Pituitary Macroadenoma in Dogs

What is a pituitary macroadenoma?

Pituitary macroadenomas are large, non-cancerous (benign) tumors of the pituitary gland. 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, although they can also occur in cats.

These tumors may be functional (hormone-secreting) or nonfunctional, and their effects depend on whether the tumor is producing hormones. The effects of nonfunctional 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, meaning they secrete hormones. 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.

What are the clinical signs of a pituitary macroadenoma?

In dogs with a functional pituitary macroadenoma, the most common sequela (an aftereffect resulting from a prior disease or condition) is Cushing’s disease. Signs of Cushing’s disease include increased appetite, increased thirst and urination, loss of hair (alopecia), dry skin, blackheads, and a pendulous (sagging) belly. Dogs with Cushing’s disease also have a weakened immune system, so recurrent/chronic infections may be observed.

Dogs with nonfunctional pituitary macroadenomas may show signs related to the tumor’s compression of nearby brain structures. Several optic 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, animals 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 pituitary macroadenoma is surgical removal. Although pituitary tumors in humans are often treated surgically, this surgery is uncommon in animals due to cost and the difficult nature of the surgery. If surgery is performed, resolution of clinical signs is rapid and complete.

Radiation therapy is a more commonly-used method to shrink pituitary tumors. Radiation is administered several times per week over a period of 4-6 weeks, with the goal of decreasing the size of the tumor and decreasing 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 therefore the tumor will continue to gradually 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 a medication that decreases cortisol production (mitotane or trilostane).