What is the conversion dose from Synthroid (levothyroxine) orally (PO) to intravenously (IV)?

Levothyroxine (Synthroid) PO to IV Conversion

When converting from oral (PO) levothyroxine (Synthroid) to intravenous (IV) administration, the IV dose should be approximately 75% of the oral dose due to increased bioavailability of the IV formulation.

Conversion Rationale

The conversion from oral to IV levothyroxine requires dose adjustment because:

1. IV administration bypasses first-pass metabolism in the gastrointestinal tract
2. IV levothyroxine has 100% bioavailability compared to approximately 60-80% for oral administration
3. This difference in bioavailability necessitates dose reduction when switching from PO to IV

Specific Conversion Algorithm

1. Calculate the IV dose using this formula:

   • IV dose = Oral dose × 0.75
   • Example: For a patient on 100 mcg oral levothyroxine, the equivalent IV dose would be 75 mcg

2. For patients with critical illness or who cannot tolerate enteral medications:

   • Start with the calculated IV dose (75% of oral dose)
   • Monitor thyroid function tests (TSH, free T4) after 5-7 days of IV therapy
   • Adjust dose if needed based on laboratory results

Special Considerations

High-Risk Patients

• Elderly patients: Start with a lower percentage (60-70% of oral dose) and titrate as needed
• Cardiac patients: Use caution as rapid IV administration may exacerbate cardiac symptoms
• Critical illness: May require closer monitoring due to altered thyroid hormone metabolism

Administration Guidelines

• IV levothyroxine should be administered as a slow IV push over 2-3 minutes
• Dilute in compatible solutions if needed according to manufacturer guidelines
• Administer at the same time each day for consistent blood levels

Monitoring Recommendations

• Check TSH and free T4 levels 5-7 days after conversion
• Monitor for signs of over-replacement (tachycardia, tremor, anxiety, insomnia)
• Monitor for signs of under-replacement (fatigue, cold intolerance, constipation)

Common Pitfalls to Avoid

• Dose equivalence error: Never use 1:1 conversion from PO to IV
• Failure to adjust: Not reducing the dose when switching from oral to IV can lead to thyrotoxicosis
• Inadequate monitoring: Thyroid function tests should be checked after conversion
• Drug interactions: IV levothyroxine may have different interaction profiles than oral formulations

This conversion ratio is well-established in clinical practice to maintain appropriate thyroid hormone levels while avoiding the risks of over-replacement when switching from oral to intravenous administration.