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Hyperthyroidism Medication for Cats (Methimazole)
Revised: August 05, 2023
Published: January 01, 2001

The most common medication prescribed to treat feline hyperthyroidism is called methimazole (trade name Tapazole or Felimazole). This medication has virtually replaced the older medication propylthiouracil or PTU because methimazole is effective without as much tendency for side effects. In the United Kingdom and Australia, carbimazole is sometimes used. Because carbimazole is converted to methimazole in the body, the information here for methimazole also holds for carbimazole.

All these medications block the production of T4 and T3. Thyroid hormones that are already in the body when medication is started are still in play, so a good 2 to 4 weeks are needed before thyroid blood tests will show the effect of treatment.

It is important to understand what the potential side effects of methimazole and its relatives are and the monitoring that is generally recommended. The thyroid nodule, which can be detected in a cat's throat, will not reduce in size with treatment and may in fact get larger.

Advantages of Using Methimazole

  • Medication is inexpensive relative to radiotherapy or surgery, at least in the short term. Methimazole is just as effective as radiotherapy or surgery as long as it is used properly.
  • Control of thyroid disease is achieved only while the pet is on medication so that if there is any problem with increased poor kidney function, treatment can be discontinued.  In other words, the effects of methimazole are reversible.
  • No hospitalization is required.
  • Side effects are relatively uncommon.
  • If an occasional dose is skipped, no harm is done.
  • If no side effects are encountered after the first three months of therapy, the chance of side effects occurring thereafter is substantially reduced.

Disadvantages of Using Methimazole

  • Medication must be given at least daily (usually twice daily). Some cats simply will not take oral tablets reliably or cooperatively at this frequency, and providing pills in the cat's food is notoriously inconsistent.  That said, however, methimazole can be readily made into a flavored liquid or chewable for easier administration by a compounding pharmacy, or it may even be possible to convert methimazole into a gel administered on the hairless inner surface of the cat’s ear (see below).

  • Approximately 15% of cats will experience some kind of side effect. The usual side effects are lethargy, loss of appetite, and vomiting. If one of these side effects occurs, medication is discontinued until the symptoms resolve. Medication is then restarted at a lower dose and gradually increased to the former dose. These side effects do not generally recur if medication is increased gradually in this way.

  • Facial itching is a more significant side effect because it means the cat cannot tolerate methimazole. The facial itching resolves with anti-itch medication and discontinuing methimazole, but cats with this side effect can be expected to have it again if medication is restarted, so another form of treatment should be given for the thyroid disease. Facial itching occurs in less than 4% of cats on methimazole.

  • Potentially serious liver disease results in an extremely small number (less than 2%) of cats taking methimazole. This toxicity can be expected to resolve after discontinuing the medication, but again, alternative therapy for the thyroid will be needed if the cat cannot tolerate methimazole without developing liver disease.

  • Bone marrow changes can also result from methimazole administration. Blood tests evaluating white blood cell patterns should be periodically performed to monitor for these changes. This side effect occurs in less than 4% of cats on methimazole, but should it occur, it necessitates a change in therapy.

  • Pre-existing kidney insufficiency can be masked in hyperthyroidism because the heart disease and high blood pressure that go with hyperthyroidism increase blood flow through the kidneys, making the kidneys more efficient; this is virtually the only positive aspect of having hyperthyroidism). Once treatment is instituted for hyperthyroidism, the kidney disease is unmasked or made worse when the kidney blood flow returns to normal. Sometimes it is necessary to find a happy medium between treating the kidneys and treating the thyroid; thus, monitoring kidney function along with thyroid levels is particularly important during methimazole therapy.

  • Kidney problems can be minimized by starting with a lower dose of methimazole and working up over weeks or months so as not to cause as abrupt a change in kidney blood flow.

  • If kidney problems become significantly worse on methimazole, medication can be discontinued. Approximately 15-22% of cats treated for hyperthyroidism will show kidney disease that was not evident prior to treatment.

  • A study published in the February 15, 2006, Journal of the AVMA by Milner et al. found that when cats with pre-existing renal disease are excused from the group and compared to cats treated with radiotherapy vs. those treated with methimazole, those treated with radiotherapy had a much longer median survival time (four years vs. two years). This may be due to difficulties in regularly medicating cats with oral medication, ultimately leading to periods of incomplete treatment.

Most side effects occur during the first three months of methimazole therapy.

Periodic blood testing to examine the T4 level, white blood cell patterns, kidney function, and liver enzymes should be done. Be sure to ask your veterinarian to review an appropriate schedule for your cat. Side effect potential can be reduced by beginning at a smaller dose and working up to the full therapeutic dose over the first couple of months of therapy.

May be given with or without food. Keep it away from light and store it at room temperature. Do not refrigerate

Transdermal Methimazole

Many cats remain untreated for this ultimately debilitating disease because their owners cannot administer the medication. For many cats, even converting the medication into liquid does not yield a comfortable alternative. Fortunately, another format for methimazole administration has emerged: transdermal gel that is applied to the hairless portion of the inner ear.

Advantages of Transdermal Therapy

  • No need to fight with the cat over oral medication.
  • Decreased potential for intestinal side effects. (No decreased potential for the non-intestinal side effects reviewed above. For those side effects, it does not help to give the medication topically.)

Disadvantages of Transdermal Therapy

  • It may take longer to achieve normal thyroid levels. (In one study, after four weeks of treatment, 82% of the cats on oral methimazole were controlled, but only 67% of the cats on transdermal therapy were controlled.)
  • A compounding pharmacy must custom-make the product.
  • Gloves are necessary to apply the product.

Is there an Alternative to Methimazole?

It seems there are always some cats who cannot take methimazole due to side effects and cannot undergo radiotherapy due to expense. An alternative medication called ipodate (an iodine-based radiographic contrast agent) has been investigated. This medication inhibits the conversion of T4 to T3 (unlike methimazole, which inhibits the production of T4). In a study of 12 hyperthyroid cats, eight responded well to ipodate with no negative side effects. More severe cases of hyperthyroidism tend not to respond. This seems like an attractive alternative, except that nearly as soon as the research was published, the product went off the market, having been surpassed in radiology by more efficient contrast media. A similar compound called iopanoic acid appears to be similar in effect to ipodate but is difficult to obtain. It is available through compounding pharmacies, if at all.

Another approach for cats who cannot take methimazole involves a combination of potassium iodate and a heart medication called propranolol. Potassium iodate blocks the thyroid gland’s uptake of iodine. Propanolol is used to control the heart disease that is common in hyperthyroid cats, and the dose depends on the patient’s heart rate. Side effects of potassium iodate consisted of upset stomach and depression in 13 out of 20 cats and were controlled by reducing the dose. While all cats in this study seemed better with medication, less than 50% achieved normal lab values; furthermore, all the cats in this study took these medications in preparation for thyroid surgery, so long-term follow-up was not conducted. Still, for a cat who cannot tolerate more conventional treatment, this may be a reasonable alternative.

Specifics on protocols for either of the above therapies are outlined in The Options for Treating Hyperthyroidism by Dr. D. Bruyette in the November 2004 issue of Veterinary Medicine.

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