When converting levothyroxine from oral to IV administration, what is the recommended dosage adjustment?

When converting levothyroxine from oral to intravenous (IV) administration, it is crucial to adjust the dosage due to differences in bioavailability between the two routes. Oral levothyroxine has a lower bioavailability compared to IV administration—typically estimated around 60-80% in most patients.

Choosing a dosage of 75% of the oral dose when switching to IV reflects this conversion reality, as it accounts for the higher absorption efficiency of the IV form. The remaining 25% reduction compensates for the fact that IV administration bypasses first-pass metabolism in the liver, leading to increased drug availability in the circulation.

This level of dosage adjustment helps maintain appropriate thyroid hormone levels while preventing potential side effects that could arise from administering too high of a dose. This standard practice ensures that the patient receives an effective yet safe amount of levothyroxine when switching routes.