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Antithyroid Medicines for Hyperthyroidism

Examples

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

Side effects of antithyroid medicines include:

  • Rash and itching.
  • Joint aches.
  • 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.

Call your doctor right away if you have any signs of side effects. Side effects can be serious. But they usually go away after you stop taking the medicine.

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

What To Think About

If you take antithyroid medicine for a long time, you may develop hypothyroidism, which means your body is making too little thyroid hormone.

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

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.

Your hyperthyroidism is most likely to come back (relapse) within 6 months after you stop taking medicine. But it can also come back years later. Be sure to have regular check-ups with your doctor.

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

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

By Healthwise Staff
Primary Medical Reviewer E. Gregory Thompson, MD - Internal Medicine
Specialist Medical Reviewer David C.W. Lau, MD, PhD, FRCPC - Endocrinology
Last Revised November 4, 2011

WebMD Medical Reference from Healthwise

Last Updated: November 04, 2011
This information is not intended to replace the advice of a doctor. Healthwise disclaims any liability for the decisions you make based on this information.

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