methimazole, Tapazole

  • Pharmacy Author:
    Omudhome Ogbru, PharmD

    Dr. Ogbru received his Doctorate in Pharmacy from the University of the Pacific School of Pharmacy in 1995. He completed a Pharmacy Practice Residency at the University of Arizona/University Medical Center in 1996. He was a Professor of Pharmacy Practice and a Regional Clerkship Coordinator for the University of the Pacific School of Pharmacy from 1996-99.

  • Medical and Pharmacy Editor: Jay W. Marks, MD
    Jay W. Marks, MD

    Jay W. Marks, MD

    Jay W. Marks, MD, is a board-certified internist and gastroenterologist. He graduated from Yale University School of Medicine and trained in internal medicine and gastroenterology at UCLA/Cedars-Sinai Medical Center in Los Angeles.

  • Medical Editor: William C. Shiel Jr., MD, FACP, FACR
    William C. Shiel Jr., MD, FACP, FACR

    William C. Shiel Jr., MD, FACP, FACR

    Dr. Shiel received a Bachelor of Science degree with honors from the University of Notre Dame. There he was involved in research in radiation biology and received the Huisking Scholarship. After graduating from St. Louis University School of Medicine, he completed his Internal Medicine residency and Rheumatology fellowship at the University of California, Irvine. He is board-certified in Internal Medicine and Rheumatology.

What is methimazole, and how does it work (mechanism of action)?

Methimazole is used to treat an overactive thyroid gland (hyperthyroidism). Grave's disease is the most common cause of hyperthyroidism. Grave's disease is an autoimmune disease resulting from antibodies that attach to receptors on thyroid hormone-producing cells in the thyroid gland and trigger overproduction of thyroid hormone. An enzyme (peroxidase) produces thyroid hormones, i.e., thyroxine (T4) and triiodothyronine (T3), by combining iodine with a protein called thyroglobulin. Methimazole prevents iodine and peroxidase from their normal interactions with thyroglobulin to form T4 and T3. This action decreases thyroid hormone production. Methimazole also interferes with the conversion of T4 to T3. Since T3 is more potent than T4, this also reduces the activity of thyroid hormones. The FDA approved methimazole in March 1999.

What brand names are available for methimazole?

Tapazole, Northyx (discontinued)

Is methimazole available as a generic drug?

Yes

Do I need a prescription for methimazole?

Yes

What are the side effects of methimazole?

Methimazole is generally well-tolerated with side effects occurring in 3 out of every 100 patients. The most common side effects are related to the skin and include: 

Other common side effects are:

Less common but more serious side effects include a decrease in white blood cells (agranulocytosis) and blood platelets (thrombocytopenia). Symptoms and signs of agranulocytosis include infections of the throat, the gastrointestinal tract, and skin with an overall feeling of illness and fever. Since platelets are important for the clotting of blood, thrombocytopenia may lead to problems with excessive bleeding. Hepatitis and death of liver cells (hepatic necrosis) have rarely been associated with methimazole.

Medically Reviewed by a Doctor on 5/11/2017
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