Updated December 16, 2014.

Written or reviewed by a board-certified physician. See About.com's Medical Review Board.

Antithyroid drugs -- also referred to as thionamides -- are medications that treat an overactive thyroid (hyperthyroidism) by blocking the thyroid gland's ability to produce thyroid hormone. In the treatment of an overactive thyroid (which most commonly results from the autoimmune condition Graves' disease), it is critical to slow down the gland's excess production of thyroid hormones, especially the hormones thyroxine (T4) and triiodothyronine (T3).

In addition to treating hyperthyroidism resulting from Graves' disease, antithyroid drugs may also be used to treat hyperthyroidism associated with toxic multinodular goiter, thyrotoxicosis, or a toxic adenoma ("hot nodule"). Antithyroid drugs are sometimes used to treat women with hyperthyroidism during pregnancy as well.

Antithyroid drugs work by making it more difficult for the body to use iodine, thereby blocking the formation of thyroid hormones in the gland.

Antithyroid drugs are one of the key treatments for hyperthyroidism and Graves' disease. The other treatments include radioactive iodine (RAI) ablation, and surgical removal of all or part of the thyroid gland.

In some cases, physicians prescribe antithyroid drugs as a long-term treatment for Graves' disease/hyperthyroidism. Some studies have estimated that approximately 30 percent of Graves' disease patients will have a remission after prolonged treatment with antithyroid drugs. Some physicians only recommend antithyroid drug treatment for a short time, followed by more permanent treatments such as thyroid surgery or RAI.

What Are the Types of Antithyroid Drugs?

Two antithyroid drugs are currently available in the United States: methimazole and propylthiouracil (which is usually abbreviated as PTU). In Europe and Asia, another antithyroid drug, carbimazole, is also available.
Methimazole: Methimazole prevents the thyroid from using iodine to produce thyroid hormone. Methimazole is sometimes also called thiamazole, and is used around the world. It is generally taken once per day. It is available under the brand-name Tapazole in the US, and also as a generic from various manufacturers.

Propylthiouracil (PTU): PTU acts by preventing the thyroid from using iodine to produce thyroid hormone, and also inhibits T4 from being converted into T3. It has a short-acting timespan, and must be taken two to three times per the day to effectively lower thyroid hormone levels. Only generic PTU is available; there are no brand names of PTU marketed in the US, and a number of manufacturers produce generic PTU.

Carbimazole: Carbimazole metabolizes to methimazole in the body. Like methimazole, carbimazole inhibits the thyroid's ability to produce thyroid hormone. The most well-known brand of carbimazole is Neomercazole. Carbimazole is quite similar in action to methimazole.

In the United States, methimazole is considered the preferred antithyroid drug in most situations.

How Should Antithyroid Drugs Be Taken?

Antithyroid drugs work best when you can keep a constant amount in your bloodstream. To maintain that constant level, it's important to take your antithyroid drug dose at the proper times, and if you are taking more than one pill a day, evenly space your doses. Generally, methimazole is taken once a day (or twice a day for those on larger doses), and PTU is taken 3 to 4 times per day, or every 6 to 8 hours.

How Quickly Do Antithyroid Drugs Work?

It's important to note that antithyroid drugs do not block the effects of thyroid hormone that was made by the gland before starting the drug. So generally, even after you begin taking an antithyroid drug, your thyroid will continue to release the hormone it has already formed, causing continued hyperthyroidism symptoms. It can take as many as six to eight weeks, therefore, for elevated thyroid hormones to begin to normalize, and symptoms of hyperthyroidism to subside.

Effectiveness of Antithyroid Drugs

An estimated 25 to 50% of patients go into remission when taking an antithyroid drug for at least six months to a year. This is most likely if...

• You have mild or subclinical hyperthyroidism.
• Your goiter (thyroid enlargment) is small or minimal.
• You are not a smoker.
• You do not have high levels of blocking antibodies.
• You are not a child, teen or young adult .
• You do not have ophthalmopathy.

People who have more serious hyperthyroidism, a large goiter, smoke, have high levels of blocking antibodies, ophthalmopathy, and/or are a child, teenager or young adult have less of a chance of permanent remission on antithyroid drug therapy.
Some studies have shown that remission rates are higher when you take antithyroid drugs for more than 18 to 24 months versus 6 to 12 months, but the findings are still controversial.

While some 30 to 40 percent of patients treated with antithyroid drugs remain in remission 10 years after they stop their drug treatment, an estimated half of the patients who have a remission will also have a recurrence.