Also found in: Dictionary.
Pharmacologic class: Thiomidazole derivative
Therapeutic class: Antithyroid drug
Pregnancy risk category D
Directly interferes with thyroid synthesis by preventing iodine from combining with thyroglobulin, leading to decreased thyroid hormone levels
Tablets: 5 mg, 10 mg
Indications and dosages
➣ Mild hyperthyroidism
Adults and adolescents: Initially, 15 mg P.O. daily in three equally divided doses at approximately 8-hour intervals. For maintenance, 5 to 15 mg/day in equally divided doses at approximately 8-hour intervals.
Children: Initially, 0.4 mg/kg/day in three divided doses at 8-hour intervals. For maintenance, approximately 0.2 mg/kg/day in three divided doses at 8-hour intervals.
➣ Moderate hyperthyroidism
Adults and adolescents: Initially, 30 to 40 mg P.O. daily in three equally divided doses at approximately 8-hour intervals. For maintenance, 5 to 15 mg/day in three equally divided doses at approximately 8-hour intervals.
Children: 0.4 mg/kg/day P.O. as a single dose or in divided doses at 8-hour intervals. For maintenance, approximately 0.2 mg/kg/day as a single dose or in three divided doses at 8-hour intervals.
➣ Severe hyperthyroidism
Adults and adolescents: Initially, 60 mg/day P.O. in three equally divided doses at approximately 8-hour intervals. For maintenance, 5 to 15 mg/day in three equally divided doses at approximately 8-hour intervals.
Children: Initially, 0.4 mg/kg/day P.O. as a single dose or in three divided doses at 8-hour intervals. For maintenance, approximately 0.2 mg/kg/day as a single dose or in three divided doses at 8-hour intervals.
• Hypersensitivity to drug
Use cautiously in:
• bone marrow depression
• patients older than age 40
• pregnant patients.
• Give with meals as needed to reduce GI upset.
CNS: headache, vertigo, paresthesia, neuritis, depression, neuropathy, CNS stimulation
GI: nausea, vomiting, constipation, epigastric distress, ileus, salivary gland enlargement, dry mouth, anorexia
Hematologic: thrombocytopenia, agranulocytosis, leukopenia, aplastic anemia
Hepatic: jaundice, hepatic dysfunction, hepatitis
Musculoskeletal: joint pain, myalgia
Skin: rash, urticaria, skin discoloration, pruritus, erythema nodosum, exfoliative dermatitis, abnormal hair loss
Other: fever, lymphadenopathy, lupuslike syndrome
Drug-drug. Aminophylline, oxtriphylline, theophylline: decreased clearance of both drugs
Amiodarone, iodine, potassium iodide: decreased response to methimazole
Anticoagulants: altered requirements for both drugs
Beta-adrenergic blockers: altered beta blocker clearance
Digoxin: increased digoxin blood level
Drug-diagnostic tests. Granulocytes, hemoglobin, platelets, white blood cells: decreased values
• Check for agranulocytosis in patients older than age 40 and in those receiving more than 40 mg/day.
• Assess hematologic studies. Agranulocytosis usually occurs within first 2 months of therapy and is rare after 4 months.
• Monitor thyroid function tests periodically. Once hyperthyroidism is controlled, elevated thyroid-stimulating factor indicates need for dosage decrease.
• Assess liver function tests and check for signs and symptoms of hepatic dysfunction.
• Monitor patient for fever, sore throat, and other evidence of infection as well as for unusual bleeding or bruising.
• Assess patient for signs and symptoms of hypothyroidism, such as hard edema of subcutaneous tissue, drowsiness, slow mentation, dryness or loss of hair, decreased temperature, hoarseness, and muscle weakness.
• Tell patient to take with meals if GI upset occurs.
• Advise patient to take exactly as prescribed to maintain constant blood level.
• Tell patient to report rash, fever, sore throat, unusual bleeding or bruising, headache, rash, yellowing of skin or eyes, abdominal pain, vomiting, or flulike symptoms.
• Caution female patient not to breastfeed while taking drug.
• As appropriate, review all other significant and life-threatening adverse reactions and interactions, especially those related to the drugs and tests mentioned above.
ClassificationTherapeutic: antithyroid agents
Time/action profile (effect on thyroid function)
|PO||1 wk||4–10 wk||wk|
Adverse Reactions/Side Effects
Central nervous system
- loss of taste
- rash (most frequent)
- skin discoloration
- agranulocytosis (life-threatening)
Drug-Drug interactionAdditive bone marrow depression with antineoplastics or radiation therapy.Antithyroid effect may be ↓ by potassium iodide or amiodarone.↑ risk of agranulocytosis with phenothiazines.May alter response to warfarin and digoxin.
Availability (generic available)
- Monitor response for symptoms of hyperthyroidism or thyrotoxicosis (tachycardia, palpitations, nervousness, insomnia, fever, diaphoresis, heat intolerance, tremors, weight loss, diarrhea).
- Assess for development of hypothyroidism (intolerance to cold, constipation, dry skin, headache, listlessness, tiredness, or weakness). Dose adjustment may be required.
- Assess for skin rash or swelling of cervical lymph nodes. Treatment may be discontinued if this occurs.
- Lab Test Considerations: Monitor thyroid function studies prior to therapy, monthly during initial therapy, and every 2–3 mo during therapy.
- Monitor WBC and differential counts periodically during therapy. Agranulocytosis may develop rapidly; usually occurs during the first 2 mo and is more common in patients over 40 yr and those receiving >40 mg/day. This necessitates discontinuation of therapy.
- May cause ↑ AST, ALT, LDH, alkaline phosphatase, serum bilirubin, and prothrombin time.
Potential Nursing DiagnosesNoncompliance (Patient/Family Teaching)
- Do not confuse methimazole with metolazone.
- Oral: Administer at same time in relation to meals every day. Food may either increase or decrease absorption.
- Instruct patient to take medication as directed, around the clock. Take missed doses as soon as remembered; take both doses together if almost time for next dose; check with health care professional if more than 1 dose is missed. Consult health care professional prior to discontinuing medication.
- Instruct patient to monitor weight 2–3 times weekly. Notify health care professional of significant changes.
- May cause drowsiness. Caution patient to avoid driving or other activities requiring alertness until response to medication is known.
- Advise patient to consult health care professional regarding dietary sources of iodine (iodized salt, shellfish).
- Advise patient to report sore throat, fever, chills, headache, malaise, weakness, yellowing of eyes or skin, unusual bleeding or bruising, rash, or symptoms of hyperthyroidism or hypothyroidism promptly.
- Instruct patient to notify health care professional of all Rx or OTC medications, vitamins, or herbal products being taken and to consult with health care professional before taking other medications.
- Advise patient to carry identification describing medication regimen at all times.
- Advise patient to notify health care professional of medication regimen prior to treatment or surgery.
- Emphasize the importance of routine exams to monitor progress and to check for side effects.
- Decrease in severity of symptoms of hyperthyroidism (lowered pulse rate and weight gain).
- Return of thyroid function studies to normal.
- May be used as short-term adjunctive therapy to prepare patient for thyroidectomy or radiation therapy or may be used in treatment of hyperthyroidism. Treatment from 6 mo to several yr may be necessary, usually averaging 1 yr.