No Waiting Period Required When Switching from Synthroid to Armour Thyroid
You can switch directly from levothyroxine (Synthroid) to Armour Thyroid without any washout period. The guidelines and drug labels do not specify or require any waiting period between these medications 1, 2.
Key Switching Principles
Direct Transition Protocol
- Stop levothyroxine and start Armour Thyroid the next day using an appropriate dose conversion
- The long half-life of levothyroxine (approximately 7 days) means residual T4 will remain in your system during the transition, preventing any gap in thyroid hormone coverage 2
- No washout period is needed because both medications provide thyroid hormone replacement—you're simply changing the formulation and T4:T3 ratio
Dose Conversion Considerations
When switching, recognize that:
- Armour Thyroid contains both T4 and T3 in approximately a 4:1 ratio by weight 1
- The T3 component is more rapidly absorbed and has a shorter half-life than T4
- Typical conversion is roughly 100 mcg levothyroxine = 60-65 mg (1 grain) of Armour Thyroid, though individual needs vary
Monitoring After the Switch
Check TSH and free T4 levels 4-8 weeks after switching 1:
- This timing allows for steady-state levels to be achieved
- The TSH may take 4-6 weeks to reflect the new treatment regimen 2
- Dose adjustments should be made based on these results and clinical response
Important Caveats
Cardiovascular Considerations
If you have underlying heart disease or are over 50-60 years old:
- Start with a lower dose of Armour Thyroid (not full replacement)
- Monitor closely for anginal symptoms, palpitations, or arrhythmias 1
- The T3 component in Armour Thyroid has more immediate cardiovascular effects than levothyroxine alone
Guideline Position on Desiccated Thyroid
Be aware that current guidelines do not recommend desiccated thyroid hormone over levothyroxine monotherapy 1. The 2012 AACE/ATA guidelines state there is no evidence supporting its preferential use. However, some patients report subjective preference for DTE despite lack of objective superiority in clinical trials 3, 4.
Practical Monitoring
After switching:
- Take Armour Thyroid consistently 30-60 minutes before breakfast on an empty stomach 1
- Avoid taking it within 4 hours of calcium, iron supplements, or antacids 2
- Watch for signs of over-replacement: increased heart rate, palpitations, anxiety, tremor, or weight loss
- Watch for signs of under-replacement: persistent fatigue, weight gain, cold intolerance
The absence of a required washout period makes the transition straightforward from a timing perspective, but careful dose conversion and subsequent monitoring remain essential for optimal outcomes.