Methimazole, available under various brand names depending on location, is the most common medication prescribed to treat feline hyperthyroidism. This medication has virtually replaced the older medication propylthiouracil, or PTU, because methimazole is effective without the same likelihood of 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.

These medications block the production of T4 and T3. Thyroid hormones are already in the body when medication is started, and are still in play, so it takes three to four weeks before thyroid blood tests 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 your cat's throat, will not reduce in size with treatment and may 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.
• Methimazole controls your pet's thyroid condition. If testing shows problems with poor kidney function, the medication can be discontinued and its effects reversed. 
• 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 will not reliably or cooperatively take oral medication at this frequency, and providing medication in the cat's food is notoriously inconsistent.

• Methimazole is available as a small tablet and, more recently, in a flavored liquid. A compounding pharmacy can also custom-make it into a chewable or a liquid in an assortment of flavors. It can be made into a gel to be administered on the hairless inner surface of the cat's ear (see below).

• Approximately 15% of cats will experience some side effects. 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. Then, medication is 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 your cat cannot tolerate methimazole. The facial itching resolves with anti-itch medication and discontinuing methimazole. However, 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 happen, it necessitates a change in therapy.
  
  
• Hyperthyroidism increases blood flow to the kidneys. When the T4 level is lowered with the successful treatment of hyperthyroidism, kidney function values may decrease. It is important to remember that the hyperthyroid treatment itself has not caused chronic kidney disease; the renal disease was present before treatment and was ‘masked’ by the increased blood flow through the kidneys in untreated hyperthyroidism. It is also important to remember that untreated hyperthyroidism itself contributes to chronic kidney disease. Under-treating hyperthyroidism to make the kidney values look better on paper is not recommended.
  
  
• Caution is recommended in initiating methimazole therapy if your cat’s kidney disease is advanced at the time of diagnosis of hyperthyroidism; in this situation, a lower dose of methimazole that is gradually increased over time, with careful monitoring of the kidney function, may be warranted.
  
  

Radioactive iodine (I131) is the gold standard therapy for treating feline hyperthyroidism. I131 kills the thyroid mass, while methimazole only controls T4 secretion. When a cat is on methimazole, the mass continues to grow; if the cat lives long enough, the mass becomes resistant to methimazole and is unable to control the hyperthyroidism. Also, with time, the mass may transform into a carcinoma. In addition, I131 replaces the need for twice-a-day pilling with methimazole.  

Periodic blood testing should be done to examine the T4 level, white blood cell patterns, kidney function, and liver enzymes. T4 monitoring is not only to ensure the hyperthyroidism is adequately controlled but to make sure unintentional hypothyroidism doesn’t happen (which also accelerates the progression of kidney disease).

Side effects 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. It may be given with or without food. Please 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.