Hyperthyroidism in cats

Last week I wrote about hypothyroidism, which is a common condition in dogs. This week, let’s talk about hyperthyroidism–an overactive thyroid. This is a condition very common in cats but pretty uncommon in dogs.

Interestingly, hyperthyroidism is a condition I am intimately familiar with because my dog did have it. When my greyhound started getting very thin and was hungry all the time, we suspected something was wrong.

It was thyroid cancer that caused his hyperthyroidism. Typically, if a dog suffers from hyperthyroidism, it’s usually due to a cancerous tumor on the thyroid.

But for today let’s talk about how hyperthyroidism affects cats because it is the most common endocrine (glandular) disorder that they suffer from.

What is hyperthyroidism?

You may remember from last week’s post that the thyroid is a small 2-lobe gland near the windpipe in your cat’s neck. It regulates the body’s metabolism by producing thyroid hormones.

Hyperthyroidism (overactive thyroid) is an excessive production and release of T3 and T4 thyroid hormones into the bloodstream.

What causes hyperthyroidism?

The most common cause of hyperthyroidism is the formation of small benign nodules (tumors) that are created by an abnormal production of cells in the thyroid. These nodules on the thyroid are called adenomas. They cause the thyroid to be enlarged.

In cats with hyperthyroidism, both lobes will be enlarged 70% of the time. In only 1 to 2% of hyperthyroid cases in cats will these nodules be a malignancy (cancer).

Hyperthyroidism is not specific to a breed of cat, and it’s not more prevalent in one sex or the other. It generally occurs in older cats with the average age of onset between 12 and 13.

It’s not clear what actually causes the adenomas that ultimately lead to hyperthyroidism, but there is some speculation that it has to do with diet and the environment.

Interestingly, hyperthyroidism in cats has increased dramatically in the last 30 years.

What’s caused the incidence of hyperthyroidism to increase?

Prior to the 1970s, hyperthyroidism in cats was very rare.

According to Dr. Mark Peterson, who wrote an article on the topic in the Journal of Feline Medicine and Surgery, there is no one definitive factor in the increase of thyroid tumors but there does seem to be two likely culprits—nutritional excesses or deficiencies, and thyroid disrupting compounds present in the environment.

The nutritional excesses and deficiencies he’s talking about are soy isoflavones and iodine.

Soy isoflavones can be found in many commercial cat foods. They’re used as a cheap source of protein and are linked to thyroid tumor growth in humans and animals.

The other factor is the iodine content in cat food. The recommended guidelines for iodine in cat food have been lowered in the last 30 years. There seems to be a link between a low iodine diet and thyroid tumor growth in cats.

Not only is less iodine a factor, but also from one commercial cat food to the next the amount of iodine varies dramatically. And it’s the wide variations in your cat’s iodine intake that’s caused by changing their food that seems to be a factor in thyroid tumor growth over time.

It’s probably no coincidence that hyperthyroidism has been on the rise since iodine guidelines have changed.

The major environmental factors that Dr. Peterson speaks of in his article are bisphenol A (BPA) in the lining of cat food cans, certain chemical flame-retardants found in places like furniture and carpet, and environmental pesticides and herbicides.

These are all endocrine disrupters and exposure to them is believed to contribute to thyroid tumor growth.

What are the signs of hyperthyroidism?

Symptoms of hyperthyroidism may be subtle at first but a problem will become obvious to you over time.

Here are some of the most common symptoms to watch out for:

Weight loss

Increased food consumption


Increased water consumption/urination


Increased activity/behavior changes

Unkempt hair coat/hair loss




Panting or labored breathing

Your cat may take on a somewhat disheveled appearance because their coat may become matted or greasy, and they’ll seem thin.

Hyperthyroidism can result in serious health problems if it goes untreated for too long. It’s important to get your cat to the vet as soon as you suspect a problem, or your cat could develop a serious heart condition that can lead to heart failure and ultimately death.

Your vet will be able to feel if there is a thyroid tumor by palpating the area. And as with hypothyroidism, your vet will run blood work. They may run additional tests because some times thyroid hormone levels will show in the normal range on a blood test.

Since some of the symptoms of hyperthyroidism are the same symptoms of other diseases, your vet will want to rule out things like diabetes, kidney failure, heart disease or liver disease. This means more tests. But better to be safe than sorry.

Can hyperthyroidism be treated?

Fortunately hyperthyroidism is treatable.

There are three treatment options; medication, surgery or radioactive iodine therapy. Each of these treatments has plusses and minuses. Some are curative. Others minimize symptoms. A conversation with your vet will give you the information you need to decide on the best treatment plan for your kitty.

Initially, your vet may also suggest a high protein, highly digestible diet to counteract the poor absorption of nutrients and high metabolism brought on by hyperthyroidism.

Your cat will likely be able to return to their normal diet, unless they have complications that require something special, once symptoms are controlled by treatment.

The good news is that with treatment, your cat can live a normal healthy life even with hyperthyroidism.

Have you had a cat that suffered from hyperthyroidism? How did you know there was something wrong? Share your experience in the comment section above.

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