Hyperthyroidism is Increasingly Common Among Older Cats

If you have a kitty, you have probably heard of feline hyperthyroidism. Chances are even better if your Fluffy is a senior. That’s because feline hyperthyroidism is the No. 1 endocrine disorder of aging cats.

Hyperthyroidism is caused by an overactive thyroid gland. The opposite condition is an underactive thyroid gland, or hypothyroidism. People can be affected by either condition. Interestingly, for our four-legged family members, hyperthyroidism is virtually exclusive to senior cats, and hypothyroidism to dogs.

In fact, hyperthyroidism is so common in aging cats that standard wellness guidelines recommend screening for this disorder annually once a cat turns 7 years old.

The initial signs of an overactive thyroid gland can be subtle. As the disease progresses, your kitty may begin to show more obvious signs of a problem.

Early detection is key to successfully managing this disease and Fluffy’s overall health. Although some consequences of hyperthyroidism are reversible once treatment is established, others are not.

Excess thyroid hormone affects virtually every organ in the body. It increases metabolism, causing weight loss despite increased appetite. Fat and muscle are burned away. If left untreated, hyperthyroidism is a wasting disease.

Initially, you may not be aware that your cat has lost a few ounces. Or, you may erroneously think that Fluffy’s diet is finally taking effect.

Your veterinarian may examine Fluffy and feel an enlarged thyroid gland in his throat. This is usually due to a benign growth of thyroid cells. These abnormal cells don’t listen to the cat’s body’s signals to turn off hormone production.

Rarely, the enlarged thyroid gland may be due to cancer. This occurs in 1 to 2 percent of hyperthyroid cats. These cats initially have signs similar to other hyperthyroid cats, but the abnormal cells eventually metastasize, causing tumors elsewhere, such as in the lungs.

As metabolism increases, the heart works harder. This muscle pump changes in size and dimension due to the constant stimulation. Eventually, this leads to heart failure.

The kidneys take a toll as hyperthyroid blood is pounded into their delicate filters. Ironically, hyperthyroidism can initially mask the signs of kidney disease, but the damage is occurring nevertheless.

Blood pressure may climb and affect a variety of organs. For example, cats may become blind from retinal detachment.

Uncontrolled hyperthyroidism increases anesthetic risk. Safe anesthesia protocols include screening all senior cats for hyperthyroidism before performing elective anesthesia.

Fortunately, hyperthyroidism can be easily diagnosed with a simple, inexpensive blood test. Occasionally, thyroid levels can be in the “gray zone.” These cases warrant monitoring until their thyroid trend can be determined.

The good news is that most cases of hyperthyroidism can be successfully managed.

Daily, lifelong medicine controls hyperthyroidism in many cats. Most kitties tolerate this quite well. However, 10 to 15 percent become too ill from side effects of the drug to continue. Risks include severe nausea, liver disease and blood cell destruction. Cats need to be monitored closely for life-threatening risks, especially when starting or increasing this drug.

Unlike drug treatment, radioactive iodine therapy is a permanent remedy for hyperthyroidism. Certain government-approved centers provide this treatment by injecting affected cats with radioactive iodine. The sick thyroid cells attract this medicine, which in turn destroys them. Not all cats are good candidates for this procedure, but for those that are, the results can be marvelous.

Another option is to surgically remove the sick thyroid gland. This is also a permanent treatment, but involves placing a potentially unstable patient under anesthesia. Sometimes, the adjacent parathyroid gland is also damaged or removed, causing possibly severe calcium derangements.

In recent months, a novel therapeutic diet has been developed by a major pet food manufacturer to treat hyperthyroidism. The food is safe and effective, but cats must stay on it forever and cannot have treats or other nibbles of food. This can be especially challenging in households with multiple cats.

Research is ongoing to better understand what causes some cats to become hyperthyroid.  We know that the incidence of this disorder is increasing.

This is partly due to the increasing lifespan of cats over the past few decades. Since hyperthyroidism is a disorder of senior cats, we expect to see the incidence rise with a larger senior feline population.

But the incidence of hyperthyroidism is increasing faster than the senior cat population. And indoor cats have a higher risk than cats that go outside.

Various environmental factors, including canned cat food, flea products and cat litter have been studied as possible causes, but none has been found to be associated with hyperthyroidism.

A more recent theory suggests flame retardants in furniture and rugs as a possible culprit. More research is needed to determine whether there is actually a cause and effect relationship.

The bottom line: You can help Fluffy by having her thyroid checked every year after age 7.

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Dr. Heidi Bassler practices at Bassler Veterinary Hospital

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