Methimazole

Hyperthyroidism (overactive thyroid) is a condition in which your child's thyroid gland makes too much thyroid hormone. Too much thyroid hormone leads to overactivity of your child's metabolism and can cause weight loss, a rapid or irregular heartbeat, sweating, diarrhea, nervousness, irritability, anxiety and decreased school performance.

The majority of hyperthyroidism is caused by Graves' disease, a kind of hyperthyroidism caused by the body's own immune system.

What is methimazole?

Methimazole is an anti-thyroid medicine used to treat hyperthyroidism caused by Graves' disease. Methimazole blocks production of thyroid hormone by the thyroid gland.

You may hear methimazole called by its brand name Tapazole.

Methimazole comes as a tablet.

What do you need to know before giving methimazole to your child?

Tell your child's health-care provider if your child might be pregnant as methimazole should not be given to pregnant people.

During treatment for hyperthyroidism, your child will need blood tests to monitor the level of their thyroid hormones periodically.

How is methimazole given?

• Please take methimazole as prescribed.
• The tablets should be swallowed whole. If you need to, you may split the tablet and mix it with water for administration.
• Methimazole can be given with or without food. Give it with food or milk if it causes an upset stomach.

What should you do if your child misses a dose of methimazole?

• Give the missed dose as soon as you remember.
• If it is almost time for the next dose, skip the missed dose. Give the next dose at the regular time.
• Do not give your child two doses to make up for one missed dose.

How long does it take for methimazole to work?

Methimazole will begin to have its peak effect approximately four to six weeks after starting the medication. Your child's health-care team may send your child for repeat lab work at that time. If there is continuation of symptoms of hyperthyroidism, please contact your health-care team.

What are the possible side effects of methimazole?

Your child may have some of these side effects while they take methimazole. Check with your child's health-care provider if your child continues to have any of these side effects and they do not go away, or they bother your child:

• skin rash or itching
• changes in taste
• stomach ache, nausea or throwing up
• muscle soreness
• feeling drowsy or sleepy
• feelings of numbness or tingling

Call your child's health-care provider if your child has any of these side effects:

• unusual bleeding or bruising
• pinpoint bleeding into the skin (will look like small red or purplish dots over skin)
• pain in the upper-right side of the abdomen
• sore or swollen joints

Stop methimazole right away AND call your primary care provider or endocrine doctor if your child has any of these side effects:

• high fever (greater than 39°C)
• oral ulcers (sores or areas of broken skin in mouth)

Most of the following side effects are not common, but they may be a sign of a serious problem. Call your child's health-care provider right away or take your child to Emergency if your child has any of these side effects:

• swelling of the face, lips or tongue
• wheezing or difficulty breathing
• rash or hives (red raised spots on the skin
• dark urine, yellowing of the skin or eyes (jaundice)
• inability to pass urine, a change in how much urine is passed, or blood in the urine
• coughing up blood
• fast breathing or difficulty breathing

What safety measures should you take when your child is using methimazole?

There are some medicines that should not be taken together with methimazole. In some cases, the dose of methimazole or the other medicine may need to be adjusted, or the schedule for giving the medicine(s) may need to be adjusted. It is important that you tell your health-care provider and pharmacist if your child takes any other medications (prescription, over the counter or herbal) including:

• warfarin (Coumadin)
• medications that can suppress the bone marrow (e.g., Clozapine)
• prednisolone (Pediapred)

What other important information should you know about methimazole?

• After two years of treatment with methimazole (or if you experience complications), your child's health-care provider may suggest that your child try stopping the medication. About 30% of children are able to successfully remain off methimazole, while the remainder require more permanent treatment. It is difficult to predict in advance who will be able to come off medication.
• Keep a list of all medications your child is on and show the list to the health-care provider or pharmacist.
• Do not share your child's medicine with others, and do not give anyone else's medicine to your child.
• Make sure you always have enough methimazole to last through weekends, holidays and vacations. Call your pharmacy at least two days before your child runs out of medicine to order refills.
• Do not keep any medications that are out of date. Check with your pharmacist about the best way to throw away outdated or leftover medications.

Keep methimazole out of your child’s sight and reach and locked up in a safe place. If your child takes too much methimazole, call your local Poison Information Centre at one of these numbers. These calls are free. 

• Call 1-844 POISON-X, or 1-844-764-7669, from anywhere in Canada except Quebec.
• Call 1-800-463-5060 if you live in Quebec.