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What is Hyperthyroidism?

Hormones produced by the thyroid gland affect your cat's metabolic rate. When a cat produces too many thyroid hormones, she is said to have hyperthyroidism. Hyperthyroidism is common in cats, especially middle-aged and senior cats. On its own, hyperthyroidism is simple to treat, though it requires surgery or medical treatment. However, hyperthyroidism in cats is linked to the development of other conditions as well, such as heart disease, hypertension, and kidney or brain damage. Regular veterinary check-ups are recommended for cats with overactive thyroids to prevent and intervene early on these complications of hyperthyroidism. 

Symptoms of Hyperthyroidism in Cats

Thyroid hormones regulate your cat's metabolic rate, so they play a role in all her body's systems. Signs and symptoms of hyperthyroidism in cats can vary widely. Schedule an appointment with a veterinarian if your cat exhibits any of the following symptoms:

  • Weight loss
  • Increased appetite
  • Excessive thirst
  • Increased volume or frequency of urination
  • Vomiting
  • Diarrhea
  • Hyperactivity
  • Greasy coat
  • Increased fecal volume
  • Depression

These symptoms may worsen over time if hyperthyroidism goes untreated; however, veterinary intervention works quickly to resolve most primary symptoms of hyperthyroidism. 

Causes of Hyperthyroidism in Cats

Hyperthyroidism in cats is usually caused by a benign tumor, also known as an adenoma, that produces thyroid hormones. Less often, hyperthyroidism is caused by a malignant, or cancerous, tumor called an adenocarcinoma. Only your veterinarian is qualified to determine whether the tumor causing your cat's hyperthyroidism will require surgery to remove it.

Diagnosis of Hyperthyroidism in Cats

Your veterinarian will begin the diagnostic process with a thorough physical examination of your cat, including the collection of a complete medical history. The veterinarian will palpate, or apply pressure to, your cat's neck to detect swelling or soreness associated with a swollen thyroid gland. 

Urine and blood samples will be collected, and your veterinary team will perform a urinalysis, complete blood count and chemistry profile to rule out other hormonal disturbances and confirm a diagnosis of hyperthyroidism. The final diagnosis will be made once blood tests confirm the presence of an excess of thyroid hormones. 

Since heart disease, vascular disease and organ damage often follow hyperthyroidism in cats, your veterinarian will likely perform a comprehensive examination on your cat to detect abnormality. Urinalysis can test for kidney function, while blood tests can provide information on the functioning of all organ systems. 

Treatment of Hyperthyroidism in Cats

There are three common interventions prescribed for cats with hyperthyroidism: administration of radioactive-iodine therapy, surgical removal of the thyroid gland, and management with medication.

Radioactive Iodine Therapy

Radioactive iodine therapy is often the first line of treatment for cats with hyperthyroidism. For this therapy, your veterinarian will inject your cat with a radioactive iodine solution, which targets and destroys affected thyroid tissue. This treatment usually corrects hyperthyroidism in cats within a month. This treatment cures hyperthyroidism in 95 percent of cats, however, your cat will require hospitalization for approximately three days while her levels of radiation fall. After three days she will be safe to handle again. 


Another solution available for cats with hyperthyroidism is surgical removal of the entire thyroid gland. While this procedure is generally recognized as safe, it does pose a threat to the nearby parathyroid gland. Like radioactive-iodine therapy, surgery is a curative solution to hyperthyroidism in cats. 


The final mode of treatment for hyperthyroidism in cats is medication. Unlike radioactive-iodine therapy and surgery, medication does not cure hyperthyroidism. Rather, it reduces the production of thyroid hormones by the thyroid gland for the long-term management of hyperthyroidism. Medication for hyperthyroidism in cats is inexpensive, but requires administration multiple times per day and often causes unpleasant side effects. It also taxes organ systems that might already be affected by hyperthyroidism, such as the kidneys. Your veterinarian should monitor your cat's kidney function while she is taking antithyroid medication. 

Recovery of Hyperthyroidism in Cats

Hyperthyroidism can cause a cat's heart to enlarge or thicken, which can lead to heart disease or heart failure. Hypertension (high blood pressure), another side effect of hyperthyroidism, can lead to heart damage, as well as damage to the kidneys, eyes and central nervous system. For these reasons, cats with hyperthyroidism should be monitored by a veterinarian even if the thyroid condition is resolved. 

Cats who have undergone radioactive iodine therapy may require a few days of hospitalization while radiation levels fall. Rarely, treatment with radioactive iodine may damage too much thyroid tissue, leading to an underactive thyroid gland. Likewise, the surgical removal of the thyroid gland can cause hypothyroidism. Cats suffering from this condition are prone to obesity, hair loss, lethargy, and low body temperatures. See your doctor right away if you suspect treatment for hyperthyroidism has caused hypothyroidism in your cat.