What is the appropriate dose of levothyroxine (T4) to start when converting a patient from NP thyroid (thyroid extract) 120 mg?

Converting NP Thyroid 120 mg to Levothyroxine

When converting from NP Thyroid 120 mg to levothyroxine, the appropriate starting dose is approximately 100-112 mcg of levothyroxine daily.

Conversion Rationale

NP Thyroid is a desiccated thyroid extract (DTE) containing both T4 and T3 in an approximate ratio of 4:1. The conversion to levothyroxine (T4 only) requires careful consideration to maintain euthyroidism while avoiding both under and overtreatment.

Understanding the Composition

• NP Thyroid 120 mg contains approximately:
  • T4 (levothyroxine): ~76 mcg
  • T3 (liothyronine): ~18 mcg

Conversion Algorithm

1. Calculate equivalent T4 dose:

   • Start with the T4 content in NP Thyroid 120 mg (~76 mcg)
   • Account for the T3 component by adding an additional 25-30 mcg of T4 to compensate for the lost T3 activity
   • This yields approximately 100-112 mcg of levothyroxine

2. Dosing considerations:

   • For young, healthy adults: Start with the calculated dose (100-112 mcg daily)
   • For elderly patients (>60 years): Start with a lower dose of 50-75 mcg daily and titrate upward
   • For patients with cardiac disease: Start with 25-50 mcg daily and titrate more slowly 1

Monitoring After Conversion

1. Initial follow-up:

   • Check TSH and free T4 after 6-8 weeks of treatment 1
   • Adjust dose as needed based on results

2. Dose adjustments:

   • If TSH remains elevated: Increase dose by 12.5-25 mcg 1
   • If TSH is suppressed (<0.1 mIU/L): Decrease dose 2

3. Long-term monitoring:

   • Once stable, monitor annually or sooner if clinical status changes 1

Special Considerations

• Elderly patients: Start with lower doses (25-50 mcg daily) and titrate gradually to avoid cardiac complications 1
• Cardiac disease: Use lower starting doses and more gradual titration to avoid exacerbating cardiac conditions 1
• Adrenal insufficiency: If present, must be treated with glucocorticoids before starting thyroid hormone replacement 1

Common Pitfalls to Avoid

1. Overtreatment: Approximately 25% of patients on levothyroxine are inadvertently maintained on doses high enough to suppress TSH, which can lead to adverse effects on bone mineral density and cardiovascular health 1

2. Absorption issues: Levothyroxine should be taken on an empty stomach, and administration should be separated from other medications by at least 4 hours 1

3. Inadequate monitoring: Failure to check thyroid function tests at appropriate intervals can lead to prolonged periods of under or overtreatment

4. Patient symptoms: Some patients may report persistent symptoms despite normalization of TSH. This could be due to the loss of T3 from the desiccated thyroid extract 3. If symptoms persist despite normalized TSH, consider evaluation for other causes.

By following this approach, most patients can be safely transitioned from NP Thyroid to levothyroxine with minimal disruption to their thyroid status while prioritizing long-term morbidity and mortality outcomes.