[me-thim-uh-zohl, -thahy-muh-]

Methimazole (also known as Tapazole) is an antithyroid drug similar in action to propylthiouracil, and part of the thioamide group.


Methimazole is a drug used to treat hyperthyroidism, a condition that usually occurs when the thyroid gland is producing too much thyroid hormone. It is also taken before thyroid surgery or radioactive iodine therapy, to lower thyroid hormone levels and minimize the effects of thyroid manipulation.

Mechanism of action

Thioamides inhibit many steps in the synthesis of thyroid hormones, including the addition of iodide to thyroglobulin by the enzyme thyroperoxidase, a necessary step in the synthesis of thyroxine.

Notably, they do not inhibit the action of the sodium-dependent iodide transporter located on follicular cells' basolateral membranes. Inhibition of this step requires competitive inhibitors such as perchlorate and thiocyanate.

Adverse effects

The chance of side effects from methimazole are about 5%. It is important to monitor any symptoms of fever or sore throat while taking methimazole; this could indicate the development of agranulocytosis, an uncommon but severe side effect resulting from a drop in the white blood cell count (specifically, neutropenia, a deficiency of neutrophils). A complete blood count (CBC) with differential is performed to confirm the suspicion, in which case the drug is discontinued. Administration of recombinant human granulocyte colony-stimulating factor (rhG-CSF) may increase recovery.

Other side effects include

  • skin rash
  • itching
  • abnormal hair loss
  • upset stomach
  • vomiting
  • loss of taste
  • abnormal sensations (tingling, prickling, burning, tightness, and pulling)
  • swelling
  • joint and muscle pain
  • drowsiness
  • dizziness
  • decreased white blood cells
  • decreased platelets

Drug interactions

Adverse effects may occur for individuals who:

  • Take anticoagulants ('blood thinners') such as warfarin (Coumadin), beta blockers such as propranolol (Inderal), diabetes medications, digoxin (Lanoxin), theophylline (Theobid, Theo-Dur), and vitamins.
  • Have ever had any blood disease, such as decreased white blood cells (leukopenia), decreased platelets (thrombocytopenia), or aplastic anemia, or liver disease (hepatitis, jaundice).
  • Pregnant, or going to become pregnant, or are breast-feeding. An alternative anti-thyroid drug, propylthiouracil is often substituted during pregnancy or breast-feeding. If pregnancy occurs while taking methimazole, switching to propylthiouracil may be an alternative. Early studies suggested that methimazole may harm the fetus, although more recent studies suggest this may not be the case.
  • Are going to have surgery, including dental surgery.

See also

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