How do you convert liothyronine (T3) 5 mcg to levothyroxine (T4)?

Converting Liothyronine 5 mcg to Levothyroxine

When converting liothyronine (T3) 5 mcg to levothyroxine (T4), use a 1:3 ratio, which means 5 mcg of liothyronine is equivalent to approximately 15 mcg of levothyroxine.

Conversion Rationale

The conversion between liothyronine and levothyroxine is based on their relative potencies and pharmacodynamic effects:

• According to pharmacodynamic equivalence studies, the therapeutic substitution ratio of liothyronine to levothyroxine is approximately 1:3 1
• This means that 1 mcg of liothyronine is roughly equivalent to 3 mcg of levothyroxine in terms of TSH suppression effects

Conversion Process

1. Identify the current liothyronine dose: 5 mcg
2. Apply the 1:3 conversion ratio: 5 mcg × 3 = 15 mcg levothyroxine
3. Consider monitoring TSH levels 6-8 weeks after conversion to ensure appropriate dosing

Important Clinical Considerations

Monitoring After Conversion

• Monitor TSH every 6-8 weeks while titrating hormone replacement to achieve TSH within the reference range 2
• FT4 can help interpret ongoing abnormal TSH levels, as TSH may take longer to normalize
• Once adequately treated, repeat testing every 6-12 months or as indicated by symptom changes

Potential Risks of Inappropriate Dosing

• Overdosing with levothyroxine can increase risk for:

  • Osteoporosis and fractures
  • Abnormal cardiac output
  • Ventricular hypertrophy 2
  • Heart failure and stroke, particularly with long-term use 3

• Underdosing may result in persistent hypothyroid symptoms

Special Populations

For patients with thyroid cancer requiring TSH suppression:

• The target TSH level should be reviewed by the treating endocrinologist 2
• TSH suppressive treatment with levothyroxine is beneficial in high-risk thyroid cancer patients to decrease progression of metastatic disease 2

Practical Application

When converting from liothyronine to levothyroxine:

1. Calculate the equivalent dose (5 mcg T3 = 15 mcg T4)
2. Consider starting at the calculated dose if the patient is otherwise healthy
3. For elderly patients (>70 years), frail patients, or those with cardiac disease, consider starting at a lower dose (e.g., 12.5 mcg) and titrating upward
4. Schedule follow-up in 6-8 weeks to check TSH levels and adjust dosing as needed
5. Monitor for symptoms of hypothyroidism or hyperthyroidism during the transition period

Remember that this conversion is an estimate, and individual patient response may vary, necessitating dose adjustments based on clinical response and laboratory parameters.