NP Thyroid to Levothyroxine Conversion
The standard conversion ratio is approximately 60-65 mg of NP Thyroid (desiccated thyroid extract) equals 100 mcg of levothyroxine, though individual titration based on TSH monitoring is essential. 1
Conversion Protocol
Initial Conversion Ratio
- Use a 60 mg DTE to 100 mcg levothyroxine conversion as your starting point 1
- For example, if a patient takes 120 mg NP Thyroid daily, convert to approximately 200 mcg levothyroxine 1
- This ratio accounts for the T4/T3 content difference: DTE contains both T4 and T3 in a 4:1 ratio, while levothyroxine is pure T4 1
Age-Specific Considerations
- For patients under 65 years without cardiac disease: Start with the full calculated conversion dose 2, 3
- For patients over 65 years: Reduce the calculated dose by 25% initially (e.g., if calculation suggests 200 mcg, start at 150 mcg) 4
- For patients over 70 years or with cardiac disease: Start at an even lower dose (12.5-50 mcg less than calculated) and titrate more slowly 2, 3
Critical Safety Considerations
- Never make the switch abruptly in elderly patients or those with cardiac disease, as levothyroxine may unmask cardiac ischemia 2, 5
- Check baseline TSH and free T4 before conversion to establish current thyroid status 2
- Rule out adrenal insufficiency before initiating or increasing levothyroxine, especially in patients with autoimmune conditions, as thyroid hormone can precipitate adrenal crisis 2, 6
Monitoring After Conversion
Initial Monitoring Period
- Recheck TSH and free T4 at 6-8 weeks after conversion to assess adequacy of replacement 2, 6, 3
- The peak therapeutic effect may not be attained for 4-6 weeks 3
- Target TSH should be 0.5-4.5 mIU/L with normal free T4 2, 6
Dose Adjustments
- Adjust levothyroxine in 12.5-25 mcg increments based on TSH results 2, 6
- For patients under 70 without cardiac disease, use 25 mcg increments 6
- For elderly or cardiac patients, use smaller 12.5 mcg increments 6
- Wait a full 6-8 weeks between dose adjustments to allow steady state 2, 6
Long-Term Monitoring
- Once TSH is stable in target range, monitor TSH every 6-12 months 2, 6
- Recheck sooner if symptoms change or new medications are started 2
Why Patients May Prefer DTE
Patient Preference Data
- In a randomized crossover trial, 48.6% of patients preferred DTE over levothyroxine, while only 18.6% preferred levothyroxine 7
- Patients on DTE lost an average of 3 pounds compared to levothyroxine 7
- Those who preferred DTE reported significantly better subjective symptoms on quality of life questionnaires 7
Managing Patient Expectations
- Explain that levothyroxine is the FDA-approved standard of care and DTE remains outside formal FDA oversight 1
- Some patients may experience transient symptoms during conversion that resolve within 6-8 weeks 2
- If symptoms persist after 3-4 months with normalized TSH on levothyroxine, consider a trial of LT4+LT3 combination therapy rather than returning to DTE 1
Common Pitfalls to Avoid
- Do not use a 1:1 grain-to-mcg conversion (e.g., 1 grain = 60 mg ≠ 60 mcg levothyroxine) 1
- Avoid starting elderly or cardiac patients at full replacement doses, which can precipitate myocardial infarction or arrhythmias 2, 5
- Do not adjust doses more frequently than every 6-8 weeks, as levothyroxine requires this time to reach steady state 2, 6
- Never assume the conversion will be perfect—approximately 25% of patients require dose adjustments after initial conversion 2
- Do not ignore persistent symptoms with normal TSH—these patients may benefit from referral to endocrinology for consideration of combination T4+T3 therapy 1, 5
Special Populations
Pregnant Patients
- Increase levothyroxine dose by 25-50% immediately upon pregnancy confirmation if converting a pregnant patient from DTE 3, 5
- Monitor TSH every 4 weeks during pregnancy with target TSH <2.5 mIU/L in first trimester 3
- DTE is not recommended during pregnancy—levothyroxine monotherapy is the only appropriate treatment 5