How to switch a 30-year-old female with hypothyroidism from Armour thyroid (thyroid desiccated) 30 mg daily and Liothyronine (T3) 10 mcg twice daily (bid) to an appropriate dose of levothyroxine (T4)?

From the Guidelines

To switch a 30-year-old female with hypothyroidism from Armour thyroid 30 mg daily and Liothyronine 10 mcg twice daily to levothyroxine, I recommend starting with levothyroxine 100 mcg once daily. This conversion is based on the T4 content of the current regimen, considering the patient's age and lack of risk factors such as cardiac disease or multiple comorbidities, as suggested by 1. The calculation involves understanding that Armour thyroid 30 mg contains approximately 19 mcg of T4 and 4.5 mcg of T3, while the patient is also taking 20 mcg of T3 daily. Since T3 is roughly four times as potent as T4, the 24.5 mcg total T3 is equivalent to about 98 mcg of T4. Combined with the 19 mcg of T4 already in the Armour thyroid, this approximates to 100 mcg of levothyroxine. Key points to consider in this transition include:

• Discontinuing both previous medications and starting levothyroxine the next day.
• Taking levothyroxine on an empty stomach 30-60 minutes before breakfast or 3-4 hours after the last meal of the day.
• Checking thyroid function tests (TSH, free T4) 6-8 weeks after the switch to ensure adequate replacement, as the goal is to achieve a TSH level within the normal range, considering the patient's current TSH of 0.393, free T4 of 0.32, and free T3 of 3.1.
• Being aware that some patients may experience symptoms during the transition as T3 levels decrease, since levothyroxine provides only T4, which the body must convert to the active T3 form, and considering a small dose of liothyronine if persistent symptoms occur despite normal lab values, as guided by clinical judgment and patient response. The dose of levothyroxine can be estimated using ideal body weight for a dose of approximately 1.6 mcg/kg/d, as suggested by 1, but in this case, the conversion from the current regimen provides a more direct approach to determining the initial levothyroxine dose.

From the FDA Drug Label

The recommended starting daily dosage of levothyroxine sodium tablets in adults with primary, secondary, or tertiary hypothyroidism is based on age and comorbid cardiac conditions, as described in Table 1 Full replacement dose is 1.6 mcg/kg/day. Titrate dosage by 12.5 to 25 mcg increments every 4 to 6 weeks, as needed until the patient is euthyroid.

To switch a 30-year-old female with hypothyroidism from Armour thyroid 30 mg daily and Liothyronine 10 mcg twice daily to an appropriate dose of levothyroxine, we need to calculate the full replacement dose based on her weight.

• Assuming an average weight of 60 kg for a 30-year-old female, the full replacement dose would be 1.6 mcg/kg/day * 60 kg = 96 mcg/day.
• However, this dose may need to be adjusted based on her individual response and laboratory parameters.
• It's also important to note that Liothyronine (T3) is being used in conjunction with Armour thyroid, so the conversion to levothyroxine (T4) may require a gradual tapering of the T3 dose to avoid adverse effects.
• The patient's dosage should be titrated by 12.5 to 25 mcg increments every 4 to 6 weeks, as needed until she is euthyroid, based on serum TSH or free-T4 levels 2.
• Key considerations for dosing include:
  • Patient's age
  • Body weight
  • Cardiovascular status
  • Concomitant medical conditions
  • Concomitant medications
  • Co-administered food
  • Specific nature of the condition being treated
• The peak therapeutic effect of a given dose of levothyroxine may not be attained for 4 to 6 weeks.

From the Research

Switching to Levothyroxine

To switch a 30-year-old female with hypothyroidism from Armour thyroid 30 mg daily and Liothyronine 10 mcg twice daily to an appropriate dose of levothyroxine, consider the following:

• The patient's current dose of Armour thyroid is 30 mg daily, which is equivalent to approximately 60-90 mcg of levothyroxine, as the conversion rate is roughly 1:2 to 1:3 for desiccated thyroid to levothyroxine 3.
• The patient is also taking Liothyronine 10 mcg twice daily, which may need to be considered when calculating the levothyroxine dose, as some patients may require combination therapy 4, 5, 6.
• The patient's TSH level is 0.393, free T4 is 0.32, and free T3 is 3.1, indicating that she is currently euthyroid on her current medication regimen.

Calculating the Levothyroxine Dose

To calculate the levothyroxine dose, consider the following:

• The patient's weight and body mass index (BMI) are not provided, but the general starting dose for levothyroxine is 50-100 mcg daily for full replacement therapy 7, 3.
• Given the patient's current dose of Armour thyroid and Liothyronine, a starting dose of levothyroxine could be estimated to be around 75-100 mcg daily, with adjustments made based on her TSH and free T4 levels.
• It is essential to monitor the patient's TSH and free T4 levels regularly to adjust the levothyroxine dose and ensure that she remains euthyroid.

Considerations for Combination Therapy

Consider the following when deciding whether to use combination therapy with levothyroxine and Liothyronine:

• Some patients may require combination therapy, especially those with polymorphisms in the type 2 deiodinase gene 4, 5, 6.
• The patient's current dose of Liothyronine is 10 mcg twice daily, which may need to be adjusted or discontinued when switching to levothyroxine monotherapy.
• The decision to use combination therapy should be based on the patient's individual needs and response to treatment, as well as the results of clinical trials and expert consensus statements 6.