Can Cats Treated With Methimazole for Hyperthyroidism Be Dehydrated?

"That methimazole didn't agree with me."
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If your cat's diagnosed with hyperthyroidism, your vet might prescribe methimazole pills for treatment. He'll have to take this medication once or twice daily - not fun if your kitty's hard to pill. While methimazole itself doesn't cause dehydration, side effects can. You have other treatment options.


If your older cat's losing weight while eating like there's no tomorrow, take him to the vet to rule out hyperthyroidism. It's a common condition in older felines, resulting from a benign growth on his thyroid gland along with overproduction of the thyroid hormone. Other symptoms include increased drinking and peeing, hyperactivity, grumpiness, poor hair quality, vomiting and diarrhea. Your vet will check circulating thyroid levels through blood tests.


Marketed under the brand name Tapazole for people and Felimazole for felines, methimazole is available in pill and transdermal gel form. If your cat's a pill for pilling, you can try the transdermal gel, which you apply to the hairless part of his ear. However, the gel isn't as strong as the pill. If your cat's prescribed methimazole for hyperthyroidism, he'll be on the medication for the rest of his life. It's a treatment, not a cure.

Methimazole Side Effects

If your cat can't tolerate methimazole, side effects generally develop within the first few months of treatment. These include vomiting and diarrhea, which can cause dehydration in your cat. Other side effects include appetite loss and lethargy. If Puffy displays these symptoms, your vet might take him off the drug for a short period, and then put him back on it at a lower dose. If more serious side effects such as severe facial itching, kidney and liver failure and changes in bone marrow occur, then methimazole's a no-no for your cat.

Other Options

If your cat can't tolerate methimazole, there are other options. If your cat's in good enough health for surgery, your vet can perform a thyroidectomy, removing the glands. There's risks with any surgery, but this can cure the disease. The recommended treatment is radioactive iodine therapy, a relatively simple procedure that will likely require some travel on your part. Because it involves radioactive material, only certain veterinary facilities can offer the procedure. It consists of an injection of radioactive iodine, which destroys the thyroid tissue without the cat. The downside is that Puffy must stay at the facility for several days until his radioactivity levels go down, so you can't visit him. You'll also have to take some precautions for a few weeks once he's home, including special litter disposal and keeping him away from kids and pregnant women. If surgery or radioactive iodine therapy aren't options for you, your vet can prescribe a special, low-iodine diet. Your cat can't eat anything else, which might be a problem if you have other cats in your home.

Always check with your veterinarian before changing your pet’s diet, medication, or physical activity routines. This information is not a substitute for a vet’s opinion.

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