What is the appropriate levothyroxine (T4) dose to switch a 30-year-old female with hypothyroidism from Armour thyroid (desiccated thyroid) 60 mg daily and liothyronine (T3) 10 mcg twice daily (bid)?

From the Guidelines

The appropriate levothyroxine (T4) dose for a 30-year-old female switching from Armour thyroid 60 mg daily plus liothyronine (T3) 10 mcg twice daily would be approximately 100-112 mcg daily, based on the estimated full replacement dose using ideal body weight of approximately 1.6 mcg/kg/d, as suggested by 1. To determine the ideal starting dose, we need to consider the patient's current thyroid hormone replacement regimen. Armour thyroid 60 mg contains approximately 38 mcg of T4 and 9 mcg of T3, and the additional liothyronine provides 20 mcg of T3. Since T3 is approximately 3-4 times more potent than T4, the T3 component (total 29 mcg) is equivalent to roughly 87-116 mcg of T4. Combined with the T4 component, this equals approximately 125-154 mcg T4 equivalent.

• Key considerations for the dose conversion include:
  • The patient's age and lack of comorbidities, which suggest that full replacement can be estimated using ideal body weight, as noted in 1.
  • The potential for temporary symptoms of hypothyroidism during the transition period as the body adapts to the new regimen.
  • The need for monitoring with thyroid function tests (TSH, free T4) after 6-8 weeks of therapy to assess if dose adjustments are needed. Given these factors, starting with levothyroxine 100 mcg daily, taken on an empty stomach 30-60 minutes before breakfast, is a reasonable approach, allowing for adjustments as needed based on follow-up thyroid function tests, and considering the guidance provided by 1.

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 For patients at risk of atrial fibrillation or patients with underlying cardiac disease, start with a lower dosage and titrate the dosage more slowly to avoid exacerbation of cardiac symptoms. 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 60 mg daily and liothyronine 10 mcg twice daily to levothyroxine, the full replacement dose is estimated to be 1.6 mcg/kg/day. Given the patient's current dosage of Armour thyroid and liothyronine, the equivalent levothyroxine dose can be estimated, but the exact dose is not directly provided in the label. However, a common conversion is to use the following approximate equivalencies:

• 60 mg of Armour thyroid is roughly equivalent to 90-100 mcg of levothyroxine
• 20 mcg of liothyronine is roughly equivalent to 50 mcg of levothyroxine So the total equivalent levothyroxine dose would be approximately 140-150 mcg. It's recommended to start with a lower dose, such as 100-125 mcg daily, and titrate the dosage by 12.5 