IV Levothyroxine Dosing for NPO Patient
For a patient who is NPO and normally takes Synthroid (levothyroxine) 88 mcg PO daily, the appropriate IV dose is 70-75% of the oral dose, which equals approximately 65 mcg IV daily.
Conversion Rationale
When converting from oral to intravenous levothyroxine, a dose reduction is necessary due to differences in bioavailability:
- Oral levothyroxine has approximately 60-80% bioavailability
- IV levothyroxine has 100% bioavailability
- Therefore, IV doses should be approximately 70-75% of the oral dose
Dosing Calculation
For this patient:
- Current oral dose: 88 mcg daily
- Conversion factor: 0.75 (75%)
- Calculated IV dose: 88 mcg × 0.75 = 66 mcg
Since levothyroxine IV formulations typically come in standard strengths, the most appropriate dose would be 65 mcg IV daily.
Administration Considerations
- IV levothyroxine should be administered as a slow IV push over 2-3 minutes or diluted in compatible IV fluids
- The medication should be protected from light during administration
- Daily administration is recommended to maintain consistent thyroid hormone levels
Monitoring
- Monitor vital signs during administration, particularly heart rate
- For patients remaining NPO for extended periods, consider checking TSH and free T4 levels after 2-3 weeks to ensure adequate replacement
- Watch for signs of hypothyroidism (fatigue, cold intolerance) or hyperthyroidism (tachycardia, tremor)
Special Considerations
- If the patient has cardiac disease, consider starting at a lower dose (50-60 mcg) and titrating up
- For elderly patients, a more conservative approach with 60-65 mcg may be appropriate
- Once the patient can resume oral intake, convert back to the original oral dose of 88 mcg daily
Common Pitfalls to Avoid
- Avoid administering IV levothyroxine too rapidly, which may cause cardiovascular side effects
- Do not mix IV levothyroxine with other medications in the same syringe or IV line
- Remember that IV levothyroxine is more potent than the oral form due to 100% bioavailability
When the patient is able to resume oral intake, they should return to their previous oral dose of 88 mcg daily.