What is the equivalent dose of Levothyroxine (T4) for a patient taking 150 mg of Armour thyroid (desiccated thyroid extract) daily?

Equivalent Levothyroxine Dose for 150 mg Armour Thyroid

The equivalent levothyroxine dose for a patient taking 150 mg of Armour thyroid (desiccated thyroid extract) daily is approximately 150 mcg of levothyroxine.

Conversion Rationale

When transitioning from desiccated thyroid extract (DTE) to levothyroxine (T4), it's important to understand the hormonal composition differences:

• Armour thyroid contains both T4 and T3 hormones in a ratio of approximately 4:1
• 150 mg of Armour thyroid contains approximately 95 mcg of T4 and 22.5 mcg of T3
• The T3 component has approximately 3-4 times the metabolic potency of T4

Dosing Algorithm:

1. Start with a 1:1 conversion ratio (150 mcg levothyroxine for 150 mg Armour)
2. Monitor TSH and free T4 after 6-8 weeks
3. Adjust dose as needed based on laboratory values

Monitoring and Adjustment

• Check TSH and free T4 levels 6-8 weeks after initiating levothyroxine 1
• Target TSH should be within the reference range (typically 0.4-4.5 mIU/L)
• If TSH remains elevated, increase levothyroxine dose by 12.5-25 mcg 1
• Recheck thyroid function tests 6-8 weeks after any dose adjustment

Special Considerations

• Age and cardiac status: For patients over 60 years or with known cardiovascular disease, consider starting at a lower dose (75-100 mcg) and titrating upward gradually 1
• Pregnancy: Women who become pregnant while on levothyroxine should increase their weekly dosage by 30% (taking one extra dose twice weekly) 2
• Medication interactions: Certain medications can affect levothyroxine absorption or metabolism, requiring dose adjustments

Common Pitfalls to Avoid

1. Bioequivalence issues: Different levothyroxine brands may have different bioavailability; maintain the same brand when possible 3
2. Overtreatment: Avoid excessive dosing that suppresses TSH below 0.2 mIU/L, which increases risk of atrial fibrillation and bone loss 3
3. Compounded T4/T3 combinations: These have inconsistent quality and can lead to iatrogenic hypothyroidism or hyperthyroidism 4
4. Inadequate monitoring: Failure to check thyroid function tests 6-8 weeks after dose changes can lead to prolonged under or over-replacement

Maintenance Therapy

Once the appropriate maintenance dose is established:

• Continue monitoring TSH and free T4 annually if stable 1
• The typical maintenance dose for most adults ranges between 75-250 mcg daily 3
• Take levothyroxine on an empty stomach, 30-60 minutes before breakfast or 3-4 hours after the last meal of the day

By following this approach, most patients can successfully transition from Armour thyroid to levothyroxine with appropriate dose adjustments based on laboratory monitoring.