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Hyperthyroidism is the most common endocrine disorder in cats. It is usually caused by benign changes to the thyroid gland, located in the neck, causing overproduction of thyroid hormone. This leads to an increase in metabolic rate causing weight loss and increased appetite. It presents in middle-aged and older cats.

Feline Hyperthyroidism Symptoms


Senior Cat with Feline Hyperthyroidism

  Senior cats are more likely to develop
feline hyperthyroidism.

Typically owners notice the following clinical signs in their cat:

  • Weight loss
  • Intermittent vomiting
  • Ravenous appetite
  • Unkempt hair coat

As it progresses, hyperthyroidism in cats can lead to hypertension (high blood pressure) and thyrotoxic cardiomyopathy (enlargement of the heart to meet the increased metabolic demands). With treatment of the disease these conditions are usually reversible.


Diagnosis is made by clinical signs and confirming high blood levels of one of the thyroid hormones, thyroxine (or T4).

Treatment for Feline Hyperthyroidism

Treatment is usually successful. There are several different types of treatment. First, your veterinarian will want to evaluate your pet and his/her blood results to determine the best choice.

  • Oral medication — a drug methimazole (Tapazole) can control thyroid hormone levels. Disadvantages are that some cats have side effects to it, and it must be given lifelong. Periodic blood tests must be done to keep the dosage regulated and to monitor for side effects.

  • Radioactive iodine — this is given by injection and destroys abnormal thyroid tissue without affecting other organs. Because it involves radioactive material, only certain facilities are licensed to use it. Disadvantages are that it is expensive, cats may experience side effects, and that cats may have to be kept at the treatment facility for hospitalization during treatment. Advantages are that it is usually a curative solution.

  • Surgery — removal of the thyroid lobe/gland can be effective. But because the disease is usually in older cats, there are increased risks with surgery. It also requires a surgical specialist which can be more costly.

  • Diet change — recently a new food has been available (Hills Y/D) which is iodine restricted. If affected cats eat only this food, they aren’t able to produce the thyroid hormones which cause problems. Disadvantages are that cats can often be finicky and don’t always adjust to a diet change well. It has been found to be a safe diet for normal cats as well, so that in multiple cat households it can still be used.


With treatment, prognosis for hyperthyroid cats is good and most cats return to a normal state of health. Cats which are treated medically must receive medication routinely and have regular follow-up blood tests.

Early Detection

There are no preventive measures for hyperthyroidism, but early detection helps so that potentially irreversible changes don't occur. Middle-aged and senior cats should be examined by a veterinarian at least yearly with blood or urine tests when recommended.

Dr. Jeanette Asuncion is an associate at Illiana Veterinary Hospital in South Holland, IL. Originally from the Detroit suburbs, Dr. Asuncion earned a DVM degree from Michigan State's College of Veterinary Medicine in 1990.