Converting Synthroid (Levothyroxine) to Armour Thyroid (Desiccated Thyroid Extract)
Direct Conversion Is Not Recommended as Standard Practice
Levothyroxine monotherapy remains the standard of care for hypothyroidism, and switching to desiccated thyroid extract (DTE) should only be considered for patients who remain symptomatic despite adequate levothyroxine therapy with normalized TSH 1.
When Conversion May Be Considered
- DTE should only be considered as a trial therapy for patients who have "unambiguously not benefited from LT4" despite proper dosing and adherence 2.
- The decision to switch should be made after confirming that TSH is within the reference range (0.5-4.5 mIU/L) on levothyroxine, ruling out other causes of persistent symptoms, and ensuring the patient understands this is an off-guideline approach 1.
Approximate Conversion Ratios
Historical Thyroid Hormone Content Data
- One grain (60-65 mg) of desiccated thyroid contains approximately 38 mcg T4 and 9 mcg T3 3.
- More recent measurements using radioimmunoassay found that 1 grain of Armour Thyroid contains approximately 64 mcg T4 and 12 mcg T3 (T4/T3 molar ratio of 4.3:1) 4.
- The mean daily dose of DTE needed to normalize serum TSH contains approximately 11 mcg T3, though some patients may require higher doses 2.
Practical Conversion Approach
- A rough starting conversion is 100 mcg levothyroxine ≈ 60 mg (1 grain) desiccated thyroid extract 2, 3.
- For example, a patient on 100 mcg Synthroid could be started on 60 mg (1 grain) Armour Thyroid 2.
- A patient on 50 mcg Synthroid could be started on 30 mg (½ grain) Armour Thyroid 2.
Critical Safety Considerations
Monitoring Requirements
- Recheck TSH and free T4 in 6-8 weeks after any dose change, as this represents the time needed to reach steady state 1.
- Monitor for symptoms of hyperthyroidism including tachycardia, tremor, heat intolerance, or weight loss, as the T3 component in DTE can cause transient hypertriiodothyroninemia 2.
- Once stable, repeat TSH testing every 6-12 months or sooner if symptoms change 1.
Cardiovascular Risks
- For patients over 70 years or with cardiac disease, start with lower doses and titrate more cautiously, as rapid T3 delivery can unmask cardiac ischemia or precipitate arrhythmias 1.
- Prolonged TSH suppression (<0.1 mIU/L) increases risk for atrial fibrillation 3-5 fold, especially in elderly patients, and increases fracture risk in postmenopausal women 1.
Regulatory and Quality Concerns
- DTE remains outside formal FDA oversight, and consistency of T4 and T3 contents is monitored by manufacturers only, raising concerns about batch-to-batch variability 2.
- Bioavailability between different thyroid preparations can vary significantly (0.8 to 1.25-fold), necessitating TSH rechecking when switching brands 5.
Common Pitfalls to Avoid
- Never assume dose equivalence is exact—the T4:T3 ratio in DTE (approximately 4:1) differs substantially from the physiologic ratio, and individual responses vary 2, 4.
- Do not switch patients who are doing well on levothyroxine—there is no evidence that DTE is superior to levothyroxine for patients who are biochemically and clinically euthyroid 1, 2.
- Avoid making the switch in patients with cardiac disease, atrial fibrillation, or osteoporosis without careful risk-benefit assessment, as the T3 component poses higher cardiovascular and bone risks 1.
- Do not use DTE in pregnant women—levothyroxine monotherapy is the only recommended treatment during pregnancy, as T3 supplementation provides inadequate fetal thyroid hormone delivery 1.
Evidence Quality
- The evidence supporting DTE use is limited to observational studies and small trials—one observational study of 400 patients with mean follow-up of 9 years showed no increased mortality or cardiovascular morbidity compared to levothyroxine alone, but this does not establish superiority 2.
- Clinical guidelines consistently recommend levothyroxine as first-line therapy, with combination therapy (LT4+LT3 or DTE) reserved only for symptomatic patients who have failed adequate levothyroxine trials 1, 2.