What is the levothyroxine equivalent dose for a 180 mg tablet of Armour Thyroid (desiccated thyroid extract)?

Armour Thyroid 180 mg to Levothyroxine Conversion

For a patient taking Armour Thyroid 180 mg (3 grains), the equivalent levothyroxine dose is approximately 270–300 mcg daily, though individual titration based on TSH monitoring is essential.

Understanding Desiccated Thyroid Extract Composition

• Armour Thyroid (desiccated thyroid extract) contains both T4 and T3 in a fixed ratio of approximately 4:1, which differs from the body's natural production and peripheral conversion 1
• Each grain (60 mg) of desiccated thyroid extract contains approximately 38 mcg T4 and 9 mcg T3 1
• Therefore, 180 mg (3 grains) provides approximately 114 mcg T4 and 27 mcg T3 1

Conversion Rationale and Calculation

The traditional conversion ratio suggests that 1 grain (60 mg) of desiccated thyroid is roughly equivalent to 100 mcg of levothyroxine 2. Using this ratio:

• 180 mg Armour Thyroid = 3 grains
• 3 grains × 100 mcg levothyroxine per grain = 300 mcg levothyroxine

However, because desiccated thyroid provides direct T3 (which is 3–4 times more potent than T4), some patients may require slightly less levothyroxine than the straight mathematical conversion suggests 1.

Recommended Conversion Strategy

Start with levothyroxine 270–300 mcg daily and titrate based on TSH response after 6–8 weeks 3, 4.

• The goal is to normalize serum TSH to 0.4–4.0 mIU/L (or 0.5–4.5 mIU/L per some guidelines) 4, 5
• Recheck TSH and free T4 at 6–8 weeks after the conversion, as this represents the time needed to reach steady-state levels 3, 5
• Adjust the levothyroxine dose by 12.5–25 mcg increments based on TSH results 5

Special Considerations for This Conversion

Why Levothyroxine Monotherapy Is Preferred

• Levothyroxine monotherapy provides uniform levels of both T4 and T3 through peripheral conversion without diurnal variation, making it the preparation of choice for most patients 2
• The body's peripheral conversion from T4 to T3 is assumed to provide the exact amount of T3 needed by every organ 6
• Desiccated thyroid extract remains outside formal FDA oversight, and consistency of T4 and T3 contents is monitored only by manufacturers 1

Patient-Specific Dosing Adjustments

• For patients <70 years without cardiac disease, the full calculated dose can be initiated 5, 2
• For patients >70 years or with cardiac disease, start with 25–50 mcg daily and titrate gradually by 12.5–25 mcg every 6–8 weeks to avoid precipitating cardiac ischemia or arrhythmias 5, 2
• The usual required dosage of levothyroxine is approximately 1.6 mcg/kg body weight, but this may need adjustment based on age and comorbidities 6, 4

Factors Affecting Levothyroxine Absorption

• Intestinal absorption of levothyroxine is around 80%, but only in an acidic environment 6
• Absorption is significantly decreased by proton-pump inhibitors, antacids, atrophic gastritis, and Helicobacter pylori infection 6
• Levothyroxine should be taken on an empty stomach, 30–60 minutes before breakfast, and at least 4 hours apart from iron, calcium supplements, or antacids 5
• The bioavailability of different levothyroxine generics might vary between 0.8 and 1.25, so thyroid function should be rechecked if switching brands 6

Monitoring Protocol After Conversion

• Monitor TSH and free T4 every 6–8 weeks during dose titration 5, 3
• Once the target TSH is achieved (0.4–4.5 mIU/L with normal free T4), repeat testing every 6–12 months 5, 4
• Free T4 can help interpret ongoing abnormal TSH levels during therapy, as TSH may take longer to normalize 5

Common Pitfalls to Avoid

• Never assume the patient will feel the same immediately after conversion – some patients report persistent symptoms when switched from combination T4+T3 therapy (like desiccated thyroid) to levothyroxine monotherapy, even with normalized TSH 1
• Avoid excessive dose increases that could lead to iatrogenic hyperthyroidism, which increases risk for osteoporosis, fractures, atrial fibrillation, and cardiovascular complications 5, 2
• Do not adjust doses too frequently before reaching steady state – wait the full 6–8 weeks between adjustments 5, 3
• Approximately 25% of patients on levothyroxine are unintentionally maintained on doses sufficient to fully suppress TSH, increasing serious complication risks 5

If Patient Remains Symptomatic on Levothyroxine

• For patients who remain symptomatic despite normalized TSH on levothyroxine monotherapy, a trial of combination therapy with LT4+LT3 can be considered 1
• Reduce the LT4 dose by 25 mcg/day and add 2.5–7.5 mcg liothyronine once or twice daily as an appropriate starting point 1
• Trials following almost 1000 patients for almost 1 year indicate that therapy with LT4+LT3 can restore euthyroidism while maintaining normal serum TSH 1