What is the equivalent IV dose of levothyroxine (Synthroid) for a patient who is NPO (nil per os) and normally takes 100 mcg of Synthroid daily?

IV Levothyroxine Dosing for NPO Patient on 100 mcg Synthroid

For a patient who normally takes 100 mcg of oral levothyroxine (Synthroid) daily and is currently NPO, the appropriate intravenous dose is 80 mcg (80% of the oral dose).

Rationale for IV Levothyroxine Conversion

When converting from oral to intravenous levothyroxine administration, several key principles must be considered:

1. Bioavailability difference: Oral levothyroxine has approximately 80% bioavailability compared to IV administration, which has 100% bioavailability
2. Dose equivalence: Due to this difference in bioavailability, IV doses should be approximately 80% of the oral dose
3. Absorption factors: When taken orally, about 20% of levothyroxine is lost due to incomplete intestinal absorption and first-pass metabolism

Dosing Algorithm

For converting from oral to IV levothyroxine:

• Calculate 80% of the patient's stable oral dose
• For this patient on 100 mcg oral levothyroxine: 100 mcg × 0.8 = 80 mcg IV daily

Administration Considerations

• IV levothyroxine should be administered as a slow IV push over 2-3 minutes
• Administer at approximately the same time each day for consistency
• Monitor for signs of thyroid dysfunction during the transition period

Special Considerations

• Duration: IV levothyroxine should be continued until the patient can resume oral intake
• Monitoring: If the NPO status is prolonged (>1-2 weeks), consider checking TSH and free T4 levels to ensure adequate replacement
• Resuming oral therapy: When the patient can take oral medications again, resume the previous oral dose of 100 mcg daily

Potential Pitfalls

1. Overdosing: Using a 1:1 conversion from oral to IV can lead to thyrotoxicosis due to the higher bioavailability of IV formulation
2. Underdosing: Failure to provide adequate thyroid replacement can lead to progression of hypothyroid symptoms
3. Administration errors: IV levothyroxine should not be given as a rapid bolus as this may cause cardiac complications

This approach ensures maintenance of appropriate thyroid hormone levels while avoiding the risks associated with either over-replacement or under-replacement during the NPO period.