Disorders of the Pituitary Gland in Dogs
The pituitary gland is located near the center and bottom of the brain. It produces a number of critical hormones that control many parts of the body, including several other endocrine glands. Because of this central role, it is sometimes called a “master gland.” Because large numbers of hormones are produced by the pituitary gland, a variety of different conditions can be caused by pituitary disease or tumors. The specific illness and signs depend on the cause and the area(s) of the pituitary gland that is affected.
Cushing disease, also called hyperadrenocorticism, is caused by too much cortisol. Cushing disease is common in dogs but not in other species. Miniature Poodles, Dachshunds, Boxers, Boston Terriers, and Beagles are at increased risk. In most affected dogs (85 to 90%), the cause is a small, benign pituitary tumor. In the remaining 10 to 15% of dogs, the cause is a tumor of the adrenal gland itself.
This condition tends to occur in middle-aged to older dogs. Common signs include increased thirst and urination, increased appetite, heat intolerance, lethargy, a “potbelly,” panting, obesity, weakness, thin skin, hair loss, and bruising. Rarely, calcinosis cutis develops, a condition in which minerals are deposited in the skin and can appear as small, thickened “dots” on the abdomen.
Diagnosis of Cushing disease can be difficult because laboratory test results may be inconclusive and false-positive test results are common in dogs that have other diseases. Retesting may be needed 3 to 6 months later. Once the disease is confirmed, additional laboratory tests are usually done to determine whether the cause is a tumor of the pituitary gland or of the adrenal gland. The pituitary and adrenal glands can be further evaluated using abdominal x-rays, ultrasonography, or more sophisticated methods of diagnostic imaging such as computed tomography (CT) or magnetic resonance imaging (MRI).
Hyperadrenocorticism can be treated with mitotane, a drug that acts on the adrenal glands to decrease the production of cortisol. Dogs should be monitored for signs that the cortisol level has dropped too low, such as reduced appetite, vomiting, and diarrhea. After 7 to 10 days of treatment, a test may be performed to determine if cortisol levels are low enough. Often, mitotane treatment is continued, with blood tests done periodically to monitor cortisol levels. Gradually increasing doses of mitotane are often needed to keep the disease under control. Adverse effects of mitotane can include vomiting and loss of appetite, lack of coordination, weakness, seizures, and mildly low blood sugar. These effects sometimes stop if the daily dose is divided into 2 equal parts and given 8 to 12 hours apart.
Another medication called trilostane has been shown to be effective in treating pituitary-dependent Cushing disease in some studies, and it may have fewer adverse effects. Your veterinarian can recommend the best treatment for your dog based on your pet's condition.
If adrenal gland tumors are present, surgical removal is sometimes an option. In some cases, radiation treatment of the pituitary gland is a possibility, although it may take several months for signs of the disease to resolve.
In adult-onset panhypopituitarism, the pituitary gland and nearby tissues, including the hypothalamus, are compressed or damaged. As a result of this damage, the secretion of many of the anterior pituitary hormones becomes inadequate or absent. This disrupts a number of other hormone-producing glands, resulting in a variety of signs.
Inactive pituitary tumors develop most commonly in adult to older dogs. All breeds appear to be equally affected. Other conditions, infections, or injuries that lead to destruction of pituitary tissue can also cause panhypopituitarism.
Affected dogs are often depressed and uncoordinated, collapse with exercise, and lose weight. Occasionally, they show a change in attitude, do not respond to people, and tend to hide. In chronic cases, the animal may become blind because the growing pituitary tumor puts pressure on the optic nerves. Animals with panhypo-pituitarism appear dehydrated despite drinking more water. Dogs may urinate in large volumes and break housetraining.
Inactive pituitary tumors usually become quite large before they cause obvious signs or death. The entire hypothalamus may be compressed and replaced by tumor, and the thyroid and adrenal glands may be smaller than normal.
External beam radiation therapy offers the best means of treatment for dogs with large pituitary tumors through reduction in the size of the mass. In dogs with severe signs and very large tumors, however, the response is often inadequate and the outlook for these dogs is very poor.
In juvenile-onset panhypopituitarism, also called pituitary dwarfism, the front portion of the pituitary gland does not fully develop or is disrupted by a tumor. This affects several other hormone-producing glands, leading to a variety of signs. In particular, the lack of growth hormone causes the young animal to be dwarfed.
Pituitary dwarfism is most common in German Shepherds and has been seen in the Spitz, Miniature Pinscher, and Karelian Bear Dog. It is inherited and occurs equally in male and female dogs.
Dwarf pups appear the same as their normal littermates up to about 2 months of age. After that, they grow slower than their littermates and keep their puppy coat. Primary guard hairs do not develop. Hair is gradually lost on both sides of the body, and hair loss often becomes complete except for the head and tufts of hair on the legs. Permanent teeth do not come in, or come in late. Closure of the growing ends of the bones can be delayed as long as 4 years. The testes and penis of male dogs are small. In female dogs, heat cycles are irregular or absent. Because the pituitary gland affects the production of other hormones in the body, the levels of thyroid hormones and cortisol are reduced, and the thyroid and adrenal glands show signs of deterioration. Affected dogs have a shortened life span.
Unfortunately, effective treatment of pituitary dwarfism is not possible in dogs. Daily thyroid hormone replacement should be started as soon as there is evidence of secondary hypothyroidism. Canine growth hormone is not yet available for use as a therapy, and only porcine or human growth hormone has been tried. Because of the differences between canine and human growth hormone, the formation of antibodies against the human growth hormone is common in dogs, making this treatment ineffective. Porcine growth hormone appears to work better, but it is expensive and is not widely available. Recently, the use of progestogens to treat congenital growth hormone deficiency in dogs has been reported. This treatment was reported to slightly increase body size and hair regrowth in a few young dogs, but it has many potential adverse effects.
The outlook for dogs with pituitary dwarfism is poor if treatment is not attempted. By 3 to 5 years of age, affected dogs are usually bald, thin, mentally dull, and lethargic. These changes are due to the progressive loss of pituitary function, as well as the continuing expansion of pituitary cysts.
Despite its name, diabetes insipidus is not related to the more commonly known diabetes mellitus, and it does not involve insulin or sugar metabolism.
Diabetes insipidus is caused by problems with antidiuretic hormone or vasopressin, a pituitary gland hormone responsible for maintaining the correct level of fluid in the body. Either the pituitary gland does not secrete enough of this hormone (called central diabetes insipidus), or the kidneys do not respond normally to the hormone (called nephrogenic diabetes insipidus).
Affected dogs urinate in large volumes and drink equally large amounts of water. The urine is very dilute even if the animal is deprived of water. (Normally, urine becomes more concentrated when an animal is dehydrated.)
If the animal is not dehydrated and does not have kidney disease, a water deprivation test can be done to assist in diagnosing diabetes insipidus. A second test that measures the animal’s response to treatment with antidiuretic hormone is then performed to distinguish between central and nephrogenic diabetes insipidus.
Increased urination may be controlled using desmopressin acetate, a drug that acts in a way similar to antidiuretic hormone. Water should not be restricted. Treatment is usually lifelong.