Methimazole is a thiourea-class (Thiouracil) antithyroid medication primarily used to treat hyperthyroidism. Its mechanism of action involves inhibiting the activity of thyroid peroxidase, thereby reducing the synthesis of thyroid hormones (T3/T4) and alleviating hyperthyroid symptoms. This medication is commonly used for conditions such as Graves' disease, toxic multinodular goiter, and can also serve as an adjunct therapy prior to radioactive iodine treatment or surgery.
Compared to other antithyroid drugs, methimazole has a longer half-life, allowing for less frequent dosing. However, this may increase the risk of severe hematologic side effects. Physicians will decide on its use based on the patient's age, liver and kidney function, and severity of the condition.
The scientific name of methimazole is "Methimazole." Common brand names in Taiwan and Hong Kong include:
Methimazole is classified as a "Thyroid Antagonist," specifically a thiourea compound. These drugs primarily act on thyroid follicular cells, blocking the iodination of tyrosine and coupling reactions, thus inhibiting T3/T4 production. Its pharmacological action is similar to propylthiouracil but with higher bioavailability.
The primary indications include:
In specific cases such as pregnant women or children with hyperthyroidism, physicians will weigh the benefits and risks before prescribing. It is important to note that this medication is not suitable for acute respiratory distress caused by goiter enlargement.
Minor side effects include:
Serious side effects requiring immediate medical attention include:
Contraindications: Patients allergic to methimazole or its excipients should avoid use. Those with severe hepatic or renal impairment may need dose adjustments or alternative treatments. Pregnant women require strict monitoring, as the drug may cause fetal hypothyroidism or edema, classified as pregnancy category C (potential risk to the fetus).
Black Box Warning: This medication may cause fatal blood disorders. Full blood counts should be monitored every two weeks during the first six months of therapy. If symptoms such as fever, oral ulcers, or unexplained bleeding occur, the medication should be discontinued immediately and medical attention sought.
Concurrent use with the following medications may increase side effect risks:
Supplements such as kelp extracts or iodine-rich nutritional products may reduce efficacy. Patients should inform their healthcare provider of all medications and supplements they are taking prior to starting therapy.
The initial dose is usually 30-40 mg daily, divided into 2-3 doses. Based on the control of the condition, the physician will gradually adjust to a maintenance dose (5-15 mg daily). The medication should be swallowed whole, not chewed, and can be taken before or after meals, but at the same time each day to maintain stable blood levels.
Regular follow-up visits are necessary, with thyroid function tests (TSH, T3/T4) and complete blood counts every 4-6 weeks. The goal is to restore normal thyroid function, and continued therapy for 6-12 months can reduce relapse risk.
If a dose is missed, take it as soon as remembered. If it is close to the next scheduled dose (within 3 hours), skip the missed dose and resume the regular schedule. Do not take a double dose to make up for the missed one.
Overdose may cause symptoms such as nausea, vomiting, or altered consciousness. Immediate actions include:
Hospitals may administer activated charcoal to adsorb unabsorbed drug or perform hemodialysis to reduce toxicity. Patients should store medication out of children's reach to prevent accidental ingestion.
Methimazole can be taken on an empty stomach or with meals. It is recommended to take it at the same time each day to maintain consistent drug levels. Currently, there are no strict dietary restrictions. If gastrointestinal discomfort occurs, taking the medication with food may help. Avoid drinking strong tea or coffee, as they may affect drug absorption.
If a rash or allergic reaction occurs, should the medication be stopped immediately?Rash is a possible side effect of methimazole. If symptoms are mild and without other complications, contact your healthcare provider for assessment rather than stopping the medication on your own. If severe rash, fever, or joint pain indicating possible drug allergy (Stevens-Johnson syndrome) occurs, discontinue immediately and seek medical attention, as such reactions can be life-threatening.
When used with anticoagulants (e.g., warfarin), what interactions should be monitored?Methimazole may enhance the anticoagulant effect of warfarin, increasing bleeding risk. Regular monitoring of coagulation parameters (such as INR) is necessary, and doses should be adjusted accordingly. Patients should avoid self-adjusting doses and watch for signs of bleeding or bruising.
Does long-term use of methimazole lead to hypothyroidism?While methimazole is used to suppress overactive thyroid function, long-term use may result in hypothyroidism. About 10-20% of patients may develop this after several months of therapy. Regular monitoring of TSH and free T4 levels is essential. If hypothyroidism is diagnosed, dose adjustments or alternative treatments may be necessary.
Can methimazole be continued during pregnancy?Methimazole is generally considered safer during early pregnancy but should be used under close medical supervision. During later pregnancy, propylthiouracil (PTU) may be preferred due to potential fetal risks associated with methimazole observed in animal studies. Regardless of the medication chosen, thyroid function and fetal health should be closely monitored throughout pregnancy.
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