methimazole

Absorption: Rapidly absorbed following oral administration.

Distribution: Concentrated in the thyroid gland.

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

Half-Life: 3–5 hr.

Time/Action Profile ⬆ ⬇

( effect on thyroid function)

ROUTE		ONSET		PEAK		DURATION
PO		1 wk		4–10 wk		1–2 wk

Contraind./Precautions ⬆ ⬇

Contraindicated in:

Hypersensitivity.

Use Cautiously in:

↓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).

Adv. 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

Interactions ⬆ ⬇

Drug-drug:

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.

Route/Dosage ⬆ ⬇

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

Assessment ⬆ ⬇

Monitor 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, weakness). Dose adjustment may be required.

Lab Test Considerations:

Monitor thyroid function tests prior to therapy, monthly during initial therapy, and every 2–3 mo during therapy.
- Monitor WBC and differential periodically during therapy. Agranulocytosis may develop rapidly; usually occurs during the 1st 2 mo and is more common in patients >40 yr and those receiving >40 mg/day. If agranulocytosis or pancytopenia occurs, discontinue methimazole and monitor bone marrow indices.
- May ↑ AST, ALT, LDH, alkaline phosphatase, serum bilirubin, and PT. If clinically significant hepatotoxicity, including transaminases >3 times the upper limit of normal, occurs, discontinue methimazole.

Implementation ⬆ ⬇

Do not confuse methimazole with metolazone or methazolamide.
PO: Administer at same time in relation to food every day.

Patient/Family Teaching ⬆ ⬇

Explain purpose and side effects of medication. Advise patient to read Patient Information before starting therapy.
Instruct patient to take missed doses as soon as remembered; take both doses together if almost time for next dose; check with health care provider if >1 dose is missed. Consult health care provider prior to discontinuing medication.
Instruct patient to monitor weight 2–3 times weekly and notify health care provider 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 provider 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, or symptoms of hyperthyroidism or hypothyroidism promptly.
Advise patient to notify health care provider if signs or symptoms of vasculitis (new rash, hematuria, ↓ urine output, dyspnea, hemoptysis, headache, fatigue, numbness and tingling in extremities, weakness) occur.
Instruct patient to notify health care provider of all Rx or OTC medications, vitamins, or herbal products being taken and to consult with health care provider before taking other medications.
Advise patient to carry identification describing medication regimen at all times.
Advise patient to notify health care provider of medication regimen prior to treatment or surgery.
Rep: Advise women of reproductive potential to notify health care provider 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.

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Pronunciation ⬇

Classifications ⬆ ⬇

Indications ⬆ ⬇

Action ⬆ ⬇

Pharmacokinetics ⬆ ⬇