Converting from Tirosint to Synthroid: Recommended Approach
When converting a patient from Tirosint (levothyroxine sodium soft gelatin capsules) to Synthroid (levothyroxine sodium tablets), a 1:1 dose conversion is recommended as they contain the same active ingredient, though bioavailability differences may necessitate TSH monitoring and potential dose adjustments.
Understanding the Formulations
Tirosint and Synthroid both contain levothyroxine sodium as their active ingredient, but differ in their formulations:
- Tirosint: Soft gelatin capsule formulation
- Synthroid: Tablet formulation
Conversion Process
Initial Conversion
- Start with the same dose of Synthroid as the patient was taking of Tirosint (1:1 conversion)
- For example, if a patient was taking Tirosint 100 mcg daily, initially prescribe Synthroid 100 mcg daily
Monitoring After Conversion
- Check TSH and free T4 levels 6 weeks after switching formulations 1
- Do not check levels earlier as a new equilibrium requires approximately 6 weeks to establish
Dose Adjustments
- If TSH is outside the target range after 6 weeks, adjust the Synthroid dose accordingly
- After any dose adjustment, recheck TSH and free T4 after another 6 weeks
- Once stable, annual monitoring is usually sufficient 1
Special Considerations
Absorption Differences
- Tirosint may have better absorption in patients with certain gastrointestinal conditions
- Research shows that Tirosint absorption is less affected by proton pump inhibitors (PPIs) than tablet formulations 2
- Patients taking PPIs, antacids, or those with conditions affecting gastric acidity (atrophic gastritis, H. pylori infection) may require higher doses of Synthroid compared to their previous Tirosint dose 3
Bioequivalence Considerations
- Bioavailability can differ between levothyroxine preparations 1, 3
- The bioavailability of different levothyroxine generics might vary between 0.8 and 1.25 3
- Despite these potential differences, studies have shown clinical interchangeability between different brands of levothyroxine when properly monitored 4
Target Laboratory Values
- For primary hypothyroidism: Target TSH within normal reference range
- Optimal free T4 levels are typically in the upper half of the normal reference range 1
- Avoid TSH values below 0.2 mIU/L to reduce risk of atrial fibrillation and bone loss 5
Potential Pitfalls
- Failure to monitor: Always check TSH and free T4 6 weeks after switching formulations
- Premature testing: Testing before 6 weeks may lead to inappropriate dose adjustments
- Overlooking medication interactions: Consider all medications that may affect levothyroxine absorption
- Ignoring comorbidities: Patients with gastrointestinal disorders may experience different absorption patterns between formulations
- Switching without informing patients: Educate patients about the switch and the importance of follow-up monitoring
By following this approach and carefully monitoring thyroid function tests after conversion, most patients can be successfully transitioned from Tirosint to Synthroid with appropriate dose adjustments as needed.