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- What is methimazole, and how does it work (mechanism of action)?
- What is methimazole used for?
- What are the side effects of methimazole?
- What is the dosage for methimazole?
- Which drugs or supplements interact with methimazole?
- Is methimazole safe to take if I'm pregnant or breastfeeding?
- What else should I know about methimazole?
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?
Do I need a prescription for methimazole?
What is methimazole used for?
Methimazole is used for treating hyperthyroidism. It also is used for decreasing symptoms of hyperthyroidism in preparation for surgical removal of the thyroid gland or before inactivating the thyroid gland with radioactive iodine. Long-term use of methimazole may lead to a remission of the hyperthyroidism.
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.
What is the dosage for methimazole?
The initial adult dose of methimazole is:
- 15 mg/day for mild hyperthyroidism
- 30-40 mg/day for moderately severe hyperthyroidism
- 60 mg/day for severe hyperthyroidism
The daily dose is divided into three doses administered every 8 hours.
The maintenance dose is 5-15 mg/day.
The usual initial children's dose is 0.4 mg/kg given in 3 divided doses administered 8 hours apart, and the maintenance dose is half the initial dose.
Which drugs or supplements interact with methimazole?
Warfarin (Coumadin) works by reducing the activity of vitamin K and therefore the formation of vitamin K dependent clotting factors. Methimazole may increase the activity of warfarin by further reducing the activity of vitamin K in the body.
Hyperthyroidism increases elimination of beta blockers. Correcting hyperthyroidism will cause less elimination of beta blockers. Therefore, the dose of beta blockers may require reduction when hyperthyroidism is corrected. A similar reaction may also occur with digoxin (Lanoxin) and theophylline (Respbid, Slo-Bid, Theo-24, Theolair).
Is methimazole safe to take if I'm pregnant or breastfeeding?
Methimazole is excreted in breast milk and may potentially cause harm to the infant.
What else should I know about methimazole?
What preparations of methimazole are available?
Tablet: 5, 10 mg
How should I keep methimazole stored?
Methimazole should be stored at room temperature, 15 C and 30 C (59 F and 86 F).
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Related Disease Conditions
There are several types of thyroid disorders including hypothyroidism, hyperthyroidism, goiters, thyroid nodules, and thyroid cancer. Symptoms vary by condition. Diagnosis is made with blood tests, scans, ultrasound, or biopsy. Treatments depend on the disorder and can include medication or surgery.
Hyperthyroidism is an excess of thyroid hormone due to an overactive thyroid gland. Symptoms can include increased heart rate, weight loss, heart palpitations, frequent bowel movements, depression, fatigue, fine or brittle hair, sleep problems, thinning skin, and irregular vaginal bleeding. Graves' disease is the most common cause of hyperthyroidism. Many other health problems or taking excess thyroid hormone medication can cause an overactive thyroid gland. Treatment for the condition is with medication, radioactive iodine, thyroid surgery (rarely), or reducing the dose of thyroid hormone. No diet has been shown to treat hyperthyroidism or its symptoms and signs.
Graves' disease is an autoimmune disease that affects the thyroid. Some of the symptoms of Graves' disease include hand tremors, rapid heartbeat, trouble sleeping, enlarged thyroid, thinning of the skin or fine brittle hair. Causes of Graves' disease are thought to be multifactorial such as genes, gender, stress, and infection. Treatment for Graves' disease is generally medication.
Thyroiditis is the inflammation of the thyroid gland. The inflamed thyroid gland can release an excess of thyroid hormones into the blood stream, resulting in a temporary hyperthyroid state. Some forms of thyroiditis can be diagnosed based on tenderness and enlargement of the thyroid gland. A thyroid scan sometimes is used in making the diagnosis. Thyroiditis can also be diagnosed with a biopsy of the thyroid gland.
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