Tag: hyperthyroidism

Science

Hyperthyroidism

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Meredith Hemphill

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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.

Graphic of cat thyroid gland
The thyroid gland – Image from Cornell Feline Health Center

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:

Graphic of cat with hyperthyroidism
Characteristic appearance of hyperthyroid cat – Image from Cornell Feline Health Center
  • 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.

Siamese cat in meadow
Siamese have a reduced rate of hyperthyroidism – Image by rihaij from Pixabay

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.

Two veterinarians holding white cat
Photo by Gustavo Fring from Pexels

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

Cat Care

In Search of the Best Cat Food, Part II: Wet or Dry?

mehshadoweye

mehshadoweye

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White cat eating dry cat food
Image by Deedee86 from Pixabay

There is a head-spinning variety of cat foods out there, but the most basic distinctions can be drawn based on moisture content. Dry cat food, the familiar dehydrated kibble, contains 6-10% water (1, 2). Semi-moist food is 15-30% water (2). Canned food has a moisture content of at least 75% (1, 2). Generally speaking, the higher the moisture content, the higher the price and the lower the convenience, so dry food is a very popular choice. But is it the best choice for our cats’ health?

Because kibble is already dehydrated, it doesn’t dry out if left in the bowl for long periods of time, so it lends itself to the convenience of free-feeding. Cats usually like to eat lots of small meals, and let’s face it, people like to be lazy, so this arrangement is often mutually agreeable. We’ll discuss in another part of this series why free-feeding may not be ideal, at least for some cats, but regardless it is often what busy people resort to, and dry food makes that easy. Dry food can also be bought in bulk and will last a long time if stored properly in a cool, dry place. And, again, it’s usually cheap.

Human concerns are not the only reason why someone would feed their cat dry food, however. It has long been thought that kibble helps clean cats’ teeth, preventing and/or treating dental disease. Veterinarians sometimes recommend it for this purpose (3). The idea is that chewing the hard bits of kibble mechanically scrapes the teeth clean of plaque buildup (3), and there has been some scientific evidence supporting the idea that dry food does, in fact, prevent periodontal disease (4). More widely, however, the evidence does not support the idea that dry food cleans cats’ teeth, and there is good reason why it wouldn’t (3).

As mentioned in part one, cats are obligate carnivores. They evolved to eat meat exclusively. As such, their teeth are made for biting and tearing, not chewing (3). Plant matter includes tough cell walls that need to be broken down, so animals that are meant to eat plants do a lot of chewing (3). Carnivores just bite off manageable chunks and swallow them whole (3, 5). That is usually what cats do with kibble. They swallow it whole. Sometimes they bite the kibble, but when they do, the hard but brittle kibble shatters on the biting edges of the teeth and doesn’t do anything to scrape the teeth clean (3, 6).

Yawning gray tabby cat
These chompers aren’t for chewing – Image by Martina Misar-Tummelsthammer from Pixabay

As far as the nutrition of dry food goes, it isn’t great news, either. Dry cat food is typically high in carbohydrates, on average 35-50% by calories (6). Cats neither need carbohydrates, nor can they digest them well, due to their biology as obligate carnivores. They need a diet high in animal protein with a moderate amount of fat (see part one).

Not all dry foods are created equal, of course, but even grain-free dry foods can still be high in carbohydrates if they simply replace grains with potatoes or peas (6). It’s also common for much of the protein in dry food to come from plant matter, but plant proteins do not contain all of the amino acids that cats need (6). The missing nutrients are supposed to be added back in, otherwise the food won’t meet required nutrition standards, but pet nutritionists are still learning exactly what cats need.

With all that being said, is dry cat food just the worst? Well, no, trying to make a cat vegan or feeding them a diet of human food would probably be the worst. When I looked up veterinarian opinions on whether dry or wet food was better, there were mixed opinions. Some were adamant that cats should only be fed canned/wet food (i.e. 3, 6). Others said it didn’t matter (i.e. 5), or it depended upon the cat. I didn’t come across anyone who was a staunch defender of dry food, but there were definitely professionals who had no quarrel with it. I’ll touch on semi-moist food at the end, since it is kind of a weird middle ground, but first let’s take a look at wet food.

Wet food, which as far as I know only comes in cans, can vary in quality but is always notable for its high water content, up to 84% (7). Cats should consume about 20-70 ml of water per kg of body weight per day (8). But cats are descended from Felis lybica, a desert wildcat, so they have a low thirst drive. They tend to obtain most of their water from their food. Prey would normally contain 70-75% water (6). Studies have demonstrated that cats can avoid dehydration without drinking any water at all as long as they are given food with at least 67% moisture content (7, 9, 10).

