Feline hyperthyroidism is a commonly diagnosed disease, particularly in middle aged to older cats. While surgical removal of the thyroid gland and radio-iodine treatment of the thyroid gland are also treatment options for feline hyperthyroidism, Tapazole is the most commonly used treatment for hyperthyroidism in cats.
Tapazole contains methimazole as the active ingredient. Methimazole effectively blocks the production of the thyroid hormones, T4 and T3. Overproduction of these thyroid hormones is responsible for causing the clinical signs seen in feline hyperthyroidism..
Previously, propylthiouracil (PTU) was one of the medications used to treat feline hyperthyroidism. PTU is no longer widely used in veterinary medicine because methimazole has been found to have fewer side effects.
In the United Kingdom and Australia, carbimazole is used because methimazole is not easily available. Carbimazole is closely related to methimazole and is converted to methimazole in the cat so effects are similar.
Why is Methimazole So Widely Used?
Tapazole, or methimazole, is widely used because it is a safe and effective drug when used to treat hyperthyoidism in cats. Methimazole is quite inexpensive in comparison to radiotherapy or surgical treatment and, unlike radiotherapy and surgical options, hospitalization is not required for treatment with Tapazole or methimazole.
Though medicating with Tapazole or methimazole does not cure hyperthyroidism, the disease is usually fairly well controlled with the medication. In the event of unwanted side effects, the Tapazole or methimazole treatments can be discontinued and the side effects quite often resolve.
While side effects are relatively uncommon, when they do occur they are most commonly seen within the first three months of treatment with Tapazole or methimazole. After three months of treatment with no side effects, the possibility that side effects will occur is greatly reduced.
While it is preferable to give Tapazole or methimazole on a regular basis, if a few doses are missed, it is rarely harmful to the cat.
Disadvantages of Tapazole or Methimazole
While Tapazole or methimazole treatment is usually quite effective in treating hyperthyroidism for cats, administration involves at least daily and often twice daily dosing. Medicating an unwilling cat can be challenging. However, methimazole can be compounded into a flavored liquid medication which may be easier to administer to most cats.
Another dosing option is a transdermal gel which can be placed on the inner surface of the cat's ear or the medication can be incorporated into a patch applied to the cat's skin. However, these gels and patches may not be as effective as oral dosing in some cats. Investigation in this area is still ongoing.
Administration of methimazole or Tapazole will be necessary throughout the lifetime of your cat. If discontinued, the symptoms of hyperthyroidism will return for your cat.
Side Effects Associated with Tapazole and Methimazole
Tapazole and methimazole can produce the following side effects:
- lethargy
- loss of appetite
- vomiting
- facial itching
- liver failure
- bone marrow changes
- kidney failure
In most cases, discontinuation of Tapazole or methimazole medication results in the resolution of the side effect.
- In cases of lethargy, appetite loss and vomiting, medication is usally started at a lower dose and gradually increased to the proper dose once side effects have resolved.
- In the case of liver failure, facial itching or bone marrow changes, alternative therapy for hyperthyroidism may be recommended by your veterinarian.
- In the case of kidney failure, Tapazole or methimazole medication dosages may need to be lowered or discontinued altogether.
Your veterinarian will need to monitor your cat's blood T4 level periodically to make sure your cat is receiving the proper dose of Tapazole or methimazole. In addition, white blood cell counts, liver enzymes and kidney function will need to be monitored for your cat.
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