What is the equivalent intravenous (IV) dose of levothyroxine (T4) for a patient taking 75 micrograms (mcg) of levothyroxine orally (PO) who is unable to take oral medications?

Levothyroxine PO to IV Dose Conversion

For a patient taking 75 mcg levothyroxine orally who cannot take oral medications, reduce the dose by 20-30% when converting to IV administration, giving approximately 50-60 mcg IV daily. This dose reduction accounts for the complete bioavailability of IV levothyroxine compared to approximately 70-80% oral absorption 1, 2.

Conversion Algorithm

Standard Conversion Formula

• Reduce the oral dose by 20-30% for IV administration to account for complete bioavailability of the IV route 1
• For 75 mcg PO: 50-60 mcg IV daily is the appropriate equivalent dose 1
• The IV formulation bypasses first-pass metabolism and gastrointestinal absorption variability, resulting in 100% bioavailability compared to 70-80% for oral tablets 1, 2

Administration Guidelines

• Administer IV levothyroxine as a single daily dose, typically given over several minutes 1
• IV levothyroxine should be reconstituted according to manufacturer instructions and used immediately 1
• Monitor TSH and free T4 levels in 6-8 weeks after conversion to assess adequacy of the new dose 3, 4

Alternative Routes When IV Unavailable

Intramuscular Administration

• IM levothyroxine can be given at the same reduced dose as IV (50-60 mcg for a 75 mcg oral dose), as absorption is similarly complete 1
• This route is effective when IV access is problematic 1

Rectal Administration

• Rectal levothyroxine has been successfully used in case reports when oral and parenteral routes are unavailable 5
• Diluted oral tablets can be administered rectally, though bioavailability data is limited 5
• This route demonstrated clinical and biochemical effectiveness in published case studies 5

Oral Liquid Formulation

• Liquid oral levothyroxine may be better absorbed than tablets in patients with malabsorption or those receiving enteral nutrition 1, 2
• Consider this option before converting to parenteral routes if the gastrointestinal tract is functional 1

Critical Monitoring After Conversion

Short-Term Monitoring

• Recheck TSH and free T4 in 6-8 weeks after route conversion to ensure the dose adjustment maintains euthyroidism 3, 4
• For patients with cardiac disease or atrial fibrillation, consider repeating testing within 2 weeks rather than waiting the full 6-8 weeks 3

Dose Adjustment Strategy

• Adjust by 12.5-25 mcg increments based on TSH results after the initial monitoring period 3, 4
• Target TSH should remain in the reference range (0.5-4.5 mIU/L) with normal free T4 levels 3

Special Populations Requiring Modified Approach

Elderly Patients or Those with Cardiac Disease

• Use the lower end of the conversion range (50 mcg IV for 75 mcg PO) to minimize cardiac stress 3, 4
• Elderly patients with coronary disease are at increased risk of cardiac decompensation even with therapeutic doses 3

Myxedema Coma

• In myxedema coma, higher IV loading doses (200-400 mcg) are recommended regardless of previous oral dose, followed by 50-100 mcg IV daily 6
• When IV levothyroxine is unavailable, oral loading doses of 300-500 mcg have been effective in case series, though this is off-label 6

Thyroid Cancer Patients Requiring TSH Suppression

• Maintain the same degree of TSH suppression when converting routes 3
• For patients requiring TSH <0.1 mIU/L, the IV dose may need to be at the higher end of the conversion range (60 mcg IV for 75 mcg PO) 3

Common Pitfalls to Avoid

• Never use a 1:1 conversion from oral to IV, as this will result in iatrogenic hyperthyroidism due to complete IV bioavailability 1, 2
• Do not delay monitoring beyond 6-8 weeks after route conversion, as under- or overtreatment can develop 3, 4
• Avoid crushing tablets for IV administration, as this is not sterile and can cause complications; only use pharmaceutical-grade IV formulations 1
• In patients with suspected adrenal insufficiency, ensure corticosteroids are started before initiating or increasing thyroid hormone to prevent adrenal crisis 7, 3

When to Return to Oral Therapy

• Resume oral levothyroxine at the original dose (75 mcg) once the patient can reliably take oral medications 1, 2
• Recheck TSH and free T4 in 6-8 weeks after converting back to oral administration to ensure the original dose remains appropriate 3, 4
• Approximately 25% of patients on levothyroxine are unintentionally maintained on excessive doses, so conversion back to oral therapy provides an opportunity to reassess dosing 3