Trade Name(s)

  • Tapazole Canadian Trade name

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: Concentrated in the thyroid gland.

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 wk1–2 wk


Contraindicated in:

  • Hypersensitivity.

Use Cautiously in:

  • Patients with ↓ bone marrow reserve;
  • Patients >40 yr (↑ risk of agranulocytosis);
  • OB:  Use during pregnancy only if potential maternal benefit justifies potential fetal risk. May cause congenital malformations (especially if used during 1st trimester).

Adverse Reactions/Side Effects

Derm: rash, skin discoloration, urticaria

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

Hemat: AGRANULOCYTOSIS, anemia, leukopenia, thrombocytopenia

MS: arthralgia

Neuro: drowsiness, headache, vertigo

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:  0.4 mg/kg/day in 3 divided doses.   Maintenance:  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 PT.


  • 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.
  • Rep:  Advise female patients of reproductive potential to notify health care professional if pregnancy is planned or suspected or if breastfeeding. May cause fetal harm, especially during 1st trimester. If breastfeeding, monitor infant thyroid levels weekly or biweekly.
  • 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.