What is the equivalent dose of methimazole (antithyroid medication) for carbimazole (neo-mercazole)?

Carbimazole to Methimazole Equivalent Dosing

Carbimazole 10 mg is equivalent to approximately 6 mg of methimazole. This conversion ratio of 0.6 is based on the fact that carbimazole is metabolized to methimazole in vivo 1.

Pharmacological Basis

Carbimazole is a prodrug that is rapidly converted to methimazole (also known as thiamazole) in the body. This conversion occurs both in vivo and in vitro, with carbimazole having no intrinsic antithyroid activity until it is metabolized to methimazole 1.

The equivalent dosing ratio between carbimazole and methimazole ranges from 0.6 to 1.0, with most clinical evidence supporting the 0.6 ratio 1. This means:

• 5 mg carbimazole ≈ 3 mg methimazole
• 10 mg carbimazole ≈ 6 mg methimazole
• 20 mg carbimazole ≈ 12 mg methimazole
• 40 mg carbimazole ≈ 24 mg methimazole

Clinical Applications

When switching between these medications:

1. Calculate the equivalent dose using the 0.6 conversion factor
2. Monitor thyroid function tests (TSH, FT4) after switching to ensure adequate control
3. Adjust dosing as needed based on clinical response

Dosing Considerations

Both medications can be administered as a single daily dose despite their relatively short half-lives (approximately 3-4 hours) 2, 3. This is possible because these drugs have a longer intrathyroidal than plasma half-life, allowing for extended duration of action within the thyroid gland 2.

Research has shown that:

• Single daily dosing of carbimazole (30 mg once daily) is as effective as divided doses (10 mg three times daily) 2
• Low doses of carbimazole (5-10 mg daily) can significantly reduce intrathyroidal iodide binding 4
• For initial treatment of hyperthyroidism, 20 mg/day of carbimazole is effective for mild to moderate cases, while 40 mg/day may be needed for severe hyperthyroidism 5

Important Clinical Considerations

• Both drugs can cause similar side effects, including agranulocytosis, hepatitis, vasculitis, and thrombocytopenia 6
• If symptoms of agranulocytosis develop (sore throat and fever), a complete blood count should be obtained, and the medication should be discontinued 6
• Both carbimazole and methimazole can be used during breastfeeding 6
• When treating hyperthyroidism in pregnancy, monitoring of FT4 or FTI every 2-4 weeks is recommended 6

Remember that the goal of therapy is to maintain free T4 or free T4 index in the high-normal range using the lowest possible thioamide dosage to minimize side effects while effectively controlling hyperthyroidism.