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Methimazole

Pronunciation

Dosage Form: tablet

Methimazole Description

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, corn starch, and talc.

The molecular weight is 114.17, and the molecular formula is C4H6N2S. The structural formula is as follows:

Methimazole - Clinical Pharmacology

Methimazole inhibits the synthesis of thyroid hormones and thus is effective in the treatment of hyperthyroidism. The drug does not inactivate existing thyroxine and triiodothyronine that are stored in the thyroid or circulating in the blood nor does it interfere with the effectiveness of thyroid hormones given by mouth or by injection.

Methimazole is readily absorbed in the gastrointestinal tract, metabolized in the liver, and excreted in the urine.

Indications and Usage for Methimazole

Methimazole is indicated:

In patients with Graves’ disease with hyperthyroidism or toxic multinodular goiter for whom surgery or radioactive iodine therapy is not an appropriate treatment option
To ameliorate symptoms of hyperthyroidism in preparation for thyroidectomy or radioactive iodine therapy

Contraindications

Methimazole is contraindicated in the presence of hypersensitivity to the drug or any of the other product components.

Warnings

First Trimester Use of Methimazole and Congenital Malformations

Methimazole crosses the placental membranes and can cause fetal harm when administered in the first trimester of pregnancy. Rare instances of congenital defects, including aplasia cutis, craniofacial malformations (facial dysmorphism; choanal atresia), gastrointestinal malformations (esophageal atresia with or without tracheoesophageal fistula), omphalocele and abnormalities of the omphalomesenteric duct have occurred in infants born to mothers who received Methimazole in the first trimester of pregnancy. If Methimazole is used during pregnancy or if the patient becomes pregnant while taking this drug, the patient should be warned of the potential hazard to the fetus.

Because of the risk for congenital malformations associated with use of Methimazole in the first trimester of pregnancy, it may be appropriate to use other agents in pregnant women requiring treatment for hyperthyroidism. If Methimazole is used, the lowest possible dose to control the maternal disease should be given.

Agranulocytosis

Agranulocytosis is a potentially life-threatening adverse reaction of Methimazole therapy. Patients should be instructed to immediately report to their physicians any symptoms suggestive of agranulocytosis, such as fever or sore throat. Leukopenia, thrombocytopenia, and aplastic anemia (pancytopenia) may also occur. The drug should be discontinued in the presence of agranulocytosis, aplastic anemia (pancytopenia), ANCA-positive vasculitis, hepatitis, or exfoliative dermatitis, and the patient’s bone marrow indices should be monitored.

Liver Toxicity

Although there have been reports of hepatotoxicity (including acute liver failure) associated with Methimazole, the risk of hepatotoxicity appears to be less with Methimazole than with propylthiouracil, especially in the pediatric population. Symptoms suggestive of hepatic dysfunction (anorexia, pruritus, right upper quadrant pain, etc.) should prompt evaluation of liver function (bilirubin, alkaline phosphatase) and hepatocellular integrity (ALT, AST). Drug treatment should be discontinued promptly in the event of clinically significant evidence of liver abnormality including hepatic transaminase values exceeding 3 times the upper limit of normal.

Hypothyroidism

Methimazole 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 (see PRECAUTIONS, Pregnancy).

Precautions

General

Patients who receive Methimazole should be under close surveillance and should be cautioned to report immediately any evidence of illness, particularly sore throat, skin eruptions, fever, headache, or general malaise. In such cases, white-blood-cell and differential counts should be obtained to determine whether agranulocytosis has developed. Particular care should be exercised with patients who are receiving additional drugs known to cause agranulocytosis.

Information for Patients

Patients should be advised that if they become pregnant or intend to become pregnant while taking an antithyroid drug, they should contact their physician immediately about their therapy.

Laboratory Tests

Because Methimazole may cause hypoprothrombinemia and bleeding, prothrombin time should be monitored during therapy with the drug, especially before surgical procedures. Thyroid function tests should be monitored periodically during therapy. Once clinical evidence of hyperthyroidism has resolved, the finding of a rising serum TSH indicates that a lower maintenance dose of Methimazole should be employed.

Drug Interactions

Anticoagulants (oral)

Due to potential inhibition of vitamin K activity by Methimazole, the activity of oral anticoagulants (e.g., warfarin) may be increased; additional monitoring of PT/INR should be considered, especially before surgical procedures.

