Switching from Synthroid (Levothyroxine) to Armour Thyroid (Desiccated Thyroid)
When switching from Synthroid (levothyroxine) to Armour Thyroid (desiccated thyroid), start with a dose of Armour Thyroid that is approximately one-fourth of the current levothyroxine dose in micrograms. This conversion is based on the T4:T3 ratio in desiccated thyroid preparations and clinical experience with combination therapy.
Conversion Ratio and Initial Dosing
- The standard conversion is to reduce the levothyroxine dose by 25 mcg and replace it with 2.5-7.5 mcg of T3 (which is contained in desiccated thyroid) 1
- For practical purposes, this typically translates to starting Armour Thyroid at a dose that provides approximately 25% of the current levothyroxine dose 2
- For example, if a patient is taking 100 mcg of levothyroxine, an appropriate starting dose of Armour Thyroid would be approximately 30-60 mg 1, 2
Monitoring and Dose Adjustment
- Check thyroid function tests (TSH and free T4) 6-8 weeks after initiating Armour Thyroid 3
- Target TSH should be within the reference range (0.5-4.5 mIU/L) with normal free T4 levels 3
- Dose adjustments should be made in small increments (typically 15-30 mg of Armour Thyroid) 1
- Once stabilized, monitor thyroid function every 6-12 months or if symptoms change 3
Special Considerations
- For patients over 70 years or with cardiac disease, start with a lower dose and titrate more gradually to avoid potential cardiac complications 3
- Patients with a history of coronary artery disease should be monitored more closely during the transition 4
- Be aware that desiccated thyroid contains both T4 and T3 in a fixed ratio of approximately 4:1, which may cause transient episodes of elevated T3 levels 5, 1
Potential Benefits and Risks
- Some patients who remain symptomatic on levothyroxine monotherapy may experience improvement in quality of life with desiccated thyroid 2
- A study of patients switched to desiccated thyroid showed significant improvements in ThyPRO scores (measuring thyroid symptomatology) and EQ-5D-5L scores (measuring quality of life) 2
- However, the higher T3 content in desiccated thyroid may cause palpitations in some patients due to transient supranormal T3 levels 5
Common Pitfalls to Avoid
- Avoid excessive dosing that could lead to iatrogenic hyperthyroidism, which increases risk for osteoporosis, fractures, and cardiac complications 3
- Do not adjust doses too frequently before reaching steady state (wait 6-8 weeks between adjustments) 3
- Be aware that the consistency of T4 and T3 contents in desiccated thyroid products may vary as they remain outside formal FDA oversight 1
- Monitor for signs of overtreatment including tachycardia, tremor, heat intolerance, or weight loss 3
By following these guidelines and closely monitoring thyroid function tests, most patients can successfully transition from levothyroxine to desiccated thyroid with appropriate dose adjustments.