By Aimee Castor, DVM

City Cat Mobile Vet Service



Hyperthyroidism is a very common illness that affects older cats. It is usually caused by a benign tumor of the thyroid gland that is similar to goiter in humans that releases an excessive amount of thyroid hormone, called thyroxine, into the bloodstream. The median age of diagnosis in cats is 13 years of age. Signs are variable from cat to cat. The following signs are listed from most common to least common:


The majority of hyperthyroid cats have high blood pressure. If left untreated hyperthyroidism can lead to kidney disease, liver disease, and congestive heart problems.  Hyperthyroid cats also have increased risks associated with anesthesia.  Diagnosis is achieved by measuring the thyroid level in the blood.  Occasionally the thyroid level is normal even though the cat is truly hyperthyroid. This can occur due to fluctuations of thyroid levels early in the disease or it can be due to other illnesses that falsely lowers the thyroid. If the cat is suspect hyperthyroid but the thyroid level is normal, retesting at a later date is advised, usually 3 to 6 months later.



The most common treatment used is methimazole, which is a medication that inhibits the thyroid gland from producing excessive thyroid hormone. It works well but occasionally can have side effects. The most common side effects are a decreased appetite, vomiting, and diarrhea. Rare, but more serious side effects include decreased blood cell production and liver failure. Blood work should be done approximately at 1 and 3 months after the medication is started and then every 6 months to check the blood cell count, thyroid level, and kidney toxins.


A thyroidectomy is a surgical procedure that involves removing the affected thyroid gland(s). It is rarely performed now due to the fact that the other treatments have much less complications.  Methimazole must be used 2 to 4 weeks beforehand to decrease the risk of anesthesia.  Hyperthyroid cats are more at risk for anesthetic complications if their hyperthyroidism is not controlled. After removal of the thyroid glands, a low blood calcium may occur due to damage of of the adjacent parathyroid gland. That can continue for 1 to 40 days after the surgery and if that happens the cat should be kept at the vet clinic and given calcium injections. Thyroid supplementation due to a thyroid level that is now too low may be needed for 2 to 3 months after the surgery, but rarely needed life long. Another complication that can occur after surgery is laryngeal paralysis.

Radioactive iodine therapy (I131)

I131 treatment destroys the hyper-functioning thyroid tissue without affecting the normal part of the thyroid gland. It is almost always successful. Anesthesia and medications are not needed beforehand and it has lowest incidence of side effects in comparison to the other treatments if the kidney values are normal. It is, however, expensive and the cat may need to be hospitalized for 1 to 2 weeks. Cats that have moderate to severe kidney disease may be declined by the Feline Hyperthyroid Treatment Center due to the fact that I131 are more risky in those particular cats.  Rarely cats treated by the hyperthyroid treatment center will need to be treated a second time or be supplemented with levothyroxine if the thyroid level becomes too low.