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Methimazole (the active ingredient in Tapazole) is an effective veterinary medication used for the management of hyperthyroidism in pets, a condition that causes an excessive production of thyroid hormone in the body [1, 2]. While there are two drugs available, methimazole and propylthiouracil, for medical therapy of feline hyperthyroidism, the former is more commonly used because it has a lower rate of adverse side effects compared to the latter [3].
Aside from its use for lowering the thyroid hormone concentration in blood, the medicine may also be administered before a radioactive iodine therapy or surgery of the thyroid gland [5].
Quick Information |
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 Methimazole for Cats |
Generic name: Methimazole
Brand name: Tapazole® [2] Type: Thioureylene anti-thyroid agent [1] Does FDA approve it: Yes [1] Availability: Prescription medication [2] Who can take it: Cats, dogs, humans [2] Forms: 5 mg and 10 mg white round tablets, transdermal gel, cream, or liquid [1, 2] Cost: $0.46 – $0.47 per 5mg tablet |
The production of thyroid hormones, including triiodothyronine (T3) and thyroxine (T4), is initiated by the peroxidase enzyme through the oxidation reaction of tyrosyl groups of thyroglobulin protein and iodide ion. Methimazole interferes with the regular interaction of peroxidase and iodine with thyroglobulin and reduces the formation of thyroid hormones [6, 10].
Talk to a vet to determine your cat’s daily dosage requirements and the best time to administer.
The usually recommended initial dose for cats of all weight is 2.5 mg, given twice at an interval of 8 to 12 hours per day [11]. The maximal daily dosage varies depending on the severity of the condition, from 2.5 mg for a mild case to 5 mg for a moderate case, and 10-15 mg for a severe chronic condition [11].
Blood tests are usually repeated every 28 days until the hormone level reaches a steady state, and the maintenance dosage is achieved [7].
Give the medication as instructed by the vet. As a tablet, it can be administered with or without food. However, you need to check with the vet before dividing the tablet or crushing it to mix with your cat’s food [12]. Make sure you give your pet plenty of water to drink [8].
It may also be provided as an oral liquid [4]. Since methimazole tastes bitter, you may choose the flavored varieties that are compounded in some pharmacies [22].
For the transdermal cream or gel, wear gloves and measure the dose onto your finger before gently applying it on your cat’s skin (usually to the pinna) [4]. Clean the site using a small piece of moist cloth before giving the next dose.
Do not stop applying the cream unless the vet advises you to do so [4].
If you have missed a scheduled dose, give it as soon as you can. However, it is best to skip the dose if the next dose is due within a few hours. Make sure never to provide a double dose [4].
If your cat shows any of the abovementioned signs of overdose, urgently call your vet or the ASPCA Animal Poison Center at 1-888-426-4435.
Although studies have revealed that methimazole can effectively work for 2.3 hours in hyperthyroid cats and 4.7 hours in normal cats, a single dose can actually suppress thyroid hormone levels for about 24 hours [10, 11, 20].
The following common side effects may occur within the first one to three months of treatment:
Other serious side effects may include:
Contact your veterinarian if your cat has any of the above side effects.
Make sure to discuss with your vet of any existing health conditions in your cat. Methimazole is not prescribed for cats that have:
Its use is also not recommended for pregnant and nursing cats because it may be toxic to an embryo and cause birth defects [1].
Be sure to inform the vet if your cat is on any of the following medications:
Homeopathic and herbal remedies, as well as acupuncture, are considered alternatives for a methimazole treatment. However, there is not enough evidence to prove their effectiveness [21].
