Hyperthyroidism

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Thyroid disease is fairly common in cats, especially as they get on in years. The most frequent type of thyroid disease in older cats is hyperthyroidism [1, 4]. My parents’ elderly cat, Rosie, has it. So did my roommate’s cat Coco. And so do my best friend’s two ancient gray ladies. Even though hyperthyroidism is a common illness, I will attest that it can still be mystifying at times. It can impact a cat’s entire body, and it isn’t completely understood even by scientists and veterinarians. It is diagnosable and treatable, however. Sometimes, hyperthyroidism can even be cured.

What is Hyperthyroidism?

The thyroid gland is a small organ found in all vertebrates which produces the thyroxine (T4) and triiodothyronine (T3) hormones. In mammals, the thyroid is found in the throat and has two lobes. Thyroid hormones regulate a wide variety of bodily functions. When the thyroid produces too much T3 and T4, it’s called hyperthyroidism. Hyperthyroidism increases the body’s metabolism which can cause problems with many different organs and systems.

Cats typically develop hyperthyroidism when middle-aged or older [1, 2, 7, 8, 10]. Up to 10% of cats age 10 and older may have hyperthyroidism [1, 3, 8]. It’s uncommon in cats younger than that, however [1, 2, 7, 8]. Untreated hyperthyroidism is fatal, but today the prognosis for a cat with this disease is usually optimistic.

How is Hyperthyroidism Diagnosed?

If your veterinarian suspects hyperthyroidism, they will probably do a physical examination of your cat’s neck to see if they can feel that the thyroid is enlarged [1, 2, 4, 7]. The presence or absence of a suspicious lump isn’t enough for a diagnosis, however. Most of the time, the final diagnosis can be made with a total thyroxine, or TT4, blood serum test [10]. The vast majority of hyperthyroid cats will have very high T4 levels with a TT4 test, making the diagnosis clear right away [1, 2, 7, 10].

However, cats with hypothyroidism occasionally have T4 serum levels that fall within normal limits [1, 2, 7, 10]. T4 levels fluctuate, so your veterinarian may decide to try the test again in a few weeks [1, 7]. Or they might try other thyroid hormone blood tests. A thyroid scan is another option but must be performed at a specialty veterinary facility [7, 10].

What are the Symptoms?

Because thyroid hormones affect so many parts of the body, the symptoms of hyperthyroidism are varied and sometimes nonspecific. They include:

• Weight loss/muscle wasting
• Increased appetite
• Increased thirst and urination
• Vomiting
• Diarrhea
• Hyperactivity
• Unkempt fur
• Aggression or irritability
• Fast heartrate
• Increased vocalization, especially at night
• Weakness
• Heat intolerance

Some cats demonstrate “apathetic hyperthyroidism,” where they are lethargic, anorexic, and depressed/dull instead of the more usual symptoms [1]. Hyperthyroidism is progressive, so symptoms may be very mild at first [2]. It’s also not uncommon for hyperthyroid cats to also have other diseases, especially since they tend to be older. Many of these symptoms can be symptoms of something else. The safest bet is to consult your veterinarian anytime your cat seems different or unwell.

Secondary Illnesses

An overactive thyroid can sometimes cause other organs to become diseased. The heart is particularly at risk. There is a form of heart disease known as thyrotoxic cardiomyopathy which is caused by the heart muscle working overtime to meet the demands of a metabolism on overdrive [10]. The heart muscle thickens as a result of pumping so hard, but then that makes it difficult for the heart to work properly [2, 4, 10]. This can lead to heart failure. Fortunately, successful treatment of the hyperthyroidism can improve and even reverse the cardiomyopathy [2, 7, 10].

Hyperthyroid cats frequently experience high blood pressure as a cardiac complication, too [2, 4, 10]. About 25% of the time, in fact [10]. In severe cases, the high blood pressure can damage their organs [2, 4, 10]. It can even lead to retinal detachment that renders them permanently blind [4, 10]. High blood pressure is no joke and needs to be managed alongside hyperthyroidism until the thyroid hormones are under control [2]. If the hyperthyroidism is resolved, the blood pressure usually takes care of itself [2, 10].

What Causes Hyperthyroidism?

Hyperthyroidism usually begins when the thyroid gland becomes enlarged [2, 10]. In cats, this is almost always the result of a benign tumor called an adenoma [2, 4]. In less than 2% of cases, the growth is cancerous [1, 10]. What causes the abnormal thyroid tissue growth to begin with, however, remains uncertain. Researchers have found evidence for both environmental and biological factors that may play a role.

Environment

There is a demonstrated association between diet and the chances of a cat developing hyperthyroidism. Too much iodine in the diet may make a cat susceptible [10]. Canned cat food is also associated with higher rates of hyperthyroidism [1, 5]. The results of a 1999 study suggest cats fed primarily canned food have a rate of hyperthyroidism 2 to 3 times higher than other cats [5]. Researchers still don’t know why canned food would lead to an overactive thyroid. Correlation is not causation, so there may be more going on here that we don’t understand yet.

