Carbimazole to Propylthiouracil Conversion
1 mg of carbimazole is approximately equivalent to 10 mg of propylthiouracil (PTU).
Conversion Rationale
The evidence provides clear guidance on antithyroid drug equivalencies, though it requires some calculation to arrive at the carbimazole-to-PTU ratio:
Step-by-Step Conversion
Carbimazole to Methimazole (MMI):
- 10 mg carbimazole converts to approximately 6 mg MMI 1
- Therefore, 1 mg carbimazole = 0.6 mg MMI
MMI to PTU Equivalency:
- While the guidelines don't explicitly state a direct MMI-to-PTU conversion ratio, the dosing recommendations provide insight:
Clinical Practice Standard:
- The commonly accepted conversion in clinical practice is 1 mg MMI = 10-15 mg PTU
- Using the conservative 10:1 ratio: 1 mg carbimazole (= 0.6 mg MMI) = approximately 6-10 mg PTU
- Practical conversion: 1 mg carbimazole ≈ 10 mg PTU
Important Clinical Considerations
Why This Matters for Patient Safety
Methimazole is preferred over PTU in virtually all patients except:
- First trimester pregnancy (PTU preferred due to lower teratogenicity risk) 1
- Thyroid storm 1
- Minor reactions to MMI when patient refuses radioactive iodine or surgery 1
PTU carries significantly higher hepatotoxicity risk:
- PTU causes more severe liver injury including fulminant hepatic necrosis requiring transplantation 1, 2, 3
- Adjusted risk of elevated transaminases is nearly 4-fold higher with PTU (OR 3.96) 2
- Overall liver injury risk is 2.4-fold higher with PTU 2
Dosing Pitfalls to Avoid
When converting between these medications:
- Do not use simple arithmetic conversion alone - consider the patient's current thyroid status and adjust based on clinical response
- PTU requires 2-3 times daily dosing due to shorter duration of action, while carbimazole/MMI can be given once daily 1
- Monitor free T4 at 4 weeks after any medication change 1
Baseline monitoring before initiating therapy:
- Complete blood count with differential
- Liver function tests (transaminases and bilirubin) 1