PTU-Methimazole Dose Equivalency
The equivalent dose of propylthiouracil (PTU) for 2.5 mg of methimazole is 25 mg, based on the standard 10:1 conversion ratio used in clinical practice. 1
Standard Conversion Ratio
- The established dose equivalency between PTU and methimazole is approximately 10:1, meaning 10 mg of PTU is equivalent to 1 mg of methimazole 1, 2
- For 2.5 mg of methimazole, the equivalent PTU dose would be 25 mg (2.5 mg × 10 = 25 mg) 1
Clinical Context for Dose Selection
- A comparative study demonstrated that methimazole 15 mg daily was compared against PTU 150 mg daily (maintaining the 10:1 ratio), confirming this conversion is used in clinical trials 1
- Another study used methimazole 30 mg daily versus PTU 300 mg daily, again maintaining the 10:1 equivalency ratio 2
Important Efficacy Considerations
- Methimazole is significantly more effective than PTU at equivalent doses by the 10:1 ratio, with methimazole 15 mg daily achieving euthyroidism more rapidly than PTU 150 mg daily after 4 weeks of treatment 1
- Methimazole demonstrates superior reduction in serum T3, T4, and free T4 levels compared to PTU at the 10:1 dose ratio 1
- Methimazole also shows better efficacy in reducing thyrotropin receptor antibody (TRAb) levels after 8 weeks compared to PTU at equivalent doses 1
Safety Profile Differences
- PTU carries higher risk of hepatotoxicity, with odds of liver function injury 2.4 times higher than methimazole and odds of elevated transaminases 3.96 times higher 3
- Methimazole has higher risk of birth defects during first trimester exposure (OR 1.29), making PTU preferred in early pregnancy 3, 4
- Mild hepatotoxicity occurs more frequently with PTU 300 mg daily compared to methimazole 30 mg daily in head-to-head comparisons 2
Common Pitfall to Avoid
- Do not assume equal efficacy at the 10:1 conversion ratio—methimazole is clinically more potent than this ratio suggests, particularly for severe hyperthyroidism where methimazole 30 mg daily normalizes free T4 in 96.5% of patients versus only 78.3% with PTU 300 mg daily at 12 weeks 2