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Know the Signs of Hyperthyroidism in Cats

Cane Bay Summerville - hyperthyroidism in catsHyperthyroidism in cats is unfortunately a rather common problem, particularly for our older feline friends. If your cat is aging, it’s best to know the signs of hyperthyroidism so you can catch it early before it leads to further complications.

What Is Hyperthyroidism?

The term hyperthyroidism is used to describe an overactive thyroid that produces above-average levels of the thyroid hormones T3 and T4. Cats with this disease will have an enlarged thyroid gland, though typically, this is non-cancerous and benign.

What Population Does Hyperthyroidism Typically Affect?

Most frequently, middle-aged and senior cats are afflicted. According to VCA Hospitals, only 5% of cats 10 years or younger have hyperthyroidism, as most felines develop the condition at 12 years or older.

While no specific breed is more likely to develop hyperthyroidism, several breeds are slightly less likely to, including Siamese, Burmese, Persian, Abyssinian and Himalayan.

Signs and Symptoms of Hyperthyroidism

If you notice any of the following symptoms in your cat, particularly if your cat is over the age of 10, it is best to have a veterinarian assess your furry friend. Untreated hyperthyroidism can lead to additional complications.

  • Frequent urination
  • Increased appetite and thirst
  • Matted or unkempt fur
  • Occasional vomiting and/or diarrhea
  • Restless or aggressive behavior
  • Vocalizing more frequently at night
  • Weight loss

The earliest symptoms may include weight loss, increased appetite and urination. Other symptoms may develop as the disease progresses, even leading to anorexia in more severe cases.

Secondary Issues That May Arise from Hyperthyroidism

Untreated hyperthyroidism may lead to secondary complications such as heart disease and high blood pressure, which may require further medical treatment.

Heart disease may occur, because hyperthyroidism causes the heart to work harder and pump more blood. Over time, this may create thickening of the left ventricle. If left untreated, this can lead to heart failure. However, the Cornell Feline Health Center reminds owners that treating hyperthyroidism will help prevent and treat heart disease, though specific treatment for heart disease may be necessary at times.

High blood pressure, or hypertension, may also result from hyperthyroidism. Untreated high blood pressure can eventually cause damage to organs such as the eyes, kidneys, heart and brain. Oftentimes, veterinarians will prescribe medication to help manage blood pressure. In many cases, treating the hyperthyroidism will also help treat hypertension, and your feline friend may not need high blood pressure medication forever.

Treating Hyperthyroidism

Thankfully, there are four different treatment options your veterinarian can use to help treat hyperthyroidism in cats. Medication, dietary changes, radioactive iodine and surgery are all potential treatment plans, and your veterinarian will choose the most appropriate one for your individual pet.


Medication is a lifelong method of treatment used to control hyperthyroidism rather than cure it. It can help manage the disease through twice daily oral pills. A gel may also be applied to the skin. Like with any medication, some cats may experience side effects, so speak with your vet to determine the best treatment option.

Dietary Changes

In some cases, veterinarians may recommend limiting the amount of iodine in your cat’s diet as a potential option if no other alternatives are viable. As Cornell Feline Health Center notes, research is still being done on this form of treatment, as some say restricting iodine can worsen hyperthyroidism over time.

Radioactive Iodine

Radioactive iodine is used to cure hyperthyroidism rather than simply control it. Typically, injections with radioactive iodine are administered and improvements in hormone levels are seen within one to two weeks. Not all facilities participate in this form of treatment, as the facility must be specially licensed in radioisotopes.


Surgery is another option that is used to cure hyperthyroidism in cats. It involves the complete removal of the thyroid glands and in most cases allows our feline friends to live a medication-free lifestyle. While the success rate is generally high, this option is chosen far less, as other forms of management, such as radioactive iodine and medication, are less invasive.

If you have any questions about treating hyperthyroidism in your cat, please talk to your veterinarian about the safest option.

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