Feline Hyperthyroidism: What You Need to Know

It can be pretty crazy having to deal with a pet cat that has been diagnosed with feline hyperthyroidism. Many pet owners, unfortunately, don’t have any idea about what the disease is or what the symptoms are.

Feline hyperthyroidism is a common disease that afflicts mostly middle-aged and older cats. It’s a debilitating affliction that can reduce even the healthiest of cats to a messy tangle of wispy, frayed hair and a look of overall illness.

To help you better care for a cat with this condition, here’s some information about the disease, how to identify it, and how to combat it.

Health issue due to hyperthyroidism

What is Feline Hyperthyroidism?

Your cat has two thyroid glands in its neck; on either side of the windpipe. These two glands are responsible for producing and releasing a hormone called thyroxine.

Thyroxine is the hormone which regulates your cat’s metabolism. Feline hyperthyroidism is the disease that occurs when the thyroid glands begin to overproduce thyroxine and become enlarged.

An overabundance of the hormone subsequently results in a great increase in your cat’s metabolism which stresses major body parts and functions including the heart, liver, kidneys, nervous system, and gastrointestinal tract. Secondary problems can also occur.

Hyperthyroidism can occur in both male and female cats, and according to the ASPCA, the average age of cats that contract this disease is between 12 and 13 years. Studies have shown that less than 6 percent of cats under 10 years old contract the disease.

Enlargement of the thyroid glands is usually caused by the presence of a non-cancerous tumor called an adenoma. In rare cases (fewer than 2% of cases), a malignant tumor may be involved in the perversion of the thyroid glands.

Age and a couple of environmental factors are currently believed to be the circumstances that allow cats to contract hyperthyroidism.

Possible Contributing Factors and Symptoms

The exact cause of feline hyperthyroidism is not currently known. Several possible contributing factors which scientists are investigating include too little or too much of a certain compound in a cat’s diet, and constant and repeated exposure to thyroid-disrupting chemicals in food or the environment.

Although the root of the problem isn’t yet understood, endocrinologists and veterinarians have discovered that cats that consumed large amounts of commercial fish-flavored cat food were more likely to contract the disease.

Whether or not this has to do with the amount of iodine present in the food or the low-quality of the ingredients remains to be seen.

Any cats that have been unfortunate enough to contract the disease will usually show subtle signs at first, which slowly become more severe as the disease goes from one stage to the next. The most common symptoms which are associated with feline hyperthyroidism are weight loss, increased appetite, frequent urination, and increased thirst.

The physical appearance of an afflicted cat will also greatly change. A once neat and orderly coat of fur will become unkempt, greasy, and/or matted. Hair may slowly begin to shed.

In about 50 percent of cases, a diseased cat may develop mild to moderate diarrhea and vomiting. It may avoid hot areas and actively seek out cool locations to hang out or sleep.

An especially unusual behavior for cats which may be exhibited is panting when stressed.

Although a cat with hyperthyroidism might be eating well; in some cases, it can display lethargy, general weakness, irritability, and unwarranted aggression.

An increased heart-rate along with increased activity is also among the classic signs of hyperthyroidism.


In order to first detect if your cat indeed does have hyperthyroidism, a vet will first do a physical exam and check if its thyroid is enlarged. Blood pressure and heart rate tests may also be taken to search for any red flags.

Since cats with hyperthyroidism are also weak and prone to contracting other conditions, it is very important for a vet to assess their general health.

Based on the results of the tests, a vet might then order some blood work to be done or other tests to analyze thyroid hormone levels. The reason that this is necessary is that there is a very small percentage of cats suffering from hyperthyroidism which were found to have normal levels of thyroxine in their bloodstream (which should not be the case).

Once all laboratory evaluations are complete, the vet will be able to positively identify the disease and provide you with recommendations for treatment.


There are currently four different treatment options available for feline hyperthyroidism. We’ll provide additional details about them as they each have their own plus and minus points.

The majority of the time, a vet will present you with your options if your cat has been diagnosed with feline hyperthyroidism. Knowing what these options are in advance will help you to be prepared:

  • Medication

A cat with feline hyperthyroidism can take drugs to reduce how much thyroxine is being created by the thyroid gland. Taking medicines won’t completely remove the disease, but it will at least provide you with control over the condition.

