Excessive Growth Hormone in Cats

You may suspect excessive growth hormone if you have an older cat, especially a male, whose diabetes is not easy to control with traditional methods. Other signs that acromegaly may be present include a broadened face, growing paws, and a protruding jaw or growing, lengthening teeth and gums. 

Excessive growth hormone in cats is also called acromegaly or hypersomatotropism. It is a somewhat rare condition which occurs when the cat's pituitary gland, located at the base of the brain, grows a tumor. This tumor then causes the gland to secrete large, consistent amounts of growth hormone. Acromegaly is most common in older cats ages 8 and up, and is slightly more common in male cats. It is typically caught due to two key symptoms: diabetes mellitus and a large increase in lean body mass. From there, it can lead to a broadening of face and organ masses, and potential organ failure. 

There are a number of symptoms of excessive growth hormone. Many are associated with and can be misinterpreted as mere diabetes mellitus, while others appear as the condition progresses. They include: 

• Excessive thirst and hunger
• Not using the litter box
• Change in appetite
• Weight loss then weight gain
• Change in gait, energy level, signs of depression
• Broadened face
• Change in shape of jaw and/or gums
• Change in the quality of fur
• Signs of respiratory distress

The cause of acromegaly comes from an excess secretion of the cat's growth hormone, called somatropin. This hormone comes from the anterior part of the pituitary gland, a hormone-secreting gland that exists at the base of the brain. Typically, the gland will only secrete somatotropin during the cat's childhood, unless a tumor or other injury causes it to react differently.

Often, acromegaly will be mistaken as a diagnosis of diabetes mellitus at first, as many of the symptoms are the same. However, most of the treatment options associated with diabetes mellitus will not work, or will not work well. This typically includes a change in diet and the administration of insulin. 

No single diagnostic test exists for the condition, but instead, it is discovered based on your cat's clinical history and the initial response to the treatment of the diabetes symptoms. Growth hormone assay tests on blood are not commonly available, though they do exist. However, a more common blood test that a vet may do while looking for excessive growth hormone is for serum IGF-1. This is something that is known to be in much higher concentrations in cats with acromegaly than cats with diabetes. This amount is supposed to increase after a cat starts insulin treatments, so a vet may want to test for IGF several times throughout the course of an insulin treatment protocol. 

If growth hormone levels are suspected to be high after the blood tests are complete, the vet will probably request a CT scan of the cat's brain. It is here you can see the tumor or lesion that is causing the problem if it exists. 

There are several different levels of treatment you can use to control your cat's growth hormone level, which range from giving the body hormones to control release to radiation and surgery. As with people, each of these methods comes with its own risks and rewards, both medical and financial.

Somatostatin

Somatostatin is a naturally-occurring hormone that tells the pituitary gland to slow or stop the release of growth hormone. Using somatostatin on people with growth hormone issues has a 50-60% effectiveness rate, and can even shrink a pituitary tumor in some cases. Somatostatin has been formed into a similar chemical called octreotide for treatment purposes. So far, though it shows some short-term success in cats, they are not lasting. However, the success in people has research vets continuing work to find a better formulation that will work on cats as well as it does on people. The risks with this form of treatment are low, and it is non-surgical, but it is also the least likely treatment protocol to work. 

Insulin

Increasing insulin levels is the most common, conservative and cost-effective way to manage growth hormone in cats today. It does not control the release of the growth hormone in any way, but it does control diabetes caused by the growth hormone induced swelling of internal organs. It is less risky and less expensive than other options, though there is a small risk that a cat will have a hypoglycemic episode if it suddenly becomes sensitive to insulin once again. A treatment like this would need to be continued for the lifespan of your cat. 

Surgery

In people with the same problem, surgery is the most common choice, as it is the only currently-known way to remove the problem of growth hormone completely. While in people, they may take the time to try and remove only the lesion or tumor, cats and dogs who undergo an adenectomy typically have the entire pituitary gland removed. This will mean a lifetime of daily medications to replace other pituitary hormones. Surgery is costly, but it comes with few risks outside of typical surgical bleeding and clotting anomalies. It has been known to permanently eliminate diabetes symptoms in many of the cats within weeks following surgery. There are few places who perform the procedure, and you may have to travel to another state to find one of the few animal hospitals willing to perform a feline adenectomy.

Radiation

Radiation is another common form of treating the tumor, and is a good option if it is inoperable or there are no local surgical centers. This can be a costly treatment. Radiation can come with a large number of side effects, however. This includes hair/fur loss, skin changes, and long-term blindness or deafness. However, it can also shrink or remove the tumor and increase neurologic function that is being impaired by the growth hormone. This has a success rate in reducing diabetes symptoms of over 90 percent, and of around 50% of cats have a complete reversal of symptoms. Of those, about half remain in remission a year later. 

Recovery depends greatly on the kind of treatment you choose. Insulin and an adenectomy will both require a lifetime of daily medication to manage hormone replacement or insulin needs. All treatments will require regular follow-up appointments to monitor the diabetes and/or hormone levels of your cat. This may be weekly, monthly, quarterly or yearly depending on your vet's confidence in your cat's recovery.