Once daily treatment for hyperthyroidism?

Hyperthyroidism is the most common glandular disorder of cats. It tends to strike older cats (cats greater than 10), although it can be seen in cats as young as 7 or 8. When cats become hyperthyroid, their thyroid gland in the neck secretes too much thyroid hormone. This causes the metabolism to increase, and cats will start burning calories and losing weight. Cats will try to eat a lot of food to make up for it, but they can't keep up, and will lose weight despite the ravenous appetite. A variety of other possible clinical signs are possible, but weight loss and excessive hunger are the top two signs.

There are three potential treatments:
1. surgical removal of the thyroid gland: This is rarely done anymore, because of the advent of radioactive iodine therapy
2. radioactive iodine therapy: the cat is taken to a facility licensed to use nuclear materials, and the cat is given an injection of radioactive iodine under the skin. The iodine travels to the thyroid gland and corrects the problem. The cat is now cured of the condition
3. medical therapy with methimazole (brand name: Tapazole).

Most people opt for option 3. Option 1 (surgery) requires anesthesia and is costly. Option 2 is ideal, but it too is costly (and requires the cat to stay at the treatment facility for approximately 10 days). Medical therapy with methimazole is a perfectly valid option. Administering the medication twice daily will bring the thyroid hormone levels into the normal range and the clinical signs will resolve.

A problem with this option is that it requires giving medication twice daily. Cats can be difficult to medicate. The medication can be compounded into a liquid version, which some people find easier than giving in pill form. For those cats who absolutely will not take any medication orally, methimazole can be made into a gel that can be smeared on the inside of the ear. This will be absorbed through the skin and is effective in lowering the thyroid level into the normal range.

A recent article in the Journal of Small Animal Practice describes a multi-center study of 44 cats that were diagnosed with hyperthyroidism. All of the cats were treated with a drug similar to methimazole, called carbimazole. This particular form of carbimazole was a controlled-release formulation. Cats were treated once daily. The cats were followed for about a year. Nearly all cats improved within 3 weeks after treatment, with minimal side effects.

I don't know if carbimazole is available in the United States. If it is, it would offer us another option for treating hyperthyroidism, especially in cats that are difficult to medicate orally. What would be REALLY useful, I think, would be to conduct a study to see if carbimazole could be compounded into an ointment that can be administered in the ear ONCE daily. That would be truly ideal. Rumor has it that a major manufacturer of prescription diets has developed a diet that, when fed to hyperthyroid cats, controls their hyperthyroidism. I've been hearing this for months and frankly, although this is a very reputable company, I'll believe it when I see it.

Clinical signs of hyperthyroidism in cats

• Weight loss
• Ravenous appetite
• Excessive thirst and/or excessive urination
• Unkempt hair coat/patchy hair loss
• Vomiting
• Diarrhea
• Restless or hyperactive
• Panting
• Excessive shedding

Physical examination findings associated with hyperthyroidism in cats

• Enlarged thyroid gland
• Thin
• Hyperactive or difficult to examine
• Fast heart rate
• Scraggly hair coat
• Heart murmur
• “Gallop rhythm” heard when listening to heart

SEE ALSO:  "Transdermal Medication for the Treatment of Hyperthyrodism" 


  1. I picked up my cat a little over a year ago from the local humane society. According to them she is just over 6 years old now and was just diagnosed with hyperthyroidism.

    Given how young she is, is there anything else we should be considering before continuing treatment?


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