Ask Your Vet to Run This Test Annually After Age 7

feline hyperthyroidism

Story at-a-glance

  • Hyperthyroidism (an overactive thyroid) is a large-scale problem in today’s cats, and the condition can be challenging for veterinarians to accurately diagnose
  • Two mistakes vets frequently make include dismissing possible hyperthyroidism in cats with normal T4 levels, as well as relying too heavily on free T4 levels
  • Dr. Jean Dodds, an expert on thyroid disease in pets, recommends running a complete thyroid antibody profile versus just the T4 test
  • She also recommends running the TSH (thyroid stimulating hormone) test on certain cats with thyroid dysfunction
  • There are a number of treatments available for feline hyperthyroidism, both conventional and alternative; it’s also important to take steps to help your cat avoid the disease

By Dr. Karen Shaw Becker

Sadly, feline hyperthyroidism has reached epidemic proportions in the U.S. It's the most common endocrine disorder of domestic cats, with over 10 percent of kitties over the age of 10 diagnosed with the disease. Hyperthyroidism is usually caused by a benign (or rarely, cancerous) tumor that develops on the thyroid gland, which is a small butterfly-shaped organ at the base of the throat. When the gland overproduces thyroid hormone, the result is the condition known as hyperthyroidism.

Unfortunately, accurately diagnosing hyperthyroidism in cats (and dogs) can present a challenge for veterinarians. Two of the most common mistakes vets make according to Dr. Chen Gilor, writing for the journal Veterinary Medicine:1

Mistake 1: Dismissing the potential for hyperthyroidism in cats with a normal T4

Kitties with hyperthyroidism can swing in and out of the standard reference range for T4. That's why a normal T4 shouldn't be used to rule out the disease in a cat who looks and acts hyperthyroid. Chen recommends either repeating the T4 at a later date, or running a free T4 along with a T4. If the T4 is at the high end of normal and the free T4 is high in a cat who appears hyperthyroid, we can reasonably conclude hyperthyroidism.

Mistake 2: Relying too much on free T4

Chen makes the point that free T4 is only valuable when a cat's T4 is at the high end of normal. This is because other non-thyroid-related diseases can cause high free T4. He recommends that we not rely on free T4 exclusively in diagnosing feline hyperthyroidism.

Recommendations for Thyroid Testing in Cats

According to Dr. Jean Dodds, an expert on thyroid disease in animals, one of the problems in diagnosing pets with thyroid disease is the use of standard laboratory reference ranges. There's just one reference range for cats, no matter their age.

The one-size-fits-all reference range makes little sense since we know from scientific studies that age affects thyroid levels. Young growing animals have higher metabolic demands. Mature animals are no longer growing, so their metabolic demands are less. We also know that blood serum chemistry parameters and complete blood count (CBC) parameters are different in kittens and adult cats.

Veterinarians typically run a number of blood tests during wellness exams for their patients, one of which is a total T4 (thyroxine) test. However, the results of this thyroid function test can be very misleading, because total T4 is affected by non-thyroid-related illnesses, a wide variety of drugs and excessive iodine in the diet.

Some vets treat patients based on just the T4 value, when it may or may not be appropriate. Dr. Dodds feels that to accurately diagnose thyroid conditions, we should be running a complete thyroid antibody profile. And vets should explain to clients up front that while the test is more expensive than a T4, it will also tell us what we can rule in or out.

As part of a complete thyroid antibody profile, Dr. Dodds recommends total and free T4, and total and free T3. Many in the veterinary community believe it's useless to measure total and free T3, but she strongly disagrees. The T3 values are necessary because in the case of a sick animal who has low levels in all four measures, it's much more likely to be a non-thyroid-related illness. Total T3 and free T3 are the markers that indicate a non-thyroidal condition.

The Importance of TSH Tests for Some Cats With Thyroid Dysfunction

Dr. Dodds also includes a TSH (thyroid-stimulating hormone) test for certain cats, in particular older kitties who've been treated for hyperthyroidism, often with radioactive iodine therapy. These cats have a tendency to become hypothyroid within about two to five months because the gland is no longer working.

