How should I convert an adult on carbimazole 5 mg twice daily to methimazole and what follow‑up thyroid function tests are needed?

Carbimazole to Methimazole Conversion

Direct Conversion Ratio

Convert carbimazole 5 mg twice daily to methimazole 5 mg twice daily (or 10 mg once daily), as carbimazole is rapidly metabolized to methimazole with an equivalent dose ratio of approximately 1:1. 1, 2

• Carbimazole is a prodrug that converts to methimazole (also called thiamazole) immediately after absorption, making them functionally identical medications 1
• The equivalent dose ratio ranges from 0.6:1.0 to 1:1 (carbimazole:methimazole), meaning 10 mg carbimazole daily converts to approximately 10 mg methimazole daily 2
• Switching between carbimazole and methimazole should NOT be considered as a strategy to avoid side effects, because they are the same active compound and cross-reactivity is complete 1

Dosing Schedule Options

Administer methimazole 10 mg as a single daily dose rather than divided doses, as this improves compliance without compromising efficacy. 3, 4

• Methimazole has a long intrathyroidal half-life that exceeds its plasma half-life, allowing effective once-daily dosing despite shorter serum concentrations 3
• Studies demonstrate that 30 mg carbimazole once daily achieves euthyroidism in 4.6 ± 1.4 weeks compared to 3.8 ± 1.2 weeks with divided doses (p > 0.05), confirming equivalent efficacy 3
• Single daily dosing is particularly useful for patients who struggle with medication adherence 3
• If the patient prefers divided dosing, methimazole 5 mg twice daily (every 12 hours) is acceptable 4

Thyroid Function Monitoring Protocol

Check TSH, free T4, and free T3 at 6 weeks after conversion, then every 6–8 weeks during dose titration. 5, 6

• Plasma T3 is the best indicator of clinical thyroid status during antithyroid drug therapy, as T4 falls to normal or low levels before T3 normalizes 5
• The T4:T3 ratio becomes disproportionately low during carbimazole/methimazole therapy, with many clinically euthyroid patients showing normal T3 but low T4 5
• Single measurements of T4 or free T4 index alone are NOT recommended for assessing thyroid function during therapy—always measure T3 alongside T4 5
• Once clinical hyperthyroidism resolves, a rising serum TSH indicates the need for a lower maintenance dose 6
• Thyroid function tests should be monitored periodically throughout therapy to guide dose adjustments 6

Critical Safety Monitoring

Obtain baseline complete blood count with differential and liver function tests before conversion, then monitor clinically for agranulocytosis symptoms. 6, 1

• Instruct the patient to immediately report fever, sore throat, skin eruptions, headache, or general malaise, as these may indicate agranulocytosis 6
• Agranulocytosis typically occurs during the first 3 months of treatment with an incidence of 3 per 10,000 patients 1
• Cross-reactivity for agranulocytosis occurs between carbimazole and methimazole because they are the same active compound 1
• Check prothrombin time/INR before surgical procedures, as methimazole may cause hypoprothrombinemia and bleeding 6
• Monitor for vasculitis symptoms including new rash, hematuria, decreased urine output, dyspnea, or hemoptysis, and instruct patients to report these immediately 6

Dose-Dependent Adverse Effects

The current dose of 10 mg methimazole daily (equivalent to carbimazole 10 mg daily) is low enough to minimize hematologic toxicity while maintaining efficacy. 2, 1

• Hematopoietic damage from methimazole/carbimazole is dose-responsive, with lower doses (5–10 mg daily maintenance) carrying less risk than initial high doses (40–60 mg) 2
• Common dose-dependent adverse effects include hypothyroidism (monitor TSH/free T4/T3 as above), mild neutropenia, pruritus, and rash 1
• Arthralgias develop in 1–5% of patients and may herald more serious immunologic complications—discontinue methimazole immediately if arthralgias occur 1

Drug Interactions Requiring Monitoring

Increase monitoring of INR/PT if the patient takes warfarin, as methimazole potentiates oral anticoagulants. 6

• Methimazole inhibits vitamin K activity, increasing warfarin effect and bleeding risk 6
• Beta-blocker doses may require reduction as the patient becomes euthyroid, due to decreased clearance 6
• Digitalis glycoside levels may increase when hyperthyroid patients become euthyroid—monitor digoxin levels and reduce dose if needed 6
• Theophylline clearance decreases as patients become euthyroid—reduce theophylline dose accordingly 6

Common Pitfalls to Avoid

• Do not switch from carbimazole to methimazole expecting to avoid side effects—they are the same drug and cross-reactivity is complete 1
• Do not rely on T4 or free T4 index alone to assess thyroid status during therapy—always include T3 measurement 5
• Do not delay checking CBC with differential if the patient develops fever, sore throat, or malaise—agranulocytosis requires immediate diagnosis 6, 1
• Do not continue methimazole if arthralgias develop—this may be the first sign of ANCA-positive vasculitis (though rare with methimazole compared to propylthiouracil) 1