Thyrozol 15 mg Equivalent Dose
Thyrozol (carbimazole) 15 mg is approximately equivalent to 150 mg of propylthiouracil (PTU) daily.
Dose Conversion Rationale
The conversion is based on established pharmacological equivalencies between these antithyroid medications:
Carbimazole to Methimazole Conversion
First, carbimazole must be converted to its active metabolite. 10 mg of carbimazole converts to approximately 6 mg of methimazole (MMI) 1. Therefore:
- 15 mg carbimazole = approximately 9 mg methimazole
Methimazole to PTU Conversion
The standard conversion ratio used in clinical practice is approximately 1:10 to 1:20 (MMI:PTU), with most sources supporting a 1:10 to 1:15 ratio for therapeutic equivalence 2, 3.
Using the conservative 1:15-20 ratio:
- 9 mg methimazole ≈ 135-180 mg PTU
- Practical equivalent: 150 mg PTU daily
Important Clinical Considerations
Why This Conversion Matters Less Than You Think
Methimazole (and thus carbimazole) should be the preferred agent in virtually all cases 1. The American Thyroid Association guidelines explicitly recommend methimazole over PTU except in three specific situations 1:
- First trimester of pregnancy - PTU preferred due to lower risk of congenital anomalies 4, 5
- Thyroid storm - Though recent evidence questions this preference 6
- Minor reactions to methimazole in patients refusing radioactive iodine or surgery
Critical Safety Differences
PTU carries significantly higher hepatotoxicity risk 1, 5:
- PTU causes 2.40 times higher odds of liver injury (OR 2.40,95% CI 1.16-4.96) 5
- PTU causes 3.96 times higher odds of elevated transaminases (OR 3.96,95% CI 2.49-6.28) 5
- PTU can cause fulminant hepatic necrosis requiring liver transplantation 1
- FDA issued safety alerts regarding PTU-associated severe liver injury 1
Dosing Practicality Issues
PTU requires multiple daily doses (2-3 times daily) due to shorter half-life (1-2 hours), while carbimazole/methimazole can be given once daily (half-life 3-5 hours) 1, 3. This makes:
- PTU 150 mg = 50 mg three times daily 7
- Carbimazole 15 mg = once daily dosing
Clinical Recommendation
If you are considering switching from Thyrozol 15 mg to PTU, reconsider whether this switch is truly necessary. The only compelling indication for PTU over carbimazole/methimazole is first-trimester pregnancy 1, 4. Otherwise, the increased hepatotoxicity risk, inconvenient dosing schedule, and lack of superior efficacy make PTU a suboptimal choice 2, 5.
If the switch is unavoidable, start with PTU 150 mg daily (50 mg three times daily) and monitor thyroid function tests at 4 weeks, adjusting based on response 1, 7.