Which is better for treating hyperthyroidism, methimazole or carbimazole?

From the Guidelines

Methimazole is generally preferred over carbimazole for treating hyperthyroidism, especially in the second and third trimesters of pregnancy, due to its direct onset of action and equivalent efficacy to carbimazole, which is a prodrug that converts to methimazole in the body 1.

Key Considerations

• Both methimazole and carbimazole are effective for treating hyperthyroidism by inhibiting thyroid peroxidase, but their usage can vary based on the trimester of pregnancy and regional availability.
• The typical starting dose for methimazole ranges from 10-30 mg daily, while carbimazole is usually prescribed at 15-40 mg daily, with treatment typically continuing for 12-18 months.
• Side effects are similar for both medications and include rash, joint pain, and rarely, agranulocytosis, necessitating regular blood tests to monitor thyroid function and medication response.

Clinical Decision Making

• The choice between methimazole and carbimazole should be based on the patient's specific condition, including the stage of pregnancy, as methimazole is preferred in the second and third trimesters due to potential teratogenicity associated with its use in the first trimester 1.
• Regional availability and the patient's medical history should also be considered when deciding between these two medications.

Monitoring and Adjustment

• Regular monitoring of thyroid function and adjustment of medication doses are crucial to achieve and maintain euthyroidism, reducing the risk of maternal and neonatal morbidity associated with hyperthyroidism.
• Patients should be educated about the importance of adherence to their medication regimen and the need for regular follow-up appointments to ensure optimal management of their condition.

From the Research

Comparison of Methimazole and Carbimazole

• Both methimazole and carbimazole are used to treat hyperthyroidism, with studies showing their effectiveness in achieving a euthyroid state 2, 3.
• A study comparing a single daily dose of carbimazole with divided doses found no significant difference in efficacy between the two regimens 2.
• Another study found that 20 mg/day of carbimazole is effective and has a lower risk of iatrogenic hypothyroidism compared to 40 mg/day, especially in patients with mild or moderate hyperthyroidism 4.
• Methimazole has also been shown to be effective in a single daily dose regimen, with a mean time to achieve euthyroidism of 16.7 weeks 3.
• However, both methimazole and carbimazole can cause severe side effects, such as hepatotoxicity and neutropenia, although these are rare 5, 6.

Efficacy and Safety

• The efficacy of methimazole and carbimazole in treating hyperthyroidism is well established, but the choice between the two may depend on individual patient factors and the severity of the disease 2, 4, 3.
• Regular monitoring of liver function and blood cell counts is recommended for patients taking either methimazole or carbimazole due to the risk of hepatotoxicity and neutropenia 5, 6.
• The optimal dosage regimen for carbimazole remains uncertain, but 20 mg/day appears to be a safe and effective starting dose for many patients 4.

Treatment Considerations

• Patients with severe hyperthyroidism may require higher doses of carbimazole or alternative treatments 4.
• Methimazole and carbimazole can be used in a single daily dose regimen, which may improve patient compliance and convenience 2, 3.
• The risk of side effects, such as hepatotoxicity and neutropenia, should be carefully considered and monitored when using either methimazole or carbimazole to treat hyperthyroidism 5, 6.