Hyperthyroidism In Cats

The Condition

Hyperthyroidism is a disease of older cats characterized by excessive production of thyroid hormone. The usual cause is benign enlargement of the thyroid glands. Signs may include weight loss, increased appetite, hyperactivity, increased vocalization, and vomiting. Heart failure, high blood pressure and kidney failure frequently complicate hyperthyroidism. If untreated, cats usually waste away or succumb to other complications.

Diagnostic Tests

A typical work up includes a geriatric blood panel, blood pressure reading, and urinalysis. Chest x-rays are recommended if we suspect heart disease. Occasionally more involved testing such as a cardiac ultrasound is indicated.

Treatment Options

Three options are available: radioactive iodine (iodine 131), lifetime methimazole medication and surgery. Of the three, radioactive iodine is preferred because it is well tolerated and curative. Medication is often effective when iodine 131 therapy is not possible. Surgery is reserved for special situations only.

Radioactive Iodine

We are fortunate to have Dr. Faythe Vaughan providing this service at the Feline Hyperthyroid Treatment Center in Tacoma and Shoreline, WA. Iodine 131 selectively destroys the diseased thyroid tissue while preserving normal thyroid gland. The treatment is very safe and cures 95% of cats with a single treatment. Cats typically stay at the treatment center three days and two nights. Cats are sent home only after radiation counts are at a low level. Cats rarely require replacement thyroid medication following treatment. Cost is around $1345.00 – $1645.00. For in-depth information about this service, visit their website at: Feline Hyperthyroid Treatment Center or call (206) 546-1243 to arrange an appointment. The Cat Doctor will provide the pre-treatment workup and the post-treatment follow-up.

Methimazole Medication

This can be an effective alternative to radioactive iodine. Methimazole does not cure the disease. Cats require continuous medication for life or until you decide to have Iodine 131 therapy performed. Possible side effects to methimazole are loss of appetite, lethargy, vomiting, and an itchy facial dermatitis. Uncommonly, life-threatening blood cell disorders can occur. Methimazole pills are given twice daily and prescription costs range from $20 to $35 a month. Careful monitoring of methimazole therapy is needed. If all goes well your cat is rechecked two to three times at 3-week intervals and then every 4 to 6 months. Rechecks consist of a physical examination, a complete blood count, basic organ function tests, and a thyroid test (T4). More complex cases may require additional recheck tests.


Good to excellent, especially when the hyperthyroid condition is detected early, and cured or controlled. Advanced heart disease and concurrent kidney failure worsen the long-term prognosis.A recent study demonstrated longer potential survival times in cats receiving Iodine 131 compared to cats receiving methimazole therapy. In this study, when cats with concurrent kidney disease were excluded, the median survival time for cats receiving Iodine 131 alone was 4 years compared to 2 years for cats receiving methimazole alone. Cats receiving methimazole medication followed by Iodine 131 had a median lifespan of 5.3 years.

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