Methimazole

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Methimazole
Adult Indications & Dosage
Pediatric Indications & Dosage
Contraindications
Warnings & Precautions
Adverse Reactions
Drug Interactions
Use in Specific Populations
Administration & Monitoring
Overdosage
Pharmacology
Clinical Studies
How Supplied
Images
Patient Counseling Information
Precautions with Alcohol
Brand Names
Look-Alike Names

Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]; Associate Editor(s)-in-Chief: Vignesh Ponnusamy, M.B.B.S. [2]

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Overview

Methimazole is an antithyroid agent that is FDA approved for the {{{indicationType}}} of hyperthyroidism. Common adverse reactions include skin rash, urticaria, nausea, vomiting, epigastric distress, arthralgia, paresthesia, loss of taste, abnormal loss of hair, myalgia, headache, pruritus, drowsiness, neuritis, edema, vertigo, skin pigmentation, jaundice, sialadenopathy, and lymphadenopathy.

Adult Indications and Dosage

FDA-Labeled Indications and Dosage (Adult)

Hyperthyroidism
  • Methimazole is indicated in the medical treatment of hyperthyroidism. Long-term therapy may lead to remission of the disease.
  • Methimazole is also used when thyroidectomy is contraindicated or not advisable.
  • Methimazole is administered orally. It is usually given in 3 equal doses at approximately 8-hour intervals.
  • The initial daily dosage is 15 mg for mild hyperthyroidism, 30 to 40 mg for moderately severe hyperthyroidism, and 60 mg for severe hyperthyroidism, divided into 3 doses at 8-hour intervals. The maintenance dosage is 5 to 15 mg daily.

Off-Label Use and Dosage (Adult)

Guideline-Supported Use

There is limited information regarding Off-Label Guideline-Supported Use of Methimazole in adult patients.

Non–Guideline-Supported Use

There is limited information regarding Off-Label Non–Guideline-Supported Use of Methimazole in adult patients.

Pediatric Indications and Dosage

FDA-Labeled Indications and Dosage (Pediatric)

Hyperthyroidism
  • Dosing Information
  • Initially, the daily dosage is 0.4 mg/kg of body weight divided into 3 doses and given at 8-hour intervals. The maintenance dosage is approximately 1/2 of the initial dose.

Off-Label Use and Dosage (Pediatric)

Guideline-Supported Use

There is limited information regarding Off-Label Guideline-Supported Use of Methimazole in pediatric patients.

Non–Guideline-Supported Use

Disorder of thyroid gland; Prophylaxis - Toxicity due to radiotherapy; Prophylaxis
  • Dosing Information
  • Methimazole 0.5 milligrams/kilogram body mass in 2 doses.

Contraindications

  • Methimazole is contraindicated in the presence of hypersensitivity to the drug and in nursing mothers because the drug is excreted in milk.

Warnings

  • Congenital Malformations
  • Agranulocytosis
  • Liver Toxicity
  • Hypothyroidism
  • TAPAZOLE can cause hypothyroidism necessitating routine monitoring of TSH and free T4 levels with adjustments in dosing to maintain a euthyroid state. Because the drug readily crosses placental membranes, methimazole can cause fetal goiter and cretinism when administered to a pregnant woman. For this reason, it is important that a sufficient, but not excessive, dose be given during pregnancy.

Precautions

  • General
  • Laboratory Tests

Adverse Reactions

Clinical Trials Experience

Postmarketing Experience

There is limited information regarding Postmarketing Experience of Methimazole in the drug label.

Drug Interactions

  • Anticoagulants (oral)
  • The activity of oral anticoagulants may be potentiated by anti-vitamin-K activity attributed to methimazole.
  • ß-adrenergic blocking agents
  • Digitalis glycosides
  • Theophylline

Use in Specific Populations

Pregnancy

Pregnancy Category (FDA):

  • Pregnancy Category D
  • Methimazole used judiciously is an effective drug in hyperthyroidism complicated by pregnancy. In many pregnant women, the thyroid dysfunction diminishes as the pregnancy proceeds; consequently, a reduction in dosage may be possible. In some instances, use of methimazole can be discontinued 2 or 3 weeks before delivery.


Pregnancy Category (AUS):

  • Australian Drug Evaluation Committee (ADEC) Pregnancy Category

There is no Australian Drug Evaluation Committee (ADEC) guidance on usage of Methimazole in women who are pregnant.

Labor and Delivery

There is no FDA guidance on use of Methimazole during labor and delivery.

Nursing Mothers

  • The drug appears in human breast milk and its use is contraindicated in nursing mothers.

Pediatric Use

  • Because of postmarketing reports of severe liver injury in pediatric patient treated with propylthiouracil, TAPAZOLE is the preferred choice when an antithyroid drug is required for a pediatric patient.

Geriatic Use

There is no FDA guidance on the use of Methimazole with respect to geriatric patients.

Gender

There is no FDA guidance on the use of Methimazole with respect to specific gender populations.

Race

There is no FDA guidance on the use of Methimazole with respect to specific racial populations.

Renal Impairment

There is no FDA guidance on the use of Methimazole in patients with renal impairment.

Hepatic Impairment

There is no FDA guidance on the use of Methimazole in patients with hepatic impairment.

Females of Reproductive Potential and Males

There is no FDA guidance on the use of Methimazole in women of reproductive potentials and males.

Immunocompromised Patients

There is no FDA guidance one the use of Methimazole in patients who are immunocompromised.

Administration and Monitoring

Administration

  • Oral

Monitoring

There is limited information regarding Monitoring of Methimazole in the drug label.