ß-adrenergic blocking agents

Hyperthyroidism may cause an increased clearance of beta blockers with a high extraction ratio. A dose reduction of beta-adrenergic blockers may be needed when a hyperthyroid patient becomes euthyroid.

Digitalis glycosides

Serum digitalis levels may be increased when hyperthyroid patients on a stable digitalis glycoside regimen become euthyroid; a reduced dosage of digitalis glycosides may be needed.

Theophylline

Theophylline clearance may decrease when hyperthyroid patients on a stable theophylline regimen become euthyroid; a reduced dose of theophylline may be needed.

Carcinogenesis, Mutagenesis, Impairment of Fertility

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.

Pregnancy
Pregnancy Category D

(See WARNINGS)

If Methimazole is used during the first trimester of pregnancy or if the patient becomes pregnant while taking this drug, the patient should be warned of the potential hazard to the fetus.

In pregnant women with untreated or inadequately treated Graves’ disease, there is an increased risk of adverse events of maternal heart failure, spontaneous abortion, preterm birth, stillbirth and fetal or neonatal hyperthyroidism.

Because Methimazole crosses placental membranes and can induce goiter and cretinism in the developing fetus, hyperthyroidism should be closely monitored in pregnant women and treatment adjusted such that a sufficient, but not excessive, dose be given during pregnancy. In many pregnant women, the thyroid dysfunction diminishes as the pregnancy proceeds; consequently, a reduction of dosage may be possible. In some instances, anti-thyroid therapy can be discontinued several weeks or months before delivery.

Due to the rare occurrence of congenital malformations associated with Methimazole use, it may be appropriate to use an alternative anti-thyroid medication in pregnant women requiring treatment for hyperthyroidism, particularly in the first trimester of pregnancy during organogenesis.

Given the potential maternal adverse effects of propylthiouracil (e.g., hepatotoxicity), it may be preferable to switch from propylthiouracil to Methimazole for the second and third trimesters.

Nursing Mothers

Methimazole is present in breast milk. However, several studies found no effect on clinical status in nursing infants of mothers taking Methimazole. A long-term study of 139 thyrotoxic lactating mothers and their infants failed to demonstrate toxicity in infants who are nursed by mothers receiving treatment with Methimazole. Monitor thyroid function at frequent (weekly or biweekly) intervals.

Pediatric Use

Because of postmarketing reports of severe liver injury in pediatric patients treated with propylthiouracil, Methimazole is the preferred choice when an antithyroid drug is required for a pediatric patient. (See DOSAGE AND ADMINISTRATION.)

Adverse Reactions

Major adverse reactions (which occur with much less frequency than the minor adverse reactions) include inhibition of myelopoieses (agranulocytosis, granulocytopenia, thrombocytopenia, and aplastic anemia), drug fever, a lupus-like syndrome, insulin autoimmune syndrome (which can result in hypoglycemic coma), hepatitis (jaundice may persist for several weeks after discontinuation of the drug), periarteritis, and hypoprothrombinemia. Nephritis occurs very rarely.

Minor 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.

To report SUSPECTED ADVERSE REACTIONS, contact Par Pharmaceutical at 1-866-828-9393 or FDA at 1-800-FDA-1088 or www.fda.gov/medwatch.

Overdosage

Signs and Symptoms

Symptoms may include nausea, vomiting, epigastric distress, headache, fever, joint pain, pruritus, and edema. Aplastic anemia (pancytopenia) or agranulocytosis may be manifested in hours to days. Less frequent events are hepatitis, nephrotic syndrome, exfoliative dermatitis, neuropathies, and CNS stimulation or depression.

No information is available on the median lethal dose of the drug or the concentration of Methimazole in biologic fluids associated with toxicity and/or death.

Treatment

To obtain up-to-date information about the treatment of overdose, a good resource is your certified Regional Poison Control Center. In managing overdosage, consider the possibility of multiple drug overdoses, interaction among drugs, and unusual drug kinetics in the patient.

In the event of an overdose, appropriate supportive treatment should be initiated as dictated by the patient’s medical status.

Methimazole Dosage and Administration

Methimazole is administered orally. The total daily dosage is usually given in 3 divided doses at approximately 8-hour intervals.

Adults

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.

Pediatric

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.

How is Methimazole Supplied

Methimazole Tablets, USP 5 mg - white to off-white, round, flat-faced, bevelled-edged tablets, scored with “EM/5” on one side and plain on the other.

