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5 Symptoms of Hyperthyroidism in Cats

Cat drinking a lot of water
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Feline hyperthyroidism is a common condition in older cats. Fortunately, hyperthyroidism is relatively easy to diagnose, requiring only a single blood test. This condition is also relatively easy to treat, carrying a good prognosis.

However, many cats don’t receive an early diagnosis or prompt treatment. The symptoms of hyperthyroidism in cats can be subtle and often mistaken for normal age-related changes, leading to unnecessary delays in diagnosis and treatment.

If you share your home with an older feline, take a few minutes to familiarize yourself with these five signs of hyperthyroidism in cats. Also, learn what to do if you notice signs of hyperthyroidism in your feline companion.  

Cat Hyperthyroidism: A Common Condition

Feline hyperthyroidism is a common condition in middle-aged and older cats. In fact, it’s estimated that approximately 10 percent of cats over 10 years of age are affected by hyperthyroidism. Hyperthyroidism is the most common endocrine (hormonal) disease in cats.

We don’t know exactly what causes hyperthyroidism in cats. Most cases are associated with a benign tumor of the thyroid gland. Many potential causes of these tumors have been considered, including dietary factors and environmental contaminants, but the truth is that we really don’t know why it’s such a common condition.  

5 Symptoms of Hyperthyroidism in Cats

Orange cat sleeping

Hyperthyroidism causes abnormally high levels of thyroid hormone in the bloodstream. The symptoms of hyperthyroidism are all caused by an excess of circulating thyroid hormone.

In most cases, symptoms of hyperthyroidism come on relatively gradually, over a period of weeks to months. However, given their subtle nature, it’s not uncommon for these signs to go unnoticed for a long period of time, until they become far more obvious and difficult to ignore. In this case, it can appear as though the signs of hyperthyroidism have come on relatively suddenly.

Common symptoms of hyperthyroidism in cats include the following:

Weight Loss

This is often the first sign of hyperthyroidism that owners notice, and it’s also the most common sign of hyperthyroidism. Because thyroid hormone regulates your cat’s metabolism, an increase in thyroid hormone will increase your cat’s metabolism and lead to weight loss.

Many older cats are overweight, and a cat with hyperthyroidism may first lose some of their excess body weight. Your normally-chunky cat may begin to slim down and look healthier, leading you to think that their weight loss is a positive change. However, weight loss that isn’t accompanied by calorie restriction suggests a possibility of hyperthyroidism (or other medical conditions). If left untreated, cats with hyperthyroidism will eventually begin to burn muscle, leading them to become excessively skinny.

Increased Appetite

In addition to burning fat and muscle, cats with hyperthyroidism will eat more to fuel their increased metabolism. If your cat is fed free choice, you may notice that you are needing to refill the food bowl more often than usual. If you feed your cat carefully-measured meals, you may notice that your cat is eating more quickly than usual and always begging for more food.

Even as you increase their food intake, however, cats with hyperthyroidism will continue to lose weight. Increased food intake accompanied by weight loss is a big red flag that suggests hyperthyroidism.

Increased Thirst and Urination

If your cat is drinking more water than usual and visiting the litter box more often than usual, this could be a sign of hyperthyroidism. (It could also be a sign of many other medical conditions!) Approximately 50 percent of cats with hyperthyroidism demonstrate increased thirst and urination.

Thyroid hormone can act on the brain to trigger an increase in thirst. Additionally, hyperthyroidism affects how blood flows through your cat’s kidneys, resulting in increased urine production. Your cat will drink more water to compensate for this increased urine output.

Changes in activity level

An increase in your cat’s metabolism can lead to behavioral changes. Your cat may become more active than usual, as if they’re full of “nervous energy. You might notice your cat pacing or acting restless. Muscle twitches and hyperreactivity can also occur.

Cats with hyperthyroidism also tend to become more talkative. They may be more likely to yowl (a loud, drawn-out meow), especially during the overnight hours, often appearing confused or restless.

Gastrointestinal upset

Vomiting, regurgitation, and diarrhea are relatively common symptoms of hyperthyroidism in cats.

An increased level of thyroid hormones may act on the chemoreceptor trigger zone, a part of the brain that recognizes and responds to abnormalities in the bloodstream. This causes nausea and vomiting.  

Gastrointestinal issues in cats may also be associated with increased food intake. Cats with hyperthyroidism tend to quickly scarf down large quantities of food, especially in multi-cat homes. This overindulgence can lead to regurgitation, vomiting, and diarrhea.

What to Do If You Notice Symptoms

Signs of hyperthyroidism, especially in a middle-aged or older cat, warrant a visit to your veterinarian.

Your veterinarian will begin by performing a thorough physical exam. They will look for signs of hyperthyroidism, as well as signs of other medical conditions that may cause similar symptoms. Your veterinarian may be able to palpate an enlarged thyroid gland. This finding is strongly suggestive of hyperthyroidism as the cause of your cat’s signs.

Next, your veterinarian will recommend laboratory testing. Blood tests will be used to evaluate your cat’s thyroid hormone (T4) level,  as well as your cat’s overall health and organ function. A urinalysis may be performed, to assess your cat’s kidney function and rule out other potential causes of urinary changes. Cats can have hyperthyroidism in combination with other conditions, such as feline diabetes and chronic kidney disease, so your veterinarian will perform a thorough workup before attributing your cat’s signs solely to hyperthyroidism. Your veterinarian might also check your cat’s blood pressure, as high blood pressure can also occur with hyperthyroidism.

If your cat is diagnosed with hyperthyroidism, there are several available treatment options. Most cats are treated with methimazole, a medication that is given twice daily for the remainder of your cat’s life. Alternatives include radioactive iodine therapy and dietary iodine restriction (with the use of a prescription diet). 

Your cat will be closely monitored during the early stages of treatment, with frequent physical examinations and blood tests. Once your cat’s hyperthyroidism has stabilized, recheck visits will decrease in frequency to once every 6-12 months.

With prompt and effective treatment, most cats with hyperthyroidism experience a complete resolution of clinical signs. With appropriate monitoring and treatment, these cats typically go on to live long, healthy lives.