What is the levothyroxine dose equivalent to 105 mg of Armour Thyroid (desiccated thyroid)?

Armour Thyroid 105 mg Converts to Approximately 100 mcg Levothyroxine

For a patient on Armour Thyroid 105 mg (1.75 grains), the equivalent levothyroxine dose is approximately 100 mcg daily.

Conversion Rationale

The conversion is based on the FDA-approved composition of Armour Thyroid and established clinical practice:

• Each grain (60 mg) of Armour Thyroid contains 38 mcg T4 and 9 mcg T3 1
• 105 mg = 1.75 grains
• This provides approximately 66.5 mcg T4 and 15.75 mcg T3

Calculating the Levothyroxine Equivalent

When converting from combination T4/T3 therapy to levothyroxine monotherapy, you must account for the higher potency of T3. T3 is approximately 3-4 times more potent than T4 on a microgram-for-microgram basis 2.

Using the conversion calculation:

• T4 content: 66.5 mcg
• T3 equivalent in T4: 15.75 mcg × 3 = ~47 mcg T4 equivalents
• Total T4 equivalent: approximately 100-112 mcg

Important Clinical Considerations

Why Guidelines Recommend Against DTE

Current AACE/ATA guidelines explicitly state that desiccated thyroid hormone should NOT be used for the treatment of hypothyroidism 2. This is a Grade D recommendation based on:

• Lack of systematic study of absorption and bioavailability
• Variable T4:T3 ratios between brands and batches
• Supraphysiologic T3 peaks that can occur with DTE 2

Monitoring After Conversion

When switching from Armour Thyroid to levothyroxine:

• Check TSH and free T4 in 4-8 weeks after the switch 2
• Expect TSH to potentially rise initially as the T3 component is removed
• The patient may experience transient symptoms during the adjustment period
• Continue monitoring every 4-8 weeks until stable euthyroid indices are achieved 2

Common Pitfalls to Avoid

1. Don't use a simple 1:1 grain-to-mcg conversion - This ignores the T3 content entirely
2. Anticipate patient resistance - Nearly half of patients in studies preferred DTE over levothyroxine 3, 4, often due to perceived symptom improvement
3. Watch for cardiac symptoms - The T3 in DTE can cause tachycardia and increased heart rate 5, 6; removing it may improve cardiovascular parameters
4. Consider starting slightly lower (75-88 mcg) in elderly patients or those with cardiac disease, then titrating up based on TSH 2

Patient Counseling Points

Explain to the patient that:

• Levothyroxine monotherapy is the evidence-based standard of care 7
• They may feel different initially as their body adjusts to the absence of direct T3
• Weight may increase slightly (DTE is associated with modest weight loss of 3-4 lbs) 3
• The goal is TSH normalization with symptom resolution, which typically occurs within 2-3 weeks of achieving target TSH 1

Start with 100 mcg levothyroxine daily, check TSH/free T4 in 4-8 weeks, and adjust in 12.5-25 mcg increments as needed to achieve target TSH.