Dose Equivalence Between Methimazole and Propylthiouracil
The standard dose equivalence ratio is 1:10 to 1:20, meaning 10-20 mg of propylthiouracil (PTU) is equivalent to 1 mg of methimazole (MMI). 1, 2
Standard Dosing Equivalence
Based on FDA-approved dosing for hyperthyroidism:
- Mild hyperthyroidism: 15 mg MMI daily is equivalent to 150-300 mg PTU daily 1, 2
- Moderate hyperthyroidism: 30-40 mg MMI daily is equivalent to 300-400 mg PTU daily 1, 2
- Severe hyperthyroidism: 60 mg MMI daily is equivalent to 600-900 mg PTU daily 1, 2
The most commonly used conversion ratio in clinical practice is 1:10 (1 mg MMI = 10 mg PTU), though some sources suggest ratios up to 1:20 depending on disease severity 3, 4.
Practical Conversion Algorithm
When switching from PTU to MMI: Divide the total daily PTU dose by 10-20 3, 4
- Example: A patient on 300 mg PTU daily would convert to 15-30 mg MMI daily
When switching from MMI to PTU: Multiply the total daily MMI dose by 10-20 3, 4
- Example: A patient on 15 mg MMI daily would convert to 150-300 mg PTU daily
Evidence Supporting Superior Efficacy of Methimazole
Methimazole demonstrates significantly greater potency than the 1:10 ratio suggests in clinical trials. A single daily dose of 15 mg MMI was substantially more effective than 150 mg PTU in normalizing thyroid function, with 77.1% of MMI-treated patients achieving euthyroidism compared to only 19.4% of PTU-treated patients after 12 weeks 5. This suggests MMI may be 15-20 times more potent than PTU in actual clinical practice 5, 6.
Dosing Frequency Considerations
- Methimazole: Can be administered as a single daily dose due to longer half-life 1, 4
- PTU: Traditionally given in 3 divided doses every 8 hours, though once-daily dosing has been studied with inferior results 2, 5
The ability to use once-daily dosing with MMI represents a significant practical advantage over PTU's requirement for multiple daily doses 4, 7.
Clinical Situations Where PTU is Preferred Despite Lower Potency
PTU should be used instead of methimazole in the following specific scenarios:
- Thyroid storm: PTU inhibits peripheral conversion of T4 to T3, providing additional benefit 4
- First trimester of pregnancy: PTU is preferred due to lower risk of congenital anomalies (methimazole is associated with aplasia cutis and choanal/esophageal atresia) 3, 4
- Lactating women: Both drugs enter breast milk, but PTU is traditionally preferred 3, 4
Important Caveats
Hepatotoxicity risk differs significantly between the two drugs. PTU carries a higher risk of severe hepatotoxicity, particularly at doses of 300 mg/day or higher, with most severe cases occurring at these therapeutic doses 2, 7. Mild hepatotoxicity occurs more frequently with PTU than with MMI, especially compared to lower MMI doses (15 mg daily) 7.
The 1:10 conversion ratio is a starting point, not an absolute equivalence. Clinical response varies, and thyroid function tests should be monitored every 4 weeks initially when switching between medications to ensure adequate control 5, 7, 6.
Methimazole is generally the preferred first-line agent due to once-daily dosing, lower cost, greater availability, and lower risk of major side effects at therapeutic doses 4, 7.