How is the dose of Synthroid (levothyroxine) adjusted when converting from oral to intravenous (IV) administration?

Oral to IV Levothyroxine Dose Conversion

When converting from oral to intravenous levothyroxine, reduce the dose by approximately 20-30% (or use 70-80% of the oral dose), as IV administration has significantly higher bioavailability than oral administration.

Conversion Rationale

The conversion requires dose reduction because:

• IV levothyroxine bypasses first-pass metabolism and gastrointestinal absorption variability, resulting in nearly 100% bioavailability 1
• Oral levothyroxine has approximately 70-80% bioavailability under optimal conditions (taken on empty stomach, without interfering medications) 2
• The standard conversion is to administer 50-80% of the oral dose when switching to IV route 3

Practical Conversion Algorithm

For Most Patients

• If oral dose is 100 mcg daily → IV dose should be 75 mcg daily 3
• If oral dose is 150 mcg daily → IV dose should be 100-125 mcg daily 3
• This represents approximately a 25-30% dose reduction 1

Monitoring After Conversion

• Recheck TSH and free T4 in 6-8 weeks after conversion to assess adequacy of the new dose 3
• Target TSH should remain 0.5-4.5 mIU/L for primary hypothyroidism 4, 3
• Free T4 helps interpret ongoing abnormal TSH levels during therapy, as TSH may take longer to normalize 4, 3

Critical Safety Considerations

Cardiac Patients

• For elderly patients (>70 years) or those with cardiac disease, use even more conservative IV dosing—consider reducing to 50-60% of oral dose initially 3, 2
• Monitor closely for angina, palpitations, or arrhythmias after conversion 4
• Rapid increases in thyroid hormone can unmask or worsen cardiac ischemia 4

Patients Requiring TSH Suppression (Thyroid Cancer)

• For thyroid cancer patients requiring TSH suppression, consult with endocrinology before conversion, as target TSH levels vary by risk stratification 4
• Low-risk patients: target TSH 0.5-2 mIU/L 4
• Intermediate-to-high risk: target TSH 0.1-0.5 mIU/L 4
• Structural incomplete response: target TSH <0.1 mIU/L 4

Common Pitfalls to Avoid

• Never use a 1:1 conversion from oral to IV—this will result in iatrogenic hyperthyroidism 1, 2
• Failing to reduce the dose by 20-30% risks TSH suppression (<0.1 mIU/L), which increases risk for atrial fibrillation (especially in elderly), osteoporosis, fractures, and cardiovascular mortality 4, 3
• Approximately 25% of patients on levothyroxine are unintentionally overtreated with TSH suppression, increasing serious complication risks 4, 3
• Do not adjust doses more frequently than every 6-8 weeks, as levothyroxine requires this interval to reach steady state 3, 2

When IV Administration is Necessary

IV levothyroxine is indicated when:

• Patient cannot take oral medications (NPO status, severe malabsorption, critical illness) 2
• Myxedema coma or severe hypothyroidism requiring rapid correction 2
• Gastrointestinal absorption is unreliable in critically ill patients 4

Reverting Back to Oral Dosing

When converting back from IV to oral:

• Increase the dose by approximately 25-30% (use 125-140% of the IV dose) 1
• Resume the original oral dose if it was previously well-tolerated and achieved target TSH 3
• Recheck TSH and free T4 in 6-8 weeks after conversion back to oral 3