- 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.
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)
|PO||1 wk||4–10 wk||wk|
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
Misc: fever, lymphadenopathy
* CAPITALS indicate life-threatening.
Underline indicate most frequent.
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.
- Noncompliance (Patient/Family/Teaching)
- 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.
- 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.
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