Hyperthyroidism in cats is one of the most commonly diagnosed diseases in older cats. Research suggests 10 percent of older cats will develop the disease during their lives (1).  

This disease relates to a cat’s thyroid gland, a butterfly-shaped gland that sits in the neck of cats and produces essential hormones that regulate metabolic rates and other cardiac function. 

Learn more about the symptoms and causes of hyperthyroidism in cats and explore treatment options to discuss with your veterinarian. 

What is Hyperthyroidism?

Hyperthyroidism is the overproduction of thyroid hormones from the thyroid gland in cats. It represents the most commonly diagnosed feline metabolic disorder. 

In many cats, a nodule (adenoma) or benign tumor will develop on the thyroid gland, which causes the gland to go rogue and overproduce thyroid hormones. When this happens, the cat’s body does not listen to normal feedback mechanisms that would normally signal the thyroid to shut off production of hormones when they reach adequate levels. 

The excess thyroid hormones lead to side effects including increased hunger, weight loss, increased thirst and urination, and hair and coat problems. 

What Causes Hyperthyroidism in Cats?

As mentioned, hyperthyroidism results from the overproduction of thyroid hormones caused by a nodule or benign tumor that grows on a cat’s thyroid. This develops due to an overactive thyroid gland. 

The disease may arise secondary to a poor quality diet or exposure to flame-retardant chemicals (polychlorinated biphenyl or polybrominated diphenyl ethers) that are present in food or the environment (2). There’s evidence that these chemicals are found in air fresheners, upholstered furniture, and fish-flavored or fish-containing cat foods.

Some cat breeds and certain regions are more likely to be affected by hyperthyroidism. Domestic short and longhaired cats are more likely to develop the disease. Siberian, Tonkinese, Siamese, Burmese and Abyssinian cat breeds are less likely to be affected (3).

Symptoms of Hyperthyroidism in Cats

At home, pet owners may notice weight loss, muscle weakness, or what is called cervical ventroflexion, which is when a cat’s head and neck hang towards the ground. Pet parents may also see increased hunger and thirst, vomiting, diarrhea, and changes in their cat’s hair and nails. 

Some cats—though very few—will present with signs of lethargy or lack of interest in food. Additionally, cats suffering from hyperthyroidism may be unusually aggressive. 

Symptoms to watch for include:

• Weight Loss
• Increased thirst and urination
• Increased appetite
• Hair loss (alopecia)
• Overgrowth of toenails 
• Muscle weakness
• Cardiac changes including elevated heart rate, arrhythmias, heart murmur or high blood pressure 
• Increased respiratory rate or difficulty breathing 

Diagnosing Hyperthyroidism in Cats 

To diagnose your cat with hyperthyroidism, your veterinarian will do a thorough physical exam and look for symptoms that may be difficult to detect at home including difficulty breathing, heart murmurs, elevated blood pressure or changes in the rhythm of the heart beat. 

The vet may also be able to feel a nodule or benign tumor on the thyroid, known as a goiter.   

Your veterinarian will also likely test blood cell counts and thyroid hormone levels and conduct a urinalysis. 

If a cat has cardiac abnormalities on examination, chest X-rays and possibly an ultrasound of the heart are also appropriate tests to diagnose hyperthyroidism and rule out secondary complications. An example of a secondary condition is called thyrotoxic cardiomyopathy, which is thickening of the heart muscle due to elevated thyroid levels. 

How to Treat Hyperthyroidism in Cats

To treat hyperthyroidism in cats, your veterinarian may use an injection of radioactive iodine, called I-131. 

This treatment—which is also used in humans to treat thyroid disorders and thyroid cancer—actually cures the disease in cats by shrinking the nodule on the thyroid and restoring hormone levels back to normal. 

This treatment must be performed by licensed specialists who are trained to administer radioactive iodine.  

There is also oral medication that can be used in the management of hyperthyroidism in cats. This medication is called methimazole and can be given throughout a cat’s life to stabilize hormone levels. Methimazole is available as an FDA approved drug, and when appropriate for your cat as determined by your veterinarian, compounded forms. If you have trouble giving your cat a tablet, your veterinarian may prescribe an alternate dose form, such as an oral liquid or transdermal gel, that can be made by a compounding pharmacy for easier administration. Methimazole will not cure hyperthyroidism in cats.

Cat Food for Hyperthyroidism

In cases where medication and radioactive iodine injections are not available, diets that restrict iodine and limit the thyroid’s ability to produce hormones may be suggested. However, dietary changes alone are not considered a gold standard of treatment for this disease. 

Hyperthyroidism Prognosis for Cats

Without treatment, hyperthyroidism will significantly compromise a cat’s lifespan and can be fatal, usually due to cardiac issues that develop as a result of hyperthyroidism. 

If managing the disease with oral medication, lifespan may still be shortened but not to the degree it would be without any treatment.

If cats are given the I-131 injection, their lifespan should not be impacted, since the treatment is meant to cure the disease.

Cost to Treat Hyperthyroidism in Cats

The cost to treat hyperthyroidism in cats will vary by region, especially when considering I-131 treatment.  

However, a general price range for radiation therapy, which is usually administered once, is $1,500-$3,000.  

The cost of oral medication to treat hyperthyroidism in cats is closer to $30-$50 per month. However, over the life of the pet, the cost of oral medication and required medical follow up (bloodwork, vet visits) may rival the cost of a one-time I-131 injection.

How to Prevent Hyperthyroidism in Cats

Because there are many different factors that may contribute to cat hyperthyroidism, there is no specific way to prevent the disease.   

Prevention of exposure to PCBs and PBDEs and limiting fish-based cat food may help.

Also, frequent annual visits to your veterinarian may help with early diagnosis and treatment, which limits the possibility of your cat developing secondary issues including cardiac disease and muscle weakness.  

Related Conditions

•     Hypertension in Cats
•     Thyrotoxic Cardiomyopathy
•     Heart Disease in Cats

Covetrus encourages the use of an FDA approved product whenever possible, within a valid veterinary-client-patient relationship (VCPR). However, we realize that in order to achieve a desired therapeutic outcome, a customized compounded preparation may be necessary. Covetrus compounding pharmacies, a fully PCAB accredited pharmacy with compliance to USP 795/797/800 standards along with a registered cGMP 503B outsourcing facility means you can be confident in the quality of every medication order.