Hyperthyroidism in Cats

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What is Hyperthyroidism in Cats?

Our furry felines tend to be complicated creatures. The older they get, the more likely they are to develop diseases. One of the more common diseases is Hyperthyroidism. Hyperthyroidism is a condition where the thyroid is overactive. This overactivity results in unwanted clinical signs with possible secondary diseases. 

What causes Hyperthyroidism in Cats?

Cats have two thyroid glands located in the neck. The thyroid plays a vital role in regulating the cat’s metabolic rate. The thyroid secretes a hormone called thyroxine. This hormone controls how the body is run. 

Think of thyroxine like caffeinated coffee. Too much of it will put the body in overdrive, and too little will slow it down (especially if you’re used to having it every day- or multiple times a day – guilty!). 

An enlargement of the thyroid commonly causes hyperthyroidism. This enlargement can be attributed to an adenoma (benign/non-cancerous tumor) or, less likely, a malignant (cancerous) tumor known as adenocarcinomas

Some factors are thought to cause this enlargement. Environmental exposures are considered to be one of these factors. Diets containing high levels of iodine are another cause that is said to be attributed to hyperthyroidism. However, there isn’t a definite pinpoint on what causes a cat to have these thyroid tumors even though they are prevalent. 

Thyroxine molecule, Hyperthyroidism in Cats - I Love Veterinary

What are the clinical signs of hyperthyroidism?

Clinical signs of hyperthyroidism may start smaller and become more noticeable as time goes on. It is important to note that clinical symptoms of hyperthyroidism are often clinical signs of other diseases. 

Therefore, always consult with your veterinarian to find out the underlying cause. In many cases, concurrent diseases are occurring, and multiple treatments will be necessary. This is also important when it comes to medication. 

You want to make sure that you are not treating one disease and progressing the other. Some common clinical signs of hyperthyroidism are:

  • Weight loss without appetite loss (usually appetite is increased)
  • Polydipsia (increased thirst)
  • Unkempt haircoat despite grooming
  • Increased activity
  • Tachycardia (increased heart rate)
  • Vomiting/Diarrhea in some cases
  • Increased vocalization and activity 
  • Anorexia from extended periods of weight loss

Ginger underweight cat, Hyperthyroidism in Cats - I Love Veterinary

How is hyperthyroidism in cats diagnosed?

As with any disease detection, a thorough physical exam is performed by your veterinarian. Sometimes an enlarged thyroid gland can be felt through palpation, but a blood test to measure thyroxine is needed to diagnose.

This blood test is known as a total T4, and it measures the amount of thyroxine in your cat’s blood. A cat with hyperthyroidism will show a high amount of thyroxine. However, a standard T4 can be measured even when the cat is suffering from hyperthyroidism. In this case, a blood test called a free T4 is used to confirm thyroxine presence.

This blood test measures the amount of “free” thyroxine. Free T4 is thyroxine that is not bound to certain proteins making it more specific to hyperthyroidism. This test tends to be more expensive because of its intricate nature. 

When blood tests remain normal in extreme cases, but your veterinarian is still suspicious, thyroid scintigraphy (procedure that produces scans of structures in the body using contrast to monitor, detect, and diagnose disease) can be done. This is usually unnecessary as blood tests are ordinarily diagnostic. Concurrent diseases are likely occurring when it comes to cats with hyperthyroidism. That being said, it is diligent of your veterinarian to perform a full blood panel to rule out other contributing diseases. 

Secondary diseases can occur and also contribute to Hyperthyroidism. Untreated Hyperthyroidism can lead to hypertension (high blood pressure) and thyrotoxic cardiomyopathy (disease of the heart muscle). 

Hypertension occurs from the heart being required to pump extra, which also causes a high heart rate. Heart disease can emerge from this additional activity that is expected from it. This causes enlargement and thickening of the heart’s muscle wall. 

This thickening makes it harder for the heart to pump out blood to the body. In turn, it is hard for the heart to maintain a normal electrical rhythm, resulting in arrhythmia. 

Thyroxine test, Hyperthyroidism in Cats - I Love Veterinary

Treatment of hyperthyroidism in Cats

The most used treatment of hyperthyroidism is an oral medication called Methimazole (trade name Tapazole™). Methimazole works by decreasing thyroid production. Although relatively safe, this medication is not without side effects. Some include vomiting, lethargy, decreased appetite, and in extreme cases, liver abnormalities. 

It is essential to bring your cat in for routine blood work (as it is with any chronic medication) to make sure their metabolic function remains in good standing. It’s also important to make sure the right dose is being given. To make sure of this, routine checks of T4 levels are done. A too high dose can actually cause a cat to become hypothyroid. 

The surgical removal of a tumor on the thyroid can also be performed. Most owners choose oral medication because they feel it’s safer and easier. This only rings true in cats that have other underlying diseases that have gone undiagnosed. 

Cats with untreated hyperthyroidism can have damage to their organs and are not good candidates for anesthesia. For other cats, surgical intervention may be the best fit. There are some risks to consider. 

For example, the parathyroid, which controls the calcium in your cat’s bones and blood, sits next to the thyroid gland. It is possible that it can be damaged during surgery. If this happens, your cat may need supplemental vitamin D and calcium for life. Before deciding on the procedure, you and your veterinarian should go over all the pros and cons.

Another treatment option is the radioactive Iodine (I-131) therapy. Due to its costly nature, it isn’t ideal for most cat owners. However, it is thought to be the safest and most cost-effective in the long run. 

This is because it is a one-time treatment vs. daily medication. It involves an injection of radioactive iodine that goes into the thyroid and destroys the abnormal tissue without destroying the other organs. 

Your cat is then required to stay in the hospital for usually 10-14 days or until there is no trace of radioactive material in their urine or feces. This is because whatever is not taken in by the abnormal thyroid tissue is excreted in your cat’s waste. Different states have different regulations based on how long your cat will be considered radioactive. 

Nutrition is another way to treat hyperthyroidism. Feeding a low Iodine diet can reduce clinical signs and help with thyroxine concentration. This approach requires feeding only this diet and will yield results after a more extended period of time. Your veterinarian prescribes these diets with a prescription. 

Old long-haired gray cat, Hyperthyroidism in Cats - I Love Veterinary

Are certain cats more likely to develop hyperthyroidism?

Hyperthyroidism seems to occur more in older cats non-specific of gender or breed. 


Cats that are treated for this condition have an excellent prognosis. With intervention- between surgery,  I-131, or oral tablets, your cat will be able to have their thyroid under control. Underlying diseases can cause complications. 

It’s imperative to remember that an overall workup consisting of a full physical exam, blood tests, radiographs, and sometimes an ECG and ultrasound of the heart should be done before the start of any treatment. Without these tests, your veterinarian may be only treating one condition, which would do your cat a disservice. You want to always see the whole picture and not just pieces of it. 


Cats diagnosed with hyperthyroidism can live a very long healthy life. With help from your veterinarian, you can find the right plan of treatment for your cat. Make sure to take notice of any abnormalities from your cat pre- and post-treatment. 

Any contraindications must be discussed with your vet so that the problem can be solved quickly. With medical intervention, the management of clinical signs will be well taken care of, leaving you with a balanced kitty. 

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Jaclyn is a Licensed Veterinary Technician (LVT) who has a bachelors degree in journalism. Combining her two interests of writing, and veterinary medicine is a true passion. Jaclyn has already created her own blog called The Four Legged Nurse. She is blessed with two children, a wonderful husband, and four devoted fur babies. In her free time she loves spending time with her family, reading, and riding horses.