Antithyroid Medicines for Hyperthyroidism


Generic Name Brand Name
methimazole Tapazole
propylthiouracil Propyl-Thyracil or PTU

How It Works

Antithyroid medicines cause your thyroid gland to make less thyroid hormone.

Why It Is Used

Antithyroid medicine works more quickly than radioactive iodine therapy. It also does not permanently damage your thyroid gland.

You may take antithyroid medicine before you have radioactive iodine treatment or surgery in order to bring your metabolism to normal, to make you feel better, or to reduce the chances of more serious problems.

You may also take antithyroid medicine if you have Graves' ophthalmopathy and are going to be treated with radioactive iodine therapy. If you take antithyroid medicine before you have radioactive iodine treatment, it may prevent your Graves' ophthalmopathy from getting worse.

How Well It Works

Antithyroid medicines do not always start working right away. Symptoms usually get better or go away 1 to 8 weeks after you start taking the medicine. It may take as long as 6 months for your thyroid hormone levels to become normal.

Antithyroid medicines work best if you have mild hyperthyroidism, if this is the first time you are being treated for Graves' disease, if you are younger than 50, or if your thyroid gland is only swollen a little bit (small goiter).

If your hyperthyroidism comes back after you have stopped taking the medicine, you can try taking antithyroid medicine again. But your doctor may recommend radioactive iodine treatment, because radioactive iodine is more likely to permanently cure your hyperthyroidism.

In some cases, one type of antithyroid medicine works better than the other.

  • Methimazole is chosen most often because it can be given once a day, has fewer side effects, and costs less.
  • If you are pregnant, propylthiouracil may be safer than methimazole.

Side Effects

All medicines have side effects. But many people don't feel the side effects, or they are able to deal with them. Ask your pharmacist about the side effects of each medicine you take. Side effects are also listed in the information that comes with your medicine.

Here are some important things to think about:

  • Usually the benefits of the medicine are more important than any minor side effects.
  • Side effects may go away after you take the medicine for a while.
  • If side effects still bother you and you wonder if you should keep taking the medicine, call your doctor. He or she may be able to lower your dose or change your medicine. Do not suddenly quit taking your medicine unless your doctor tells you to.

Call or other emergency services right away if you have:

Call your doctor if you have:

  • Hives.
  • Rash and itching.
  • Vomiting.
  • Liver problems (hepatitis). Signs of liver problems are yellow eyes or skin, dark urine, severe tiredness, or pain in your belly.
  • Low white blood cell count, which can make it hard for your body to fight infection. If you have a low white blood cell count, you may become sick easily and have symptoms such as fever, chills, and a sore throat.

Common side effects of this medicine include:

  • Joint aches.
  • Changes in taste.
  • Nausea.

See Drug Reference for a full list of side effects. (Drug Reference is not available in all systems.)

What To Think About

It is very important to take antithyroid medicine at the same time every day.

If the hyperthyroidism comes back, it is most likely to come back within 6 months. Be sure to have regular check-ups with your doctor.

Your doctor will have to check your thyroid hormone levels frequently to make sure you are taking the right amount of medicine. If your thyroid hormone levels are too low, your doctor may prescribe a small amount of thyroid medicine to take along with your antithyroid medicine.

Children may be hard to treat with antithyroid medicine, because they grow so fast and it is hard to know how much medicine to give them.

Taking medicine

Medicine is one of the many tools your doctor has to treat a health problem. Taking medicine as your doctor suggests will improve your health and may prevent future problems. If you don't take your medicines properly, you may be putting your health (and perhaps your life) at risk.

There are many reasons why people have trouble taking their medicine. But in most cases, there is something you can do. For suggestions on how to work around common problems, see the topic Taking Medicines as Prescribed.

Advice for women

If you are pregnant, breast-feeding, or planning to get pregnant, do not use any medicines unless your doctor tells you to. Some medicines can harm your baby. This includes prescription and over-the-counter medicines, vitamins, herbs, and supplements. And make sure that all your doctors know that you are pregnant, breast-feeding, or planning to get pregnant.

If you take this medicine while you are pregnant, your doctor will recommend that you take the smallest effective dose. After your baby is born, you can safely breast-feed while taking antithyroid medicine.


Follow-up care is a key part of your treatment and safety. Be sure to make and go to all appointments, and call your doctor if you are having problems. It's also a good idea to know your test results and keep a list of the medicines you take.

Complete the new medication information form (PDF)(What is a PDF document?) to help you understand this medication.


ByHealthwise Staff
Primary Medical ReviewerE. Gregory Thompson, MD - Internal Medicine
Specialist Medical ReviewerDavid C.W. Lau, MD, PhD, FRCPC - Endocrinology

Current as ofNovember 14, 2014

WebMD Medical Reference from Healthwise
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