Conversion of 60 mg Armour Thyroid to Levothyroxine
A 60-mg (1-grain) tablet of Armour Thyroid is approximately equivalent to 100 µg of levothyroxine, though individual dose adjustments based on TSH monitoring after 6–8 weeks are essential because desiccated thyroid contains both T4 and T3 in a fixed 4.22:1 ratio, whereas levothyroxine monotherapy relies on endogenous T4-to-T3 conversion. 1
Understanding the Pharmacologic Difference
- Armour Thyroid delivers both levothyroxine (T4) and liothyronine (T3) in a fixed 4.22:1 ratio, which differs from the body's usual 11:1 secretion ratio and from levothyroxine monotherapy that depends entirely on peripheral conversion of T4 to T3. 2, 1
- Desiccated thyroid preparations cause serum T3 to rise to supranormal values during the absorption phase, often associated with palpitations, because the T3 component is rapidly absorbed. 1
- Levothyroxine (synthetic T4) has a long half-life permitting once-daily dosing, and the extrathyroidal conversion of T4 into T3 normally provides 80% of daily T3 production, which may offer protective value during illness. 1
Standard Dose Equivalency
- The traditional conversion is that 60 mg (1 grain) of desiccated thyroid is approximately equivalent to 100 µg of levothyroxine, though this is an approximation and individual responses vary. 3
- The mean levothyroxine dose required to normalize serum TSH in hypothyroid patients is 1.6 µg/kg per day, which typically results in 75–100 µg/day for women and 100–150 µg/day for men. 1, 3
Monitoring After Conversion
- Wait 4–6 weeks after initiating levothyroxine before reassessing thyroid function tests (TSH and free T4), because this is the time required to achieve steady-state levels. 4
- Measure both TSH and free T4 to assess adequacy of replacement, with a target TSH in the reference range of 0.5–4.5 mIU/L and normal free T4. 5
- Dose adjustments should be made in 12.5–25 µg increments based on TSH results, with repeat testing every 6–8 weeks until the target range is achieved. 5
Special Considerations for the Conversion
- Patients switching from desiccated thyroid to levothyroxine may initially report different symptomatology because the direct T3 component is removed and they must rely entirely on T4-to-T3 conversion. 1
- Some patients previously on desiccated thyroid may require slightly higher levothyroxine doses to achieve the same TSH suppression, because serum free T4 concentrations on levothyroxine monotherapy tend to be slightly elevated or in the upper half of the normal reference range to generate the 20% of daily T3 production that would otherwise come from the thyroid gland itself. 1
- Once adequately treated on a stable levothyroxine dose, repeat TSH testing every 6–12 months or sooner if symptoms change. 5
Common Pitfalls to Avoid
- Do not assume a 1:1 dose equivalency between different thyroid preparations—desiccated thyroid contains both T4 and T3, whereas levothyroxine is T4 only, requiring individual dose titration. 1
- Avoid rechecking TSH too early (before 6–8 weeks after dose change), as steady-state levels have not yet been reached and premature adjustments may lead to inappropriate dosing. 5, 4
- Monitor for signs of overtreatment (TSH <0.1 mIU/L), which increases risk for atrial fibrillation, osteoporosis, fractures, and cardiovascular complications, especially in elderly patients. 5
- For patients over 70 years or with cardiac disease, start levothyroxine at a lower dose (25–50 µg/day) and titrate gradually to avoid unmasking cardiac ischemia or precipitating arrhythmias. 5