Guidelines for Converting a Patient from Levothyroxine to Liothyronine
When converting a patient from levothyroxine (T4) to liothyronine (T3), the recommended approach is to start with a low dose of liothyronine and gradually increase while monitoring thyroid function, using an approximate conversion ratio of 3:1 (T4:T3 by weight).
Initial Assessment
- Before conversion, confirm the indication for switching from levothyroxine to liothyronine, as levothyroxine remains the standard treatment for hypothyroidism 1
- Evaluate current thyroid function with TSH, free T4, and free T3 levels to establish baseline status 2
- Review patient's medical history for cardiac disease, osteoporosis, or other conditions that might increase risk with liothyronine therapy 3
- Consider consulting with an endocrinologist before initiating conversion, especially for complex cases 1
Conversion Protocol
- Discontinue levothyroxine and initiate liothyronine at a low starting dose 4
- For mild hypothyroidism: Start with 25 mcg of liothyronine daily 4
- For moderate to severe hypothyroidism: Consider starting with a lower dose of 5 mcg daily and increase gradually 4
- For elderly patients or those with cardiovascular disease: Begin with 5 mcg daily and increase by 5 mcg increments at recommended intervals 4
Dosage Titration
- Increase dosage by up to 25 mcg every 1-2 weeks for mild hypothyroidism 4
- For moderate to severe cases, increase by 5-10 mcg daily every 1-2 weeks; when reaching 25 mcg daily, may increase by 5-25 mcg every 1-2 weeks 4
- Usual maintenance dose ranges:
Monitoring
- Monitor thyroid function tests (TSH, free T4, free T3) 4-6 weeks after each dose adjustment 2
- Target a normal TSH within reference range unless specific indications for TSH suppression exist (e.g., thyroid cancer) 3
- Monitor for signs of hyperthyroidism: palpitations, anxiety, insomnia, weight loss, heat intolerance 2
- Be aware that liothyronine has a more rapid onset and shorter half-life than levothyroxine, which may cause more pronounced cardiovascular side effects 4
Conversion Ratio Considerations
- The pharmacodynamic equivalence between levothyroxine and liothyronine is approximately 3:1 by weight 5
- For example, 100 mcg of levothyroxine would roughly equate to about 33 mcg of liothyronine 5
- However, due to liothyronine's shorter half-life, the daily dose is often divided into two administrations 1
Special Considerations
- Liothyronine has a rapid onset and dissipation of action compared to levothyroxine, leading to wider swings in serum T3 levels 4
- Consider twice-daily dosing to minimize fluctuations in T3 levels 1
- Patients with cardiac disease require more careful monitoring due to potential cardiovascular effects 3
- Elderly patients should start with lower doses and increase more gradually 4
Potential Pitfalls
- Avoid rapid dose escalation, which may precipitate cardiac symptoms, especially in elderly or those with cardiovascular disease 3
- Be aware that the wide swings in serum T3 levels that follow liothyronine administration may counterbalance its advantages 4
- Monitor closely for overtreatment, which can lead to subclinical or overt hyperthyroidism with risks of atrial fibrillation and bone loss 3
- Remember that liothyronine is not recommended as first-line therapy for most hypothyroid patients 1