What is Feline Hyperthyroidism?
Hyperthyroidism is a disease common in older cats caused by an overproduction of thyroid hormones by a growth in the thyroid gland.
What Do Thyroid Hormones Do?
Thyroid hormones normally function to determine the metabolic rate of the cat’s body, or in simpler terms, these hormones tell each cell in the cat’s body how hard or fast to work. Most often, thyroid growths are benign, with only 3% to 5% being malignant. In a normal cat, the body sends a message to the thyroid gland when enough thyroid hormone is circulating in the body and the thyroid gland shuts off production of hormone until it is needed. However, in a cat with hyperthyroidism, the growth in the thyroid gland doesn’t respond and continues to make thyroid hormones.
Left untreated, hyperthyroidism will shorten your cat’s lifespan and significantly decrease his/her quality of life. Untreated hyperthyroidism leads to hypertension (increased blood pressure), kidney failure, and heart failure.
Who Gets Hyperthyroidism And Why?
This is typically a disease of older cats with the average age of diagnosis of 13 years. While many causes have been speculated, including chemicals in the lining of cans used for canned cat food, no one cause has been proven. More than likely this disease has many factors including environment, diet, and genetics playing a role. Additionally, domestic cats live longer today than they did 30 years ago and we test more and catch the development of this disease more frequently.
How is Feline Hyperthyroidism Diagnosed?
The clinical signs of hyperthyroidism include, but are not limited to, weight loss despite a great appetite, decrease in muscle tone and mass, restless behavior, chronic vomiting and diarrhea, fast heart rate even when resting, and heart murmur. Any individual cat may have one or more of these signs or may have none of them. Your vet may also be able to palpate a mass your cat’s neck where the thyroid is located.
Once at the office, your vet will draw blood and run a test to measure your cat’s thyroid hormones. The first screening test is to test the level of Total T4 (thyroid hormone or TT4), mild changes in the liver enzyme values, as well as any signs of other diseases that may be present. It is difficult to diagnose mild or occult (non-symptomatic) hyperthyroidism and additional tests may be needed. It is very important to understand that TT4 levels can be artificially suppressed by other illnesses (this is called “euthyroid”) If your cat is showing signs consistent with hyperthyroidism but his thyroid levels are normal, your vet will need to rule out other diseases. Without complete bloodwork, determining the best and safest treatment is difficult. The other common disease that older cats develop is kidney disease. High thyroid levels from hyperthyroidism can mask the markers of kidney disease in your cat’s bloodwork. Once treated for hyperthyroidism, it is common for renal (kidney) disease to become apparent. This is why your vet will recommend multiple blood tests — to monitor response to treatment and to make sure we diagnose any other conditions that need to be treated. Cats on therapeutic drugs will need to have regular blood tests to monitor their hormone levels. In rare cases, hyperthyroidism that does not respond to medication may mean a malignant thyroid neoplasia and surgery may be required.
How is Hyperthyroidism Treated?
Hyperthyroidism is typically treated in one of two ways. Most cats are treated with oral medications that block the production of thyroid hormones, but many are also now treated with radiotherapy that actually cures hyperthyroidism in most cats.
The drug used to block the production of thyroid hormones is called Methimazole and is available in pill form, but can be compounded into a liquid or a flavored treat. Methimazole is given twice daily to cats and is typically well tolerated. Side effects occur in 15% of cats and usually subside in the first three months of treatment. These include vomiting, decreased appetite, and low energy. More severe side effects are possible, but are rare. Occasionally a cat will not tolerate oral dosing of methimazole in pill form. Liquid formulations are available and increasingly popular is a transdermal form of methimazole that is applied to the ears and absorbed through the skin. This is an option that can reduce the stress of delivering medications. However, because we do not know how consistently the medication is absorbed into the skin, your vet may need to test more frequently to make sure the proper dose is being delivered.
Methimazole will not cure hyperthyroidism but it can be used safely for years to manage the disease as long as routine bloodwork is done to monitor thyroid hormone levels and liver function. Methimazole treatment is the least expensive treatment up front, but over time you may wind up spending just as much as radiotherapy would cost and there’s pilling a cat twice a day for the rest of their lives.
Radiotherapy – Radioactive Iodine I131 Treatment
Radiotherapy is the safest and most effective way to treat hyperthyroidism in an otherwise healthy cat as it provides an actual cure to the disease instead of just disease management. This cannot be done at your regular vet’s practice unless they are set up and approved to offer this service. Your cat will be admitted into an approved facility to receive an injection of radioactive iodine which is taken up by the abnormal growth of the thyroid gland only. The radioactive iodine kills abnormal thyroid tissue while leaving the normal thyroid gland untouched. During this procedure, your cat will be housed in a facility specially designed to protect against exposure to radioactive elements. Your cat’s urine and feces will be radioactive for 3 to 7 days post treatment.
Once he/she has passed most of the radioactivity out of his/her body, you will be allowed to take him/her home. For the next 1-2 weeks you will still have to dispose of your cat’s fecal matter and urine according to the vet’s directions, but your cat should not pose a health threat to you or other healthy individuals. This treatment only rarely needs to be repeated as 96% of cats are cured with the first injections. Side effects are rare, but cats can become hypothyroid and need to have thyroid supplementation if too much of the thyroid gland is destroyed. Radiotherapy is expensive and the cost is up front, but your cat may have many years of healthy living ahead of him and the cost may well be worth it.
Other Types Of Treatment
Other types of hyperthyroidism treatment includes surgery to remove the thyroid gland and different medications. These alternatives tend to be less effective and/or safe than I131 therapy or Methimazole. These alternatives should be discussed with your regular vet after ruling out the safer and more effective treatments.