One of the advantages of taking medication is that the drugs are plenteous and also very cheap. Be aware, though, that some cats may experience unpleasant side effects when on meds such as vomiting, fever or unusual lethargy.

Other more serious side effects include liver problems, blood clotting disorders, and self-induced trauma. These side effects are mild and, in most cases, will eventually stop happening.

On the other hand, if you’ve planned to put your cat on medication for the rest of its life, you’ll need to commit to keeping a certain schedule. It might seem easy at first but it can quickly become difficult to maintain the twice-daily dosage as time goes by.

Also, your cat will need to constantly have its blood and thyroxine levels analyzed which means a steady inflow of never-ending vet bills.

Methimazole has long been the drug of choice to counteract hyperthyroidism and will usually correct the condition within two or three weeks of twice-daily doses.

  • Radioactive Iodine Therapy

This type of therapy is perhaps the safest and most effective type of treatment available for feline hyperthyroidism. It works by injecting a type of radioactive iodine into your cat’s bloodstream which enters the thyroid gland and destroys the abnormal tissue (cancerous or non-cancerous tumors) without damaging the surrounding tissues or the glands themselves.

Unlike taking medication, treatment via radioactive iodine does not produce any side effects, will cure hyperthyroidism permanently, and doesn’t require anesthesia. It usually takes about one to two weeks of treatment to bring a cat’s thyroxine levels back down to normal.

There’s a catch though: since the treatment involves the usage of a radioactive substance, it can only be carried out at special facilities which are licensed to handle radioisotopes. While your cat itself will be safe from any harm, any people who come in contact with the cat during the treatment period will need to take precautionary protective measures.

If your cat undergoes radioactive iodine therapy, it’ll need to remain quarantined at a hospital for three to five days after treatment until radiation levels have fallen back down. Visitors are strictly not allowed.

The good news is that in 95 percent of all cases, feline hyperthyroidism can be completely cured after just three months of therapy. In the event that the therapy fails, it can be repeated after a certain amount of time has passed.

  • Surgical Thyroidectomy

Surgical removal of the thyroid glands is another possible treatment method. It has a good success rate and is a straightforward procedure. There are both pros and cons to having your cat undergo surgery.

The main advantage of surgery is that removing the thyroid glands will usually completely cure the disease and eliminate the need to take constant daily medication.

On the other hand, surgery requires that your cat is put under anesthesia during the operation which in itself can be risky for some cats, especially older ones. Surgical thyroidectomy can also inadvertently cause damage to the parathyroid glands which are located nearby the thyroid glands.

Most of the time, radioactive therapy and medication will produce similar results as surgery does, while being less invasive. For this reason, surgery isn’t often considered as the first treatment method for feline hyperthyroidism – except in some very specific situations wherein it is the last and only option available.

  • Dietary Therapy and Natural Remedies

Another option which is still being looked into by researchers is dietary therapy. Based off of several studies which were done on the subject, it’s believed that limiting the amount of iodine in a cat’s diet can stop the disease enough to be counted as a viable treatment method.

This is still a somewhat controversial topic as there are concerns about the long-term effects of reduced iodine in diets and whether such a diet could actually cause hyperthyroidism to become worse. As such, always consult with your vet before putting your cat on dietary therapy.

According to the AVMI, the best cat food for hyperthyroidism is wet cat food with limited carbohydrate content. The type and brand of food selected need to be very specific as some contain higher amounts of iodine than others, while others have borderline excessive amounts.

Hill’s Science Diet is a good product which has both low-iodine dry and wet cat food available.

It’s recommended to avoid marine fishes which are generally high in iodine. Check the labels of any food products that you purchase for a mention of iodized salt which should be avoided.

Additional foods to stay away from are sea salts, aged cheeses, egg yolks, and any meat or bread products that use iodized salt during manufacturing. If you do insist on feeding fish to your cat, go with freshwater fish.

Natural remedies for hyperthyroidism in cats are generally not recommended by most doctors unless a cat has specific maladies or ailments which prevent it from receiving any of the other preferred methods of treatment.

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