Armour Thyroid 45mcg Equivalence to Levothyroxine
45mcg of Armour Thyroid (desiccated thyroid extract) is approximately equivalent to 75mcg of levothyroxine (T4).
Understanding Thyroid Replacement Options
- Desiccated thyroid extract (DTE) like Armour Thyroid contains both thyroxine (T4) and triiodothyronine (T3), while levothyroxine contains only T4 1
- The T4:T3 ratio in desiccated thyroid extract is approximately 4:1, and the average daily dose contains about 11mcg of T3 2
- When converting from Armour Thyroid to levothyroxine, it's important to consider that synthetic levothyroxine is the preferred treatment for hypothyroidism due to its consistent dosing and long half-life 1
Conversion Factors and Dosing Considerations
- For patients requiring thyroid hormone replacement, the typical starting dose of levothyroxine is 1.6 mcg/kg body weight per day 3
- When transitioning from Armour Thyroid to levothyroxine, the 45mcg dose of Armour would be replaced with approximately 75mcg of levothyroxine to maintain similar therapeutic effect 2
- After changing thyroid medication, TSH levels should be monitored after 6-8 weeks to ensure proper dosing, as this is when a new equilibrium is reached 1
Monitoring and Dose Adjustment
- The American Association of Clinical Endocrinologists recommends monitoring TSH and free T4 after 6-8 weeks of the adjusted dose when changing thyroid medications 4
- Target TSH should be within the reference range for most patients to prevent complications of both under and over-replacement 4
- Patients with TSH <0.1 mIU/L have greater risk of complications than those with TSH between 0.1-0.45 mIU/L, including increased risk of atrial fibrillation and bone loss 5
Special Considerations
- Bioequivalence can differ among generic and brand name levothyroxine products, which may affect dosing requirements 1
- Some patients may require dose adjustments based on individual factors such as age, weight, and concurrent medications 3
- For obese patients, calculating levothyroxine dose based on lean body mass (approximately 2.3 mcg/kg of lean body mass) may be more accurate than using actual body weight 6
Potential Pitfalls
- Serum T3 frequently rises to supranormal values in the absorption phase with desiccated thyroid products, which can be associated with palpitations and other symptoms of hyperthyroidism 1
- Overreplacement with levothyroxine (indicated by suppressed TSH) increases risk of atrial fibrillation and bone loss, so careful dose titration is essential 5
- Traditional levothyroxine dosing formulas may overestimate thyroid hormone requirements when using contemporary levothyroxine preparations 7