They are available in:

Bottles of 100         NDC 49884-640-01

Bottles of 500         NDC 49884-640-05

Methimazole Tablets, USP 10 mg - white to off-white, round, flat-faced, bevelled-edged tablets, scored with “EM/10” on one side and plain on the other.

They are available in:

Bottles of 100         NDC 49884-641-01

Bottles of 500         NDC 49884-641-05

Store at controlled room temperature 15° to 30°C (59° to 86°F).

Dispense in tight, light-resistant container.

Manufactured and distributed by:
Par Pharmaceutical
Chestnut Ridge, NY 10977

Revised: 01/16

OS640-01-1-11

PRINCIPAL DISPLAY PANEL - 5 mg

NDC 49884-640-01

Methimazole
Tablets, USP
5 mg

Rx only

100 Tablets

Each tablet contains:
Methimazole ................................ 5 mg

USUAL DOSAGE:
See package insert for dosage information.

WARNING: This drug may cause toxic
reactions. If such reactions occur,
discontinue the drug. Constant
supervision of patient is essential.

KEEP THIS AND ALL DRUGS
OUT OF REACH OF CHILDREN.

Dispense in a tight, light-resistant
container. Keep tightly closed.

Store at controlled room
temperature 15° to 30°C (59° to 86°F).

R12/15          LA640-01-1-05

Manufactured and distributed by:
Par Pharmaceutical
Chestnut Ridge, NY 10977

PRINCIPAL DISPLAY PANEL - 10 mg

NDC 49884-261-01

Methimazole
Tablets, USP
10 mg

Rx only

100 Tablets

Each tablet contains:
Methimazole ................................ 10 mg

USUAL DOSAGE:
See package insert for dosage information.

WARNING: This drug may cause toxic
reactions. If such reactions occur,
discontinue the drug. Constant
supervision of patient is essential.

KEEP THIS AND ALL DRUGS
OUT OF REACH OF CHILDREN.

Dispense in a tight, light-resistant
container. Keep tightly closed.

Store at controlled room
temperature 15° to 30°C (59° to 86°F).

R12/15          LA641-01-1-05

Manufactured and distributed by:
Par Pharmaceutical
Chestnut Ridge, NY 10977

Methimazole 
Methimazole tablet
Product Information
Product Type HUMAN PRESCRIPTION DRUG LABEL Item Code (Source) NDC:49884-640
Route of Administration ORAL DEA Schedule     
Active Ingredient/Active Moiety
Ingredient Name Basis of Strength Strength
Methimazole (Methimazole) Methimazole 5 mg
Inactive Ingredients
Ingredient Name Strength
LACTOSE MONOHYDRATE  
MAGNESIUM STEARATE  
STARCH, CORN  
TALC  
Product Characteristics
Color WHITE (off-white) Score 2 pieces
Shape ROUND Size 6mm
Flavor Imprint Code EM;5
Contains         
Packaging
# Item Code Package Description
1 NDC:49884-640-01 100 TABLET in 1 BOTTLE
2 NDC:49884-640-05 500 TABLET in 1 BOTTLE
Marketing Information
Marketing Category Application Number or Monograph Citation Marketing Start Date Marketing End Date
ANDA ANDA040350 02/28/2012
Methimazole 
Methimazole tablet
Product Information
Product Type HUMAN PRESCRIPTION DRUG LABEL Item Code (Source) NDC:49884-641
Route of Administration ORAL DEA Schedule     
Active Ingredient/Active Moiety
Ingredient Name Basis of Strength Strength
Methimazole (Methimazole) Methimazole 10 mg
Inactive Ingredients
Ingredient Name Strength
LACTOSE MONOHYDRATE  
MAGNESIUM STEARATE  
STARCH, CORN  
TALC  
Product Characteristics
Color WHITE (off-white) Score 2 pieces
Shape ROUND Size 8mm
Flavor Imprint Code EM;10
Contains         
Packaging
# Item Code Package Description
1 NDC:49884-641-01 100 TABLET in 1 BOTTLE
2 NDC:49884-641-05 500 TABLET in 1 BOTTLE
Marketing Information
Marketing Category Application Number or Monograph Citation Marketing Start Date Marketing End Date
ANDA ANDA040350 09/28/2015
Labeler - Par Pharmaceutical (092733690)
Revised: 02/2016
 
Par Pharmaceutical
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