IV Compatibility

There is limited information regarding IV Compatibility of Methimazole in the drug label.

Overdosage

Acute Overdose

Signs and Symptoms

Management

  • To obtain up-to-date information about the treatment of overdose, a good resource is your certified Regional Poison Control Center. Telephone numbers of certified poison control centers are listed in the “Physicians' Desk Reference (PDR)”. In managing overdosage, consider the possibility of multiple drug overdoses, interaction among drugs, and unusual drug kinetics in your patient.
  • Protect the patient's airway and support ventilation and perfusion. Meticulously monitor and maintain, within acceptable limits, the patient's vital signs, blood gases, serum electrolytes, etc. The patient's bone marrow function should be monitored. Absorption of drugs from the gastrointestinal tract may be decreased by giving activated charcoal, which, in many cases, is more effective than emesis or lavage; consider charcoal instead of or in addition to gastric emptying. Repeated doses of charcoal over time may hasten elimination of some drugs that have been absorbed. Safeguard the patient's airway when employing gastric emptying or charcoal.

Chronic Overdose

There is limited information regarding Chronic Overdose of Methimazole in the drug label.

Pharmacology

Methimazole.png
Methimazole1.png
Methimazole
Systematic (IUPAC) name
1-methyl-3H-imidazole-2-thione
Identifiers
CAS number 60-56-0
ATC code H03BB02
PubChem 1349907
DrugBank DB00763
Chemical data
Formula C4H6N2S 
Mol. mass 114.17 g/mol
SMILES eMolecules & PubChem
Physical data
Melt. point 146 °C (295 °F)
Solubility in water 275[1] mg/mL (20 °C)
Pharmacokinetic data
Bioavailability 93%
Protein binding None
Metabolism Hepatic
Half life 5-6 hours
Excretion Renal
Therapeutic considerations
Pregnancy cat.

D (US)

Legal status

Prescription Only (S4)(AU) -only(US)

Routes Oral

Mechanism of Action

  • The actions and use of methimazole are similar to those of propylthiouracil. On a weight basis, the drug is at least 10 times as potent as propylthiouracil, but methimazole may be less consistent in action.

Structure

  • Methimazole (1-methylimidazole-2-thiol) is a white, crystalline substance that is freely soluble in water. It differs chemically from the drugs of the thiouracil series primarily because it has a 5- instead of a 6-membered ring.
  • Each tablet contains 5 or 10 mg (43.8 or 87.6 µmol) methimazole, an orally administered antithyroid drug.
  • Each tablet also contains lactose monohydrate, magnesium stearate, potato starch, and talc.
  • The molecular weight is 114.17, and the molecular formula is C4H6N2S. The structural formula is as follows:
This image is provided by the National Library of Medicine.

Pharmacodynamics

There is limited information regarding Pharmacodynamics of Methimazole in the drug label.

Pharmacokinetics

  • Methimazole is readily absorbed from the gastrointestinal tract. It is metabolized rapidly and requires frequent administration.
  • Methimazole is excreted in the urine.
  • In laboratory animals, various regimens that continuously suppress thyroid function and thereby increase TSH secretion result in thyroid tissue hypertrophy. Under such conditions, the appearance of thyroid and pituitary neoplasms has also been reported. Regimens that have been studied in this regard include antithyroid agents as well as dietary iodine deficiency, subtotal thyroidectomy, implantation of autonomous thyrotropic hormone-secreting pituitary tumors, and administration of chemical goitrogens.

Nonclinical Toxicology

  • In a 2 year study, rats were given methimazole at doses of 0.5, 3, and 18 mg/kg/day. These doses were 0.3, 2, and 12 times the 15 mg/day maximum human maintenance dose (when calculated on the basis of surface area). Thyroid hyperplasia, adenoma, and carcinoma developed in rats at the two higher doses. The clinical significance of these findings is unclear.

Clinical Studies

There is limited information regarding Clinical Studies of Methimazole in the drug label.

How Supplied

  • Methimazole Tablets, USP 5 mg white, round, flat-faced, bevelled-edged tablets with “VM” on one side and scored on the other.
  • They are available in:
  • Bottles of 100 NDC 0677-1945-01
  • Methimazole Tablets, USP 10 mg white, round, flat-faced, bevelled-edged tablets with “XM” on one side and scored on the other.
  • They are available in:
  • Bottles of 100 NDC 0677-1946-01
  • Store at controlled room temperature, 20°- 25°C (68°- 77°F).
  • Dispense in tight, light-resistant container.

Storage

There is limited information regarding Methimazole Storage in the drug label.

Images

Drug Images

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Drug Name:
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Drug Label Author:

This pill image is provided by the National Library of Medicine's PillBox.

Package and Label Display Panel

Methimazole02.png
This image of the FDA label is provided by the National Library of Medicine.
Methimazole03.png
This image of the FDA label is provided by the National Library of Medicine.
Methimazole04.png
This image of the FDA label is provided by the National Library of Medicine.

Patient Counseling Information

There is limited information regarding Patient Counseling Information of Methimazole in the drug label.

Precautions with Alcohol

  • Alcohol-Methimazole interaction has not been established. Talk to your doctor about the effects of taking alcohol with this medication.

Brand Names

Look-Alike Drug Names

  • methimazole® — metolazone®[3]

Drug Shortage Status

Price

References

The contents of this FDA label are provided by the National Library of Medicine.

  1. http://www.drugbank.ca/drugs/DB00763
  2. "TAPAZOLE- methimazole tablet".
  3. "http://www.ismp.org". External link in |title= (help)

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