Of course, a cat should never be without fresh water, but it goes to show just how important dietary water is to cats. Canned food replicates the water content of prey well and helps keep cats hydrated. This prevents urinary and kidney problems, which are common in cats and can be very dangerous (3, 5, 6, 11). Cats fed dry food will drink more water, but they don’t drink enough to compensate, resulting in dehydration and possibly health problems (6).

Canned food is usually higher in protein and made with more animal ingredients than dry food, although cheap, low-quality canned food may still be more plant-based. Reading labels is important, even though pet food labels are not as informative as human food labels. When doing so, keep in mind that the amounts of nutrients are listed as percentages, so the more moisture a food has, the less of other nutrients it may seem to have. Some math may be required to determine the comparative dry weight nutrient content.

Calico cat drinking from garden pool
Cats need to get most of their water from their food – Photo by Dorothea OLDANI on Unsplash

Canned food isn’t perfect. A positive correlation has been demonstrated in cats between hyperthyroidism and eating canned food (5, 12). The cause of the correlation is unknown, although there are a number of theories, one of which suggests that the actual cause is a fire-retardant chemical present mostly in indoor fabrics, not the food (13). However, until we know more, all that can be said for sure is that a correlation has been found.

Semi-moist food comes in plastic pouches or trays, which often aren’t recyclable, so I have to say it isn’t my preference just because of the environmental impact of all that plastic. However, semi-moist food provides cats with quite a bit more water than dry food while not going bad in the bowl as quickly as wet food, so it can be a good compromise between a cat’s need for dietary water a human’s busy schedule. Semi-moist food tends to feature animal ingredients as the main event, providing more protein and a more species-appropriate dish than many dry foods (1). Those pouches and trays are pre-portioned and easy to dump in the bowl, but do be careful that the portion it comes in is right for your cat if you choose this option.

Because cats are the domesticated version of a desert animal, dietary water is extremely important. If for this reason and no other, canned food, ideally a high-quality, meat-forward one, would be the best option. Not everyone can afford the most expensive, gourmet canned food, however. Cheap canned food or semi-moist food is still a good option. A 2010 study showed that cats exhibited better urinary health if water was simply added to their dry food (11). We all have to figure out the best plan for ourselves and our cats. If your cat has specific dietary needs, and you aren’t sure how that would affect what form of food would be best for them, it’s always a good idea to ask your veterinarian.

Works Cited

  1. Cornell Feline Health Center. (2017, July). Feeding your cat. Cornell University College of Veterinary Medicine. Retrieved here.
  2. Feuer, D. (2006). Your Cat’s Nutritional Needs. National Research Council.
  3. Kelsey, A. (2014). 6 reasons why dry food does not clean your cat’s teeth. Cat Behaviourist. http://www.catbehaviourist.com/blog/6-reasons-dry-food-clean-cats-teeth/
  4. Watson, A.D. (1994). Diet and periodontal disease in dogs and cats. Australian Veterinary Journal, 71(10), 313-318. DOI: 10.1111/j.1751-0813.1994.tb00905.x
  5. Barchas, E. (2017, July 31). Ask a vet: Would you recommend wet food or dry food for cats? Catster. https://www.catster.com/lifestyle/wet-dry-cat-health-canned-food-kibble-cats-ask-a-vet
  6. Pierson, L.A. (2016, November). Feeding your cat: Know the basics of feline nutrition. CatInfo.org. https://catinfo.org/
  7. Case, L.P., et al (1995). Canine and feline nutrition. Mosby, Inc.
  8. Ackerman, N. (Ed.). (2016) Aspinall’s complete textbook of veterinary nursing, 3rd edition. Elsevier.
  9. Prentiss, P.G., Wolf, A.V., Eddy, H.A. (1959). Hydropenia in cat and dog. Ability of the cat to meet its water requirements solely from a diet of fish or meat. American Journal of Physiology, 196(3), 625-632. https://doi.org/10.1152/ajplegacy.1959.196.3.625
  10. Caldwell, G.T. (1931). Studies in water metabolism of the cat. The influence of dehydration on blood concentration, thermoregulation, respiratory exchange, and metabolic-water production. Physiological Zoology, 4(2), 324-359. Retrieved from https://www.jstor.org/stable/30151145
  11. Waltham Petcare Science Institute. (n.d.) Cats benefit from increased dietary moisture. https://www.waltham.com/news-events/nutrition/cats-benefit-from-increased-dietary-moisture/5548/
  12. Edinboro, C.H., et al. (2004). Epidemiologic study of relationships between consumption of commercial canned food and risk of hyperthyroidism in cats. Journal of the American Veterinary Medical Association, 224(6), 879-886. DOI: 10.2460/javma.2004.224.879
  13. Hofve, J. (n.d.) Feline hyperthyroidism. Jackson Galaxy. https://www.jacksongalaxy.com/blog/feline-hyperthyroidism/

Published July 12th, 2020

Updated November 21st, 2020