Using cat litter has also been found to be a potential risk factor, with the same study showing triple the rate of hyperthyroidism in cats that use litter [5]. Again, although there is a potential link, there is as yet no explanation. From looking through a lot of research papers to write this article, I know that veterinary scientists are still hard at work trying to figure out feline hyperthyroidism. The disease just doesn’t want to give up its secrets easily.

Certain household chemicals are associated with higher rates of feline hyperthyroidism, too. Polybrominated diphenyl esters (PBDE), which are used as flame retardants, have been found in high levels in the blood serum of hyperthyroid cats [1]. According to the Environmental Protection Agency factsheet for PBDE, rodent studies have shown thyroid toxicity from PBDE exposure. Per- and polyfluoroalkyl substances (PFAS) are a versatile group of artificial chemicals with a wide variety of applications in industry and the home. Wang et al. found that PFAS levels, particularly perfluorooctanoic acid, were higher in hyperthyroid cats than the cats in the control group [9]. The Environmental Protection Agency lists tumor growth and thyroid hormone disruption among the effects of PFAS in animals.

Biology

Hyperthyroid cats may have defects in the way their cells communicate that lead to the adenomas and other tissue abnormalities that overproduce thyroid hormones. Cats with thyroid nodules sometimes have low levels of the G protein in their thyroids [1]. A lack of G protein can decrease the inhibition of cell growth [1]. Some cats with thyroid nodules or adenomas have a cancer-causing mutation, known as an oncogene, which could cause the excessive cell growth [1].

Certain breeds of cat are less likely to develop the disease: Siamese, Burmese, Tonkinese, Himalayan, Persian, Abyssinian, and British Shorthair [3, 5, 9, 10]. At one time, only the Siamese, Burmese, and Persian/Himalayan were known to have these decreased risk, so the benefit was thought to be associated with the color-point pattern or lighter coat colors [3]. This hypothesis ultimately didn’t hold up [3], but perhaps there is something special these breeds have in common which will one day come to light. The same study which discovered the three new resistant breeds also found that long-haired, nonpedigree cats have higher rates of hyperthyroidism [3]. This despite the extremely long-haired Persian and Himalayan’s decreased rate.

What Treatments are Available?

Radioactive Iodine

It might sound a little scary, but radioactive iodine is actually the gold standard for treating feline hyperthyroidism. This treatment can only be done in specialized veterinary clinics that are authorized to work with radioactive materials, so it may not be accessible or affordable for everyone [2, 7, 10]. It’s a great option if you can take it, though, because for about 95% of cats it cures their hyperthyroidism–even those few with cancerous thyroid tumors [1, 2].

Radioactive iodine therapy relies on the fact that the thyroid takes up iodine to synthesize thyroid hormones. The cat is injected with the radioactive version of iodine, I-131 [1, 2, 7, 10]. No anesthesia is required [2, 7, 10]. The cat’s thyroid absorbs the I-131, just like it would normal iodine, but the abnormal thyroid tissue that’s churning out T3 and T4 like crazy is hogging the iodine. So the radioactive molecules get sucked into the abnormal tissue and kill it, leaving alone the cat’s healthy tissues, including any healthy thyroid tissue [1, 2, 7, 10].

Although the procedure is quite safe for the cat, it can be dangerous for the people around the cat which is now shedding radioactive particles [2]. Cats have to be hospitalized for a week or two after radioactive iodine therapy so that their radiation levels can get low enough for release [1, 2, 7, 10]. Even then, special care has to be taken at home for a two weeks after [1].

Radioactive iodine therapy may be a good option for cats that cannot undergo anesthesia. It also works well for cats with ectopic thyroid [7]. Ectopic thyroid is thyroid tissue somewhere other than the usual location of the thyroid gland such as under the tongue or in the chest [10]. It can be in locations that would be difficult to remove surgically. Radioactive iodine therapy doesn’t usually have complications, but for the rare cat it works too well and they become hypothyroid instead [2]. The solution to this is medication with thyroid hormones.

Medication

Hyperthyroidism cannot be cured with medication, but it can be managed with it. The medicine is called methimazole [1, 4, 7, 10]. Its brand names are Felimazole and Tapazole [4, 7, 10]. Methimazole is available as an oral tablet or a gel applied to the skin [1, 2]. It works by decreasing the production and release of hormones from the thyroid [2, 4]. It’s usually given twice a day [1, 4].

Cats may be given methimazole as a long-term method of managing their disease or to stabilize them before trying a more permanent solution [1, 4, 7]. Like all drugs, methimazole can have side effects. According to the Food and Drug Administration, the most common side effects of methimazole in cats are changes in appetite, lethargy, vomiting, diarrhea, skin lesions, itchiness, unusual vocalizations, weight loss, fur abnormalities, weakness, and agitation [4]. However, a small percentage of cats develop severe side effects with methimazole [1, 4]. Namely, liver disease, anemia, low blood platelets, and low white blood cell counts [1, 4, 7, 10]. Always report medication side effects, even seemingly small ones, to your veterinarian.

