Feline Hyperthyroidism: Conventional Treatment Approach

Hyperthyroidism is one of the most common hormone disorders in middle-aged and senior cats. It occurs when the thyroid glands overproduce thyroid hormones, leading to a revved-up metabolism and various health issues.

Here are common symptoms of hyperthyroidism in cats:

• Increased appetite (often ravenous)
• Weight loss despite eating more
• Hyperactivity or restlessness
• Increased stool volume or diarrhea
• Increased thirst and urination
• Elevated heart rate (tachycardia)
• Vocalizing more than usual
• Irritability or behavior changes
• Poor coat quality (greasy, unkempt fur)
• Occasional lethargy or weakness
• Panting or breathing changes (less common)
• Possible confusion or disorientation in advanced cases (rare)

Some cats have many of these symptoms, some have none (in early stages) and some have only a few of the symptoms.

Since the thyroid hormone is an “upper” and it speeds up the metabolism, it would be the equivalent of you drinking too much coffee all the time. The metabolism speeds up, the heart has to beat faster, the body uses up calories quickly and all the other associated symptoms of agitation and anxiety show up.

That being said, I have also seen another (more rare) way that hyperthyroidism can show up in cats. In those cases, the thyroid values are elevated but the cat is showing the opposite symptoms – low appetite and weight gain. I will talk about how I treat those cats in the holistic treatment post.

Conventional Treatment Options for Feline Hyperthyroidism

1. Anti-Thyroid Medication (Oral and Transdermal)

The most common approach to treat cats with hyperthyroidism is with anti-thyroid medication (methimazole). This medication does not cure the disease, but rather it reduces the thyroid gland’s production of hormones to keep the symptoms under control.

The advantages of medication are that it’s readily available and relatively affordable for most pet parents. Many hyperthyroid cats respond well to methimazole given by mouth once or twice daily. For cats who refuse pills, methimazole can be compounded into a transdermal gel that you apply to the skin (usually on the inner ear).

While medication is easy, there are some downsides. Lifelong treatment is required (often twice-daily dosing), which can be challenging for some cats and their humans. Regular veterinary check-ups and blood tests are needed to monitor thyroid hormone levels. As cats age, their requirements for this medication may change, so labwork every 6-8 months is recommended.

Approximately 10–20% of cats may experience side effects from methimazole, including vomiting, loss of appetite, lethargy, or changes in blood cell counts. In my experience, side effects tend to be more common in cats who are started on the medication too early in the disease process, when thyroid levels are only mildly elevated and the cat isn’t showing symptoms yet. I’ve also seen negative reactions in cats who have more atypical presentations or those who were started on high doses. If your cat has had a bad reaction to medication, or if you’d prefer not to use it, I’ll be covering alternative approaches in the next blog post.

That being said, anti-thyroid medication can be a good short-term or long-term management tool. Many of my kitty patients have done well on this medicine and tolerated it long-term with no side effects.

2. Prescription Iodine-Restricted Diet

One conventional option for managing hyperthyroidism in cats is an iodine-restricted prescription diet, such as Hill’s y/d. The idea behind it is simple: the thyroid gland needs iodine to produce thyroid hormones, so by severely limiting dietary iodine, the diet aims to “starve” the thyroid of raw materials and slow hormone production. Hill’s reports that when fed exclusively, y/d can normalize thyroid levels within three weeks.

But while that may sound promising, using this diet comes with a few problems. For one, it needs to be fed exclusively. The cat can’t consume any other food or treats—not even the occasional nibble of another pet’s food—since even small amounts of regular food can supply enough iodine to override the effect of the diet.

The diet is also not particularly palatable, which may be explained by what’s in it.

Let’s take a look (photo from Hills website):

Hill’s markets y/d under the heading “Quality Nutrition,” yet here’s what the ingredient list includes: corn, chicken fat, powdered cellulose (wood pulp), egg product, and liver flavor. As obligate carnivores, cats have very specific protein and nutrient needs—and basing their diet on corn, industrial byproducts, and “flavoring” is far from ideal, even with added vitamins and minerals.

