Trade Name(s)

  • Tapazole

Ther. Class.
antithyroid agents


  • Palliative treatment of hyperthyroidism.
  • Used as an adjunct to control hyperthyroidism in preparation for thyroidectomy or radioactive iodine therapy.


Inhibits the synthesis of thyroid hormones.

Therapeutic Effect(s):

Decreased signs and symptoms of hyperthyroidism.


Absorption: Rapidly absorbed following oral administration.

Distribution: Crosses the placenta and enters breast milk in high concentrations.

Metabolism and Excretion: Mostly metabolized by the liver; <10% eliminated unchanged by the kidneys.

Half-life: 3–5 hr.

TIME/ACTION PROFILE (effect on thyroid function)

PO1 wk4–10 wkwk


Contraindicated in:

  • Hypersensitivity;
  • Lactation.

Use Cautiously in:

  • Patients with ↓ bone marrow reserve;
  • Patients >40 yr (↑ risk of agranulocytosis);
  • OB: May be used cautiously; however, thyroid problems may occur in the fetus.

Adverse Reactions/Side Effects

CNS: drowsiness, headache, vertigo

GI: HEPATOTOXICITY, diarrhea, loss of taste, nausea, parotitis, vomiting

Derm: rash, skin discoloration, urticaria

Hemat: AGRANULOCYTOSIS, anemia, leukopenia, thrombocytopenia

MS: arthralgia

Misc: fever, lymphadenopathy

* CAPITALS indicate life-threatening.
Underline indicate most frequent.



  • Additive 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.


PO: (Adults) Initial–15–60 mg/day in 3 divided doses. Maintenance–5–15 mg once daily.

PO: Children Initial–400 mcg (0.4 mg)/kg/day in 3 divided doses. Maintenance–200 mcg (0.2 mg)/kg/day in single dose or 2 divided doses.

Availability (generic available)

Tablets: 5 mg, 10 mg


  • 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 Diagnoses


  • Do not confuse methimazole with metolazone or methazolamide.
  • PO: Administer at same time in relation to meals every day. Food may either increase or decrease absorption.

Patient/Family Teaching

  • 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.

Evaluation/Desired Outcomes

  • 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.
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