Surgery

When Dr. Jean Holzworth first recognized feline hyperthyroidism in 1978, the only way to treat it was to remove the offending thyroid gland [1]. Although there are now other options, for some cats this is still the chosen course of treatment. For most cats, surgery cures their hyperthyroidism [2]. It can also be done at a regular veterinary surgery rather than the special facility required for radioactive iodine therapy, making it accessible to more people. Surgery requires a cat to go under anesthesia, however, and for older cats who often have other health problems, that can be a risky proposition [2, 10].

Surgery also carries a chance of complications. Complications aren’t likely, but they do happen. Most cats still have enough thyroid cells left after the surgery to keep up their T3 and T4 hormone levels, but some become hypothyroid and require medication [7]. Another possible complication is damage to the parathyroid glands, which are near or within the thyroid are necessary for maintaining normal calcium levels in the blood [2, 7]. If the parathyroid glands become damaged, the cat may develop low blood calcium and require supplemental calcium or vitamin D [1, 7]. Other complications include damage to nearby nerves or the larynx [1, 7].

Due to the risks associated with surgery and the availability of alternatives, it isn’t a very common choice anymore [2]. That being said, it might still be the right option for you and your cat. That’s something you would have to figure out with your veterinarian.

Diet

The research is still ongoing for diet as a means of controlling hyperthyroidism in cats [2]. For cats with health concerns that make other treatment options difficult or impossible, however, it can be the best–or only–choice despite being somewhat controversial. The principle is that limiting the amount of iodine in the cat’s diet to a small, controlled amount restricts the amount of hormones the thyroid can make. Cats fed a prescription iodine-restricted diet and nothing else should return to normal thyroid function [1, 7, 10]. Nothing else really means NOTHING else. No treats, don’t even put the prescription food in a bowl that has been used for other food or untreated water [1, 10]. The iodine-restricted diet contains only 0.2 parts per million of iodine [10]. That is a really, really small amount. It would not take much to throw off the balance.

In memory of Coco and Rosenberg

Works Cited

1. Brewer, W. G., Jr. (2020, July 6). Treating feline hyperthyroidism. Blue Pearl. https://bluepearlvet.com/medical-library-for-dvms/treating-feline-hyperthyroidism/
2. Cornell Feline Health Center. (2017, January). Hyperthyroidism in cats. Cornell University Collège of Veterinary Medicine. https://www.vet.cornell.edu/departments-centers-and-institutes/cornell-feline-health-center/health-information/feline-health-topics/hyperthyroidism-cats
3. Crossley, V. J., Debnath, A., Chang, Y. M., Fowkes, R. C., et al. (2017). Breed, coat color, and hair length as risk factors for hyperthyroidism in cats. Journal of Veterinary Internal Medicine, 31(4), 1028-1034. https://doi.org/10.1111/jvim.14737
4. Food and Drug Administration. (2019, December 3). Hyperthyroidism in cats: There’s an FDA-approved drug to treat it. FDA.gov. https://www.fda.gov/animal-veterinary/animal-health-literacy/hyperthyroidism-cats-theres-fda-approved-drug-treat-it
5. Kass, P. H., Peterson, M. E., Levy, J., James, K., et al. (1999). Evaluation of environmental, nutritional, and host factors in cats with hyperthyroidism. Journal of Veterinary Internal Medicine, 13(4), 323-329. https://doi.org/10.1111/j.1939-1676.1999.tb02189.x
6. Peterson, M. E., Castellano, C. A., and Rishniw, M. (2016). Evaluation of body weight, body condition, and muscle condition in cats with hyperthyroidism. Journal of Veterinary Internal Medicine, 30(6), 1780-1789. https://doi.org/10.1111/jvim.14591
7. Pet Health Topics. (n.d.). Hyperthyroidism in cats. Washington State University College of Veterinary Medicine. https://www.vetmed.wsu.edu/outreach/Pet-Health-Topics/categories/diseases/hyperthyroidism-in-cats
8. Stephens, M. J., O’Neill, D. G., Church, D. B., McGreevy, P. D., et al. (2014). Feline hyperthyroidism reported in primary-care veterinary practices in England: Prevalence, associated factors and spatial distribution. Veterinary Record, 175(18), 458. https://doi.org/10.1136/vr.102431
9. Wang, M., Guo, W., Gardner, S., Petreas, M., et al. (2018). Per- and polyfluoroalkyl substances in Northern California cats: Temporal comparison and a possible link to cat hyperthyroidism. Environmental Toxicology and Chemistry, 37(10), 2523-2529. https://doi.org/10.1002/etc.4239
10. Williams, K., Downing, R., and Ward, E. (n.d.). Hyperthyroidism in cats. VCA Hospitals. https://vcahospitals.com/know-your-pet/hyperthyroidism-in-cats

Published April 25th, 2021

Updated June 14th, 2023