The key feature of this diet—the thing that makes it “prescription” and expensive—is its low iodine content. Hill’s conducted studies showing that iodine restriction can help suppress hyperthyroid symptoms, which sparked an FDA-compliant formulation. But rather than digging deeper into important follow-up questions like:

• Are modern cat foods excessively high in iodine?
• Should the iodine requirements for cats be re-evaluated?
• Are certain ingredients (like kelp) sneaking in extra iodine?

They opted to produce an ultra-low iodine diet, without looking into why so many cats are becoming hyperthyroid in the first place.

It’s also important to note that pet food companies are not required to test or disclose iodine content, so we don’t really know how consistent or excessive iodine levels are in many commercial diets. We don’t fully understand the relationship between iodine and what we see happening in cats, as described in this excellent article from the Animal Endocrine Clinic: https://animalendocrine.com/yd, imbalances from limiting a key nutrient like iodine can potentially backfire and even make the situation worse in the long-run.

Hyperthyroidism first showed up in cats in the 1979, around the time that many cats were first switched to commercial diets. One of the main risk factors for hyperthyroidism is eating a commercial diet.

In short, while y/d may help suppress symptoms in some cases, it raises bigger questions about nutrition, ingredient quality, and long-term safety. We’ll explore more holistic and integrative approaches in the next section of this blog post.

All of that being said, if you have severe financial limitations, you can’t medicate your pet, or for some other reason need to feed this food, then it is an option. However, in the next post I will talk about some holistic options that may be better long-term than this diet, even if you use it temporarily to help with symptom relief.

3. Radioactive Iodine Therapy (Radioiodine I-131)

Radioactive iodine (I-131) therapy is often considered the gold-standard treatment for feline hyperthyroidism – it’s the closest thing to a cure. This treatment involves a one-time injection of a radioactive form of iodine. The thyroid gland readily takes up iodine, and so the radioactive iodine concentrates in the abnormal thyroid tissue and selectively destroys the overactive cells while sparing the rest of the body .

The vast majority of cats – roughly 95% of cases – are cured with a single I-131 treatment. In other words, after the therapy, their thyroid hormone levels return to normal and stay controlled without the need for ongoing medication in most instances.

The benefits of radioiodine are that it eliminates the thyroid tumor non-surgically, requires no anesthesia, and has a low rate of side effects. It is often recommended for cats who are difficult to medicate or handle for blood draws.

After radioiodine treatment, the main aftercare is monitoring; it’s rare, but a small percentage of cats might become hypothyroid (under-active thyroid) and need a thyroid supplement, so follow-up bloodwork is important.

Radioiodine therapy does require a specialized facility, since the use of radioactive materials is heavily regulated. Cats undergoing I-131 must stay in the hospital for a short period until the radiation levels in their body decrease to a safe level. Typically, your cat would spend about 3-5 days in an isolation ward after the injection. During that time (and for a couple weeks after going home) there will be some safety precautions to minimize others’ exposure to radiation, for example, handling your cat’s litter with gloves.

The main downside to this treatment is the up-front cost which is higher than medication or diet. However for a younger cat (less than 14 years of age), in many cases, it may be more cost-effective in the long run.

Note: There is a fourth treatment option, surgery (thyroidectomy), which involves removing the thyroid gland. Surgery can cure hyperthyroidism as well, but it’s now less commonly chosen because I-131 is widely available and less invasive. Surgery carries risks of anesthesia and potential damage to nearby parathyroid glands, and many hyperthyroid cats are elderly with concurrent issues that make surgery riskier.

