What is the equivalent dose of levothyroxine (T4) for 45mg of Armour Thyroid (desiccated thyroid)

Armour Thyroid 45mg Equivalent to Levothyroxine

45mg of Armour Thyroid (desiccated thyroid) is approximately equivalent to 75mcg of levothyroxine (T4). This is based on the T4 content of desiccated thyroid preparations.

Composition of Armour Thyroid

• One grain (60mg) of desiccated thyroid contains approximately 38mcg of T4 and 9mcg of T3 1
• Therefore, 45mg (3/4 grain) of Armour Thyroid contains approximately:
  • 28.5mcg of T4 (75% of 38mcg) 1
  • 6.75mcg of T3 (75% of 9mcg) 1

Conversion Considerations

• When converting from desiccated thyroid to levothyroxine, both the T4 and T3 content must be considered 2
• The T3 in desiccated thyroid has approximately 3-4 times the potency of T4 on a microgram basis 3
• The 6.75mcg of T3 in 45mg Armour Thyroid is roughly equivalent to an additional 20-27mcg of T4 3, 2
• Therefore, the total T4 equivalent of 45mg Armour Thyroid is approximately 48.5-55.5mcg (28.5mcg T4 + T3 equivalent) 3, 1

Clinical Dosing Recommendations

• When switching from desiccated thyroid to levothyroxine, a slightly higher dose of levothyroxine is typically needed to maintain the same TSH level 2
• Standard levothyroxine dosing for hypothyroidism is approximately 1.6-1.8mcg/kg of body weight 4
• For patients previously on desiccated thyroid, starting with 75mcg of levothyroxine would be appropriate for 45mg of Armour Thyroid 2
• TSH should be checked 6-8 weeks after switching medications to ensure proper dosing 5

Important Considerations When Switching

• Patients may experience transient symptoms when switching from combination T4/T3 therapy to T4-only therapy 2
• Some patients report better subjective well-being on combination therapy compared to T4 monotherapy 3
• Dose adjustments should be made based on TSH levels, with a target in the reference range 5
• For elderly patients or those with cardiac disease, more conservative dose adjustments (12.5mcg increments) are recommended 5

Monitoring After Medication Change

• Thyroid function tests should be repeated 6-8 weeks after changing from Armour Thyroid to levothyroxine 5
• Both TSH and free T4 should be used to guide therapy 5
• Once adequately treated, repeat testing every 6-12 months is recommended 5
• Bioequivalence can differ among levothyroxine brands, so maintaining the same brand is advisable 6

Remember that individual responses may vary, and some patients may require dose adjustments based on laboratory values and clinical symptoms after the switch.