Health Library Antithyroid Medications for HyperthyroidismFrom Healthwise

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

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

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

Antithyroid medications do not always start working right away. Usually, symptoms 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 medications 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).

Antithyroid medication may or may not make your hyperthyroidism permanently go away. Hyperthyroidism goes away in about 30% to 50% of people after they take antithyroid medication for 1 or 2 years.¹

If your hyperthyroidism comes back after you have stopped taking the medication, you can try taking antithyroid medication again. However, 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 medication works better than the other.

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

Side effects of antithyroid medications 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 experience 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 medication.

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

If you take antithyroid medication 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 medications 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 medication. If your thyroid hormone levels are too low, your doctor may prescribe a small amount of thyroid medication to take along with your antithyroid medication.

Your hyperthyroidism is most likely to come back (relapse) within 6 months after you start medication, but it can also come back years later.² This makes it very important 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 medication. After your baby is born, you can safely breast-feed while taking antithyroid medications.

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

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

Citations

1. Jameson JL, Weetman AP (2005). Disorders of the thyroid gland. In DL Kasper et al., eds., Harrison's Principles of Internal Medicine, 16th ed., vol. 2, pp. 2104–2127. New York: McGraw-Hill.

2. Cooper DS (2003). Hyperthyroidism. Lancet, 362(9382): 459–468.

Author:	Caroline Rea, RN, BS, MS	Last Updated: December 3, 2007
Medical Review:	Caroline S. Rhoads, MD - Internal Medicine David C.W. Lau, MD, PhD, FRCPC - Endocrinology & Metabolism
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