Why Is Feline Hyperthyroidism on the Rise? (Current Theories)

You might be wondering “What causes hyperthyroidism in the first place, and why does it seem so much more common now than decades ago?” The truth is that the exact cause remains unknown – likely a combination of factors – but veterinary researchers have proposed several intriguing theories. Hyperthyroidism was first recognized in cats in the late 1970s, and since then the prevalence has indeed surged. Here are some of the suspected factors that could be contributing to this increase:

• Indoor-Only Lifestyle: Many modern pet cats live exclusively indoors. Indoor cats tend to live longer, which already increases the chance of age-related diseases like hyperthyroidism. But beyond longevity, some studies have found that cats kept primarily indoors have a higher prevalence of hyperthyroidism. One reason could be chronic exposure to indoor environmental chemicals (more on that below). Another consideration is that indoor cats might have less natural sunlight exposure and lower vitamin D levels. Cats obtain vitamin D mostly through diet (they don’t synthesize it efficiently in skin), but there’s a hypothesis that chronically low vitamin D or other missing environmental factors could predispose cats to endocrine disorders. While not proven, vitamin D deficiency has been linked to thyroid problems in other species (even humans), so more research is needed.
• Dietary Factors: Diet is a major area of study. Earlier research noted that cats eating a lot of canned food, especially fish or liver flavored varieties, had a higher risk of developing hyperthyroidism. It’s unclear if the issue was the protein source itself or something in the can lining or manufacturing process. Some suspect excess iodine, or iodine fluctuations in commercial cat foods, might play a role – interestingly, both very high and very low iodine intake have been considered potential triggers for thyroid dysfunction (as we briefly discussed above). Other dietary compounds under scrutiny include soy isoflavones (plant estrogens found in some cat foods that could act as goitrogens) and certain food additives. In short, the modern pet diet, which is very different from a cat’s natural prey diet, could be inadvertently affecting thyroid health. I’ll tell you which diet I prefer in the next blog post.
• Environmental Chemicals (Endocrine Disruptors): A number of environmental toxins have been implicated as possible causes. Cats are uniquely exposed to household chemicals – they’re low to the ground and constantly groom themselves, ingesting dust and residues from the home. Researchers have found links between hyperthyroidism and exposure to flame retardant chemicals used in furniture. For example, cats with hyperthyroidism have been shown to have significantly higher levels of polybrominated diphenyl ethers (PBDEs) – a class of flame retardants – in their blood compared to healthy cats. These chemicals were common in household items from the 1970s-2000s and may still be present in older furniture and house dust. Other potential culprits include phthalates (from plastics), pesticides, and bisphenol-A (BPA) (from can linings), all of which are known or suspected endocrine disruptors. Cats that had regular exposure to flea collars or insecticidal sprays in the past were also found to have a higher risk of hyperthyroidism. It’s a stark reminder that the chemicals in our pets’ environments aren’t always benign – what we use to clean our homes, protect against pests, or even the materials in cat toys and furniture could have long-term health impacts.

In summary, the rise in feline hyperthyroidism is likely multifactorial – a combination of living indoors, dietary changes, environmental toxin exposure, and cats living to older ages. Research is ongoing, and hopefully in the future we’ll better understand how to prevent this condition. In the meantime, being mindful of your cat’s environment (reducing unnecessary chemicals, feeding a healthy balanced diet, etc.) is a good proactive measure.

Southern California Considerations for Hyperthyroid Cats

Southern California’s urban lifestyle and high coyote populations means a lot of cats are kept strictly indoors for safety, which might amplify some of the indoor risk factors we discussed (less natural sunlight, more exposure to indoor pollutants, etc.).

If your cat is a candidate for radioactive iodine therapy and this is something you would like to pursue, VCA West Los Angeles Animal Hospital (Brentwood) provides radioiodine therapy. Another option is Radiocat.

Finally, be aware of local environmental advisories – Southern California has areas of industrial pollution and unique contaminants. It’s a good practice to filter your pet’s drinking water if you’re in an area with questionable water quality, and avoid using lawn or garden chemicals that your pets could contact. While these precautions are generally applicable anywhere, our local environment makes them especially relevant.

Stay tuned for the second half of this blog post, where we’ll explore integrative and holistic therapies for managing feline hyperthyroidism. In a holistic veterinary practice like ours, we often combine the best of conventional medicine (described above) with nutritional support, herbal remedies, acupuncture, and other modalities to help hyperthyroid cats feel their best. We’ll cover those approaches next!