst and urination and a condition known as diabetes insipidus. In many cases, however, initial signs may be vague, such as lethargy and decreased appetite, progressing over time to walking in circles or seizures.

How is a pituitary macroadenoma diagnosed?

Definitive diagnosis of a pituitary macroadenoma requires brain imaging with advanced techniques such as CT or MRI. This imaging is performed under anesthesia at a specialty hospital or university.

How is a pituitary macroadenoma treated?

The most effective treatment for a pituitary macroadenoma is surgical removal, though it is uncommon in animals due to the cost and difficult nature of the surgery. If surgery is performed, the resolution of clinical signs is rapid and complete.

"Radiation therapy is a more commonly used method to shrink pituitary tumors."

Radiation therapy is a more commonly used method to shrink pituitary tumors. Radiation is administered several times per week for four to six weeks with the goal of decreasing the size of the tumor and minimizing clinical signs. Radiation therapy typically does not completely eliminate the tumor, but will often decrease the tumor’s size enough to alleviate clinical signs. Side effects of radiation include damage to the skin, as well as the risks associated with repeated anesthetic procedures.

Medications can also be used to manage the hormonal effects of functional pituitary macroadenomas. While this treatment does not address the actual tumor, and the tumor will therefore continue to increase in size, medication can be used to counteract the effects of the hormones produced by these tumors. Dogs with Cushing’s disease can be treated with medications such as trilostane (Vetoryl®) to decrease cortisol production.

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