Overview of Antithyroid Drugs

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When a thyroid gland becomes overactive—a condition known as hyperthyroidism—the gland produces too much thyroid hormone. This, in turn, can cause a variety of symptoms, including rapid pulse, high blood pressure, anxiety, insomnia, tremor, weight loss, diarrhea, and many of signs of an overactive metabolism, and overactive glands and organs.

A key treatment for hyperthyroidism is a category of prescription medications known as antithyroid drugs. Antithyroid drugs are also referred to as thioamides. They are drugs that are used to treat an overactive thyroid including when the thyroid is overactive as a result of Graves' disease, toxic multinodular goiter, or toxic nodules. 

Antithyroid drugs work by blocking the thyroid gland's ability to produce two key thyroid hormones thyroxine (T4) and triiodothyronine (T3). 

The key antithyroid drugs in the United States are available methimazole and propylthiouracil (PTU). Outside the United States, the choices are methimazole, carbimazole, and PTU.


The antithyroid drug methimazole inhibits the thyroid from using iodine -- usually from the diet or supplements -- to produce thyroid hormone. It is a medication that is generally taken once a day by patients.

Methimazole is sometimes referred to by the name thiamazole. This drug is used around the world and is the primary antithyroid drug used in the United States, where it is available under the brand name Tapazole.

Several manufacturers also offer a generic form of methimazole.


Carbimazole is a drug that, when administered, converts into the drug methimazole in the body, and is therefore quite similar in action. The most well-known brand of carbimazole is Neomercazole.

Neomercazole is a trademarked brand name of carbimazole and is manufactured by Amdipharm International (UK). The drug was authorized in 2004 and is sold in the UK and Europe, but it is not available in or prescribed in the United States. You can see a datasheet on Neomercazole here. Here are several additional resources regarding Neomercazole.

Neomercazole Ingredients: carbimazole Ph.Eur; lactose; maize starch; sucrose; magnesium stearate; sucrose; talc; gum acacia; ferric oxide; propyl hydroxybenzoate; and gelatin.

Available Strengths: 5 mg

Other Brands/Names: Carbimazole is available in some European and Asian countries under the following brand names: Atirozidina, Basolest, CG1, Carbimazole Spofa, Carbotiroid, Mertiran, Neo-Mercazole, Neo-Thyreostat, Neo-Tireol, and Tyrazol.


The way PTU slows the thyroid's overproduction of thyroid hormone is that it inhibits the thyroid gland from using iodine to produce thyroid hormone. It also inhibits the conversion of thyroid hormone T4 into T3. PTU has a short-acting timespan, so patients taking this medication usually are instructed to take the medication two to three times per the day to effectively lower thyroid hormone levels.

PTU has more side effects than the other drugs discussed here, and it is the preferred drug for hyperthyroidism only in a few situations (during early pregnancy, severe thyroid storm and in the event a patient is experiencing serious side effects from methimazole).

In the United States, only generic PTU is available; there are no brand names of PTU marketed in the United States. A number of manufacturers produce generic PTU.

More Information

Learn more about hyperthyroidism and its treatments, including antithyroid drugs, in these resources:

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Article Sources
  • Daily Med, U.S. Food and Drug Administration medication Database, Online
  • FDA Drug Safety Communication: New Boxed Warning on severe liver injury with propylthiouracil, April 21, 2010, Online
  • Ross, Douglas MD, "Patient information: Antithyroid drugs," UpToDate. Last updated: November 13, 2009