Weight Loss and My Older Cat…..What You Should Know About Feline Hyperthyroidism

By Michelle Fabiani, DVM, Diplomate ACVR

We are lucky that the average life span of our feline pets has significantly increased over the years.  Our indoor cats now live an average 12-15 years now.  So first you get a kitten and next thing you know he’s getting “up there” in the years.  So you ask, “How old is old”?  Cats are considered geriatric by the age of 8.  Since we will be taking care of an older kitty for so many years, as responsible pet owners we should know what we can do to keep them healthy and disease free as they age into their “sunset years”.  First and foremost, regular geriatric screening is a necessity!  It is important to see your veterinarian once to twice yearly for physical examinations and baseline blood work.  This will allow early detection of disease processes.  In geriatric cats, diabetes mellitus, chronic renal disease and hyperthyroidism are the three most common diseases diagnosed. 

The thyroid gland is an “H” shaped organ in the neck of cats that produces thyroid hormone. Thyroid hormone regulates metabolism.   Hyperthryoidism is caused by overproduction of thyroid hormone in the thyroid gland.  In 99% of cats with hyperthyroidism, a small benign tumor forms in one or both sides of the thyroid gland. This tumor produces more thyroid hormone than the body needs, which results in increased metabolism and ultimately weight loss. Other common clinical signs of hyperthyroidism include increased appetite, vomiting, increased vocalization and agitation.

Hyperthryoidism can sometime be challenging to diagnose due to variations in thyroid hormone production.  Most often a simple blood test identifies elevations in different forms of thyroid hormone (T3, T4, or free T4).  However, in some cats, blood tests are equivocal and an additional special test called a nuclear thyroid scan will need to be performed.  With this test, a radioactive substance called technetium is administered and the uptake in the thyroid gland is evaluated.  This test helps to differentiate between common benign and rare malignant thyroid disease and is used to determine dosing for radioiodine therapy.  At all veterinary universities throughout the United States, this test is considered the gold standard for diagnosing hyperthyroidism. 

“Ok so my cat has hyperthyroidism. What are the treatment options?”, you ask.  There are now four different ways to treat hyperthyroidism: radioiodine, surgery, medical, and dietary.  First, let’s look at radioiodine, which is most often referred to as the “treatment of choice”.  With radioiodine therapy, a very small dose of radioactive iodine is injected under the skin.  There is preferential uptake of the radioactive iodine in the thyroid tumor and thus selective tumor destruction occurs.  No matter the location of the abnormally functioning thyroid cells (which can located from the mouth to the level of the heart), they will be destroyed!  Cats will stay at the treatment facility for 3-5 post injection.  The next treatment option (and the only other one that treats the tumor) is surgery.  With surgery, there are risks associated with anesthesia. Additionally, surgical removal of the thyroid gland can leave some cats permanently hypothyroid, necessitating life-long thyroid hormone supplementation.  Another risk of surgery is not removing active thyroid tissue that is located in other areas of the body. When this happens a cat remains persistently hyperthyroid after thyroidectomy.  Methimazole is the drug used to treat hyperthyroidism. It is an oral medication that suppresses production of thyroid hormone.  It does not eliminate or slow growth of the thyroid tumor, must be given for the life of the cat at progressively increasing doses, and significant drug side effects are not uncommon.  Lastly, the latest treatment option to arrive on the scene is a diet from Hills called Y/D.  Simply explained, the thyroid needs iodine to make thyroid hormone and by feeding an iodine deficient diet, the diseased thyroid cannot produce active thyroid hormone.  This can be an option for some cats but it cannot be fed to cats with certain concurrent illnesses, it allows continuous growth of the tumor, and palatability remains a considerable obstacle for many patients. 

Make sure to see your veterinarian if you notice any signs in your cat that concern you.  Hyperthyroidism is very common in older cats and is one of few diseases that is easily curable.

NOTE: This article is copyrighted and originally published in Houston PetTalk Magazine. Used with permission. All rights reserved.