Side Effects of Methimazole in Cats and How to Use It Safely

Methimazole is a derivative of thionamide and belongs to the pharmacotherapeutic group of antithyroid preparations. This name already allows us to anticipate what its function is, which is none other than the treatment of hyperthyroidism, a disease in which the thyroid gland produces a higher quantity of hormones than normal.

Thus, methimazole works by blocking the synthesis of these hormones. In fact, in just 1-3 weeks it reduces the T4 values. Methimazole for cats is a rapidly absorbed drug that is mainly eliminated in the urine and is completely safe when used correctly with a professional's supervision.

This medication is solely used for hyperthyroidism. Therefore, if your cat is diagnosed with this disease, the veterinarian will prescribe methimazole for the following reasons:

• To stabilize thyroidism before proceeding with thyroid gland removal.
• To treat hyperthyroidism in the long term, keeping in mind that this disease is not curable, to control the cat's condition. For this reason, other options are considered, such as surgical thyroidectomy or treatment with iodine.

It should be noted that hyperthyroidism is more common in elderly cats, is due to benign or malignant thyroid disorders and usually presents in a more or less severe way. A marked weight loss, accompanied by an increase in appetite and thirst, hyperactivity, tachycardia, bad appearance of the coat, diarrhea or vomiting are clinical symptoms that should make us go to the vet. In addition, in cats over seven years of age, veterinary check-ups are recommended at least annually, precisely to detect this type of disease early.

We find several formats of methimazole in coated tablets and oral solution, so we can choose according to the presentation best tolerated by our cat. Both for stabilization before surgical thyroidectomy and for long-term treatment of hyperthyroidism, the starting dose will be the same, that is, about 5 mg per day.

It is recommended to give methimazole to the cat with food and divide the dose into two doses, one in the morning and one in the afternoon, although it can also be administered in a single dose. However, this starting dose is indicative.

Only the vet can decide how much methimazole our cat needs. Always try to give the lowest dose that achieves the desired effect. In any case, more than 20 mg per day is never prescribed. There is also transdermal methimazole, which is a good option for cats that do not accept the drugs orally or cause digestive discomfort. You only have to apply it to the inner area of the ear. Learn more in our article about how to give a cat a pill.

Cats treated with methimazole must have access to a sufficient amount of water 24 hours a day, as this drug can cause concentration of the blood, therefore they must be well hydrated. If it is always important to give our cat only drugs prescribed by the veterinarian. This is especially important in the case of methimazole, as an overdose can kill your cat. In fact, depending on the dose that is prescribed, the cat will have to be monitored and it will always be necessary to carry out regular blood tests.

In addition, before starting treatment, a blood test that includes the measurement of total serum T4 hormone should also be done. The dose is adjusted according to the result of this parameter and its evolution when following the treatment. It is also vital for your veterinarian to make sure that your cat is not allergic to methimazole, as the result can also be fatal in the worst cases.

There are some cats for whom methimazole is not recommended. They are the ones who are in the following situations:

• Systemic diseases such as diabetes or liver failure
• Autoimmune diseases
• Alterations in white blood cells
• Platelet alterations or clotting problem
• Gestation and lactation
• If the cat is taking any other medication and the veterinarian does not know, it must be reported to ensure that no interactions occur. Vaccines also count.
• Of course, it is not suitable for cats with an allergy to this active ingredient

Also, keep in mind that some other cats, such as those with kidney disease, will only be able to use methimazole once the vet carefully weighs the pros and cons of both administering the drug and not, perhpas opting for another method in your cat's specific case. This is due to the fact that methimazole reduces glomerular filtration, hence, if you decide to medicate, it is essential to subject the cat to strict control of its kidney function.

First of all, if we notice discomfort in the cat or fever, we must immediately notify the vet. In these cases, it is necessary to do a blood test for hematological and biochemical analysis. If a problem is diagnosed, the vet will prescribe the appropriate medication to control it.

On the other hand, adverse effects have been reported after the administration of methimazole in cats, although they are usually mild and resolve spontaneously without the need to stop the treatment. In cases where these are more serious, the medication will have to be stopped and an alternative treatment found. However, adverse reactions are considered to be rare. These include the following:

• Vomiting
• Decrease or loss of appetite
• Lethargy
• Intense itching
• Irritations in the head and neck area.
• Eruptions
• Jaundice, which is the yellowing of the skin and mucous membranes. It is associated with a liver problem
• Hematologic abnormalities
• If a high dose is administered, in addition to these signs, the cat may suffer from hypothyroidism, although it is rare

To make sure methimazole for cats is safe for your cat's condition, make sure it is prescribed by a veterinarian that has fully analysed your cat's condition and that has made sure they are not allergic to its active ingredient. Follow their guidelines precisely to avoid overdose or side effects.

This article is purely informative. AnimalWised does not have the authority to prescribe any veterinary treatment or create a diagnosis. We invite you to take your pet to the veterinarian if they are suffering from any condition or pain.

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