Hyperthyroidism in Cats

By: ChewyUpdated:

Thirst is a symptom of hyperthyroidism in cats

Hyperthyroidism in Cats

When it comes to cat health, hyperthyroidism is one of the most common diseases seen in older cats. If your feline friend has been diagnosed, learn what hyperthyroidism in cats is, what causes it and your treatment options.

What Is Hyperthyroidism in Cats?

Just like people, cats have a thyroid gland located in their neck where it secretes hormones that regulate the cat’s metabolism. The main hormones secreted by the thyroid gland are T3 and T4. The thyroid gland creates these hormones with iodine extracted from the bloodstream, and the metabolism of every other cell in the body is dependent upon thyroid hormones.

If thyroid hormone is low, then the metabolism is low. If the thyroid hormone is high, then metabolism is high. A cat with an overactive thyroid secretes too much thyroid hormone into the blood stream, hyping up every other system in the body. This is an easy way to remember hyperthyroidism—it makes the body too hyper!

Symptoms of Hyperthyroidism in Cats?

Hyperthyroidism causes very distinct changes in the body and behavior of a cat. These include:

  • Oodles of energy
  • More talkative or vocal
  • Increased appetite
  • Thirstier than normal
  • Vomiting
  • Diarrhea
  • Hair loss, especially on the ears and forehead
  • Excessive scratching or grooming
  • High blood pressure
  • Increased heart rate
  • Retinal detachment due to high blood pressure popping off the retinas. (Retinal detachment causes acute blindness with dilated pupils.)

What Causes Hyperthyroidism in Cats?

Hyperthyroidism is a disease seen in older cats and usually is caused by the growth of a benign tumor called an adenoma on the thyroid gland. Rarely, these tumors can be malignant. The cause of the tumor remains unknown, but theories include diet or chronic exposure to chemicals.

How is Hyperthyroidism in Cats Diagnosed?

Hyperthyroidism is diagnosed with a combination of a veterinary physical examination and laboratory testing. During the physical exam, your veterinarian also might measure blood pressure.

If your veterinarian suspects hyperthyroidism, a blood and urine sample from your cat will be submitted to the lab for testing. Several tests may be ordered, such as T4, free T4 and TSH. Your veterinarian also might order a complete blood count, blood chemistry and urinalysis. Hyperthyroidism is diagnosed by finding abnormalities in these tests.

How Is Hyperthyroidism in Cats Treated?

Hyperthyroidism shortens the quality and quantity of your cat’s life and should be treated if diagnosed. There are several options available for treatment of hyperthyroidism in cats.

Thyroid Medication for Cats

Methamizole is a medication used to treat hyperthyroidism. This medication does not cure hyperthyroidism, but it does reduce the amount of thyroid hormone released.

It typically is administered twice daily for life. If you discontinue the medication, the symptoms of hyperthyroidism will return. Some cats experience negative side effects, most commonly vomiting and diarrhea.

Cats on this type of therapy typically need blood and urine tests twice a year to monitor thyroid hormone levels and kidney function. Methamizole usually is prescribed as a pill; however, it can be formulated into a cream to rub into your cat’s ear if pilling your cat daily isn’t something you think you can or want to do.


Surgical therapy for hyperthyroidism in cats consists of surgically removing the thyroid gland that has the tumor present. This effectively cures the disease and is a good option for cats with malignant tumors and for clients who do not want to give their cats medication.

Risks include death during surgery, hypoparathyroidism (low blood calcium) and recurrent hyperthyroidism if a tumor grows on the other gland.


Dietary therapy involves switching the cat to a prescription diet formulated to reduce thyroid hormone production and support urinary, kidney and heart health. One option is Hill’s Prescription Diet y/d thyroid care dry cat food. It has been shown to clinically help some hyperthyroid cats within three weeks of starting the diet.

Radioactive Iodine Treatment

Radioactive iodine treatment is considered the best therapy choice. In this treatment, radioactive iodine is injected into the bloodstream. The thyroid gland takes up the radioactive iodine and the iodine selectively kills abnormal thyroid tissue.

Most cats have normal thyroid hormone levels 10-14 days after treatment. Cats remain hospitalized for 3-5 days after treatment to avoid exposing people to radiation.

The incidence of side effects is very low. In rare cases the treatment must be repeated, and in even rarer cases the cat becomes hypothyroid (the opposite of hyper) and a thyroid supplement is required.

Talk with your veterinarian about these options and decide together what is the best option for you and your cat.

Sarah Wooten bio


By: ChewyUpdated: