There are two antithyroid drugs that are used to treat hyperthyroidism and Graves’ disease: carbimazole and propylthiouracil. These drugs reduce the amount of thyroid hormone released into the circulation. They are the first choice for treating children with over-active thyroids. Your child will usually be given carbimazole. If they cannot tolerate carbimazole, they will be given propylthiouracil. Usually, a course of antithyroid drugs will last 18 months to three years.

Carbimazole is available in 5mg and 20mg tablets. Propylthiouracil comes in 50mg tablets only. The amount given depends on the child’s weight. Usually, the daily dose is:

  • 0.5-1mg of carbimazole for every kilogram (kg) of their bodyweight, or
  • 5-10mg of propylthiouracil for every kilogram of their bodyweight.

So a child who weighs 40 kg (about 6 st 2lb) could be given 20-40 mg of carbimazole every day or 200-400mg of propylthiouracil every day.

Antithyroid drugs can be used as part of a block and replace (BR) regimen: the anti-thyroid drug blocks the thyroid gland from producing any thyroxine. Levothyroxine is then given to your child to replace their natural thyroxine.

Side effects

If children are taking too much carbimazole, they may get some of the symptoms of an under-active thyroid. This is why it is important to have regular blood tests to check the thyroid hormone levels.

Some people experience minor side effects to antithyroid drugs, such as nausea or a rash.

There is a very rare side effect of both antithyroid drugs called agranulocytosis that causes the number of white blood cells to drop. This affects the immune system so the body can’t fight infection properly. If your child develops a sore throat, mouth ulcers, or an unexplained fever, stop giving them the tablets immediately. Get their blood count checked urgently either through your GP or your local Accident and Emergency department. It will usually be a false alarm and your child will be able to re-start their medication.

Very rarely serious liver injury has been reported as a side effect of propylthiouracil, especially during the first six months. If you notice any yellowing of the eyes or skin you should take your child to see their doctor immediately.

Top tips:

  • Check with your pharmacist before giving your child over-the-counter drugs – even things that you think are safe such as paracetamol.
  • Some medications can affect the blood test results so it is important to tell your doctor about any other medication you are taking.
  • Store in a cool, dry place, away from direct heat and light, and out of the reach of children.
  • If you’re travelling by sea or air, take your child’s levothyroxine in your hand luggage in the original container.

Antithyroid drugs FAQs