Intravenous Levothyroxine Dosing for Severe Hypothyroidism
Critical Initial Consideration
Before initiating IV levothyroxine in any patient with severe hypothyroidism or suspected central hypothyroidism, you must first rule out and treat adrenal insufficiency with corticosteroids (hydrocortisone ~10 mg/m² or equivalent) to prevent precipitating adrenal crisis. 1, 2
IV Dosing Protocol
The IV dose of levothyroxine is approximately 50-75% of the oral dose, as IV administration has higher bioavailability. 2
For Severe Hypothyroidism (Myxedema):
Start with IV levothyroxine 200-500 mcg as a loading dose, followed by 50-100 mcg IV daily until oral therapy can be resumed. 3
For patients unable to take oral medications due to impaired consciousness, malabsorption, or non-compliance, IV levothyroxine 200-500 mcg weekly has been successfully used, though daily dosing is preferred when feasible. 3
The typical maintenance IV dose ranges from 50-100 mcg daily, which corresponds to approximately 75-150 mcg oral equivalent. 2
Standard IV Conversion:
If converting from established oral therapy, reduce the oral dose by 25-30% for IV administration. For example, a patient on 100 mcg oral would receive approximately 70-75 mcg IV. 2
For patients <70 years without cardiac disease requiring full replacement, calculate approximately 1.2 mcg/kg/day IV (compared to 1.6 mcg/kg/day oral). 1, 2, 4
Special Populations Requiring Modified Dosing
Elderly or Cardiac Disease:
For patients >70 years or with known cardiovascular disease, start with 25-37.5 mcg IV daily (equivalent to 25-50 mcg oral) and titrate gradually. 1, 2
These patients are at increased risk of cardiac decompensation, angina, or arrhythmias even with therapeutic doses. 2
Critical Illness:
- In critically ill patients where enteral absorption is unreliable, IV levothyroxine ensures consistent delivery and should be continued until reliable oral intake is established. 2
Monitoring During IV Therapy
Recheck TSH and free T4 every 6-8 weeks during dose titration, but for patients with cardiac disease or atrial fibrillation, consider more frequent monitoring within 2 weeks. 2, 4
Free T4 levels help interpret ongoing abnormal TSH during therapy, as TSH may take longer to normalize. 2, 4
Monitor for signs of overtreatment including tachycardia, tremor, or cardiac arrhythmias, particularly in elderly patients. 2
Transition to Oral Therapy
Once the patient can reliably take oral medications, transition to oral levothyroxine at approximately 125-133% of the IV dose (e.g., IV 75 mcg becomes oral 100 mcg). 2
Continue monitoring TSH and free T4 6-8 weeks after transition to ensure adequate replacement. 2, 4
Critical Pitfalls to Avoid
Never start thyroid hormone before corticosteroids in suspected central hypothyroidism or hypophysitis—this can precipitate life-threatening adrenal crisis. 1, 2
Avoid excessive initial dosing in elderly or cardiac patients, as this increases risk for atrial fibrillation, myocardial ischemia, and cardiac decompensation. 1, 2
Do not use endotracheal administration—there is no established evidence for this route. 2
Approximately 25% of patients are inadvertently maintained on excessive doses leading to TSH suppression, increasing risks for osteoporosis, fractures, and cardiac complications. 2