Antithyroid agent (thionamide)
Methimazole is a medicine used to treat hyperthyroidism (overproduction of thyroid hormones), such as in Graves' disease. It is taken orally, one to three times a day, and the full effect takes several weeks.
Also known as: Tapazole
Methimazole can increase the effect of warfarin, raising the risk of bleeding. The INR should be monitored more closely during treatment.
The approved methimazole label (DailyMed) states that, through potential inhibition of vitamin K activity, the drug may increase the activity of oral anticoagulants (e.g. warfarin), recommending additional PT/INR monitoring, especially before surgical procedures. The effect is more relevant as hyperthyroidism is controlled, when drug clearance changes.
Methimazole may inhibit vitamin K activity and potentiate oral anticoagulants; monitor PT/INR, especially before surgical procedures.
Inhibition of vitamin K activity and altered hepatic clearance as the euthyroid state is reached.
Periodic INR and bleeding symptoms.
Melena, haematemesis, bruising — seek immediate care.
Monitor INR after starting, adjusting or stopping methimazole; consider reducing the anticoagulant dose.
DailyMed/FDA (NIH/NLM) — approved Methimazole label: https://dailymed.nlm.nih.gov/dailymed/drugInfo.cfm?setid=b53f84ac-4263-478c-883d-aca7ab44fef5 ; approved Warfarin label: https://dailymed.nlm.nih.gov/dailymed/drugInfo.cfm?setid=541c9a70-adaf-4ef3-94ba-ad4e70dfa057 ; Prontuário Terapêutico do INFARMED (11th ed., 2012)
As hyperthyroidism is controlled, methimazole can increase digoxin levels in the blood. Your doctor may need to reduce the digoxin dose.
The approved methimazole label (DailyMed) documents that serum digoxin levels may increase when a hyperthyroid patient on a stable digoxin regimen becomes euthyroid, recommending a digoxin dose reduction. Hyperthyroidism increases digoxin renal clearance and distribution; as the thyroid is controlled, clearance normalises and levels rise.
Serum digoxin levels may rise when a stabilised hyperthyroid patient becomes euthyroid; a digoxin dose reduction may be needed.
Normalisation of digoxin clearance with the euthyroid state, raising serum levels.
Digoxin levels, ECG and digestive/visual symptoms.
Persistent nausea, yellow/green vision, palpitations, bradycardia — assess digoxin toxicity.
Watch digoxin levels and signs of toxicity (nausea, colour vision, arrhythmias) as the patient reaches the euthyroid state.
DailyMed/FDA (NIH/NLM) — approved Methimazole label: https://dailymed.nlm.nih.gov/dailymed/drugInfo.cfm?setid=b53f84ac-4263-478c-883d-aca7ab44fef5 ; approved Digoxin label: https://dailymed.nlm.nih.gov/dailymed/drugInfo.cfm?setid=5886e233-b2da-4acb-be05-9bf40fb8e7f4 ; Prontuário Terapêutico do INFARMED (11th ed., 2012)
Methimazole can be administered with or without food, consistently.
Take at the same time every day, with or without food.
DailyMed/FDA (NIH/NLM) — approved Methimazole label: https://dailymed.nlm.nih.gov/dailymed/drugInfo.cfm?setid=b53f84ac-4263-478c-883d-aca7ab44fef5
No relevant pharmacokinetic interaction; usual moderation during treatment.
Moderate alcohol intake.
DailyMed/FDA (NIH/NLM) — approved Methimazole label: https://dailymed.nlm.nih.gov/dailymed/drugInfo.cfm?setid=b53f84ac-4263-478c-883d-aca7ab44fef5
Methimazole can cause agranulocytosis, especially in the first weeks; patients with a history of blood dyscrasias are at increased risk.
Monitor the blood count in the presence of fever or pharyngitis and stop if agranulocytosis.
DailyMed/FDA (NIH/NLM) — approved Methimazole label: https://dailymed.nlm.nih.gov/dailymed/drugInfo.cfm?setid=b53f84ac-4263-478c-883d-aca7ab44fef5
Methimazole can cause cholestasis and liver injury; use with caution in pre-existing liver disease.
Monitor liver function tests during treatment.
DailyMed/FDA (NIH/NLM) — approved Methimazole label: https://dailymed.nlm.nih.gov/dailymed/drugInfo.cfm?setid=b53f84ac-4263-478c-883d-aca7ab44fef5
Methimazole crosses the placenta and can cause congenital malformations in the 1st trimester (aplasia cutis, craniofacial and GI malformations, omphalocele).
Avoid in the 1st trimester (prefer propylthiouracil in that period); use the lowest effective dose and monitor maternal and fetal thyroid function.
Methimazole is excreted into milk in small amounts; studies have shown no toxicity in the infant, but the infant's thyroid function should be monitored.
No specific need beyond routine monitoring; untreated hyperthyroidism in pregnancy is harmful to both mother and fetus.
DailyMed/FDA (NIH/NLM) — approved Methimazole label: https://dailymed.nlm.nih.gov/dailymed/drugInfo.cfm?setid=b53f84ac-4263-478c-883d-aca7ab44fef5
Clinical and educational support tool. The information does not replace a medical prescription or the opinion of a qualified healthcare professional.
Antithyroid agent used in the treatment of hyperthyroidism (Graves disease, toxic multinodular goitre, toxic adenoma) and in preparation for thyroidectomy or radioiodine therapy. It does not inactivate existing thyroxine and triiodothyronine, so the clinical effect takes days to weeks to appear.
Inhibits thyroid hormone synthesis by blocking iodine organification and coupling of iodotyrosines in the thyroid gland (inhibition of thyroid peroxidase).
Rapidly absorbed after oral administration, with peak serum concentrations 1 to 2 hours after dosing. It is not bound to plasma proteins and accumulates in the thyroid gland.
Readily absorbed in the gastrointestinal tract, metabolised in the liver and excreted in urine; about 80% of the drug and metabolites is eliminated renally (7% unchanged).
The plasma half-life is about 3 to 5 hours (in the literature up to 5 to 13 hours); the duration of effect is longer than the half-life suggests, due to intrathyroidal accumulation.
Medicines from the same therapeutic group (ATC classification) or the same pharmacological class.