So now we have kitties — who while hyperthyroid were reactive, pacing and howling — morphing into "blobs of inactivity." Often the family thinks kitty has reached the end of her life, when she's actually hypothyroid and needs to be treated with thyroid hormones.

Dr. Dodds believes the way to monitor hypothyroidism in these cats is with the TSH test because it's very helpful in regulating doses of added T4 and T3 hormones to bring kitties back to a state of metabolic balance. Appropriate doses of these hormones return cats to their pre-hyperthyroid condition, much to the surprise and delight of their owners.

Dr. Dodds and other veterinary endocrinologists she has collaborated with have conclusively decided that it is TSH that should be monitored in cats who have become iatrogenically hypothyroid (meaning the condition was induced unintentionally by a diagnostic procedure or medical treatment).

The TSH test is also important for cats with chronic kidney disease (CKD), because CKD can affect T4 levels, lowering them into the upper limit of the normal range. TSH has proved to be the most predictive test of hyperthyroidism in cats with chronic kidney disease. Dr. Dodds also points out that suppressing the thyroid in cats with kidney failure can worsen renal (kidney) tissue profusion, so we must be very careful treating these kitties. Some actually do better being slightly hyperthyroid when they have renal disease.

My Approach to Treating Hyperthyroidism in Cats

If it's early enough in the development of your cat's hyperthyroidism, my recommendation is to start with natural therapies that avoid many of the risks and side effects associated with more traditional approaches. It's important to catch the disease early. In fact, astute proactive veterinarians begin checking a cat's thyroid level at age 7 and then compare test results annually for subtle changes. If there's a slow but consistent increase in thyroid levels, we begin natural thyroid balancing protocols immediately.

There are many natural remedies that can be beneficial in managing your cat's condition if it's caught early. These include homeopathic remedies, acupuncture, herbal remedies (Eastern, Western and Ayurvedic) and nutraceuticals. I also recommend checking your cat's thyroid levels annually after the age of 7.

My second choice when a natural approach is ineffective is thyroidectomy, which is the surgical removal of the benign tumor from your cat's thyroid gland. This operation should only be performed by a skilled soft tissue surgeon who has done many of these procedures.

Surgery cures the problem, which is why I prefer it, but there can be complications, including accidental removal of the parathyroid glands that sit on the thyroid, which can cause a whole host of other problems. However, successfully done, thyroidectomy cures the patient, which is the ultimate positive outcome. Radioactive iodine therapy is another option, but the expense and side effects often prevent people from choosing this treatment.

Another option is medical management of the disease, which means putting the cat on a medication called methimazole, which inhibits thyroid hormone production so the amount of the circulating hormone is reduced. There can be lots of side effects of methimazole, including GI upsets that cause vomiting. I've had some success avoiding GI problems by using a compounded transdermal methimazole ointment applied inside the cat's ear.

Occasionally, a patient will develop an almost immediate allergic response to the drug in the form of an intense facial itch that comes on after the first pill is administered. This reaction means the drug cannot be continued and another treatment option is needed. Other side effects, which are fortunately rare, include decreased platelets and increased liver enzymes. The downside to medical management is that cats must take methimazole for the rest of their lives, and blood levels must be routinely monitored.

And since hyperthyroidism is known to mask kidney problems in cats, it's important for veterinarians to monitor not only thyroid hormone levels but also kidney function in kitties taking methimazole. I always start holistic kidney support via herbal and nutritional supplementation when the diagnosis of feline hyperthyroidism is made. This is also an excellent time to work with a homeopathic vet.

Tips for Helping Your Cat Avoid Hyperthyroidism

Feed a nutritionally balanced, fresh, species-appropriate diet that is respectful of a cat's natural iodine intake, and never feed your cat dog food

Do not buy canned food that is not labeled BPA-free

Avoid feeding your cat a fish-based diet, or any food containing soy products

Rid your environment of flame-retardant chemicals

Provide your cat with an organic pet bed

Purchase a high-quality air purifier for your cat's environment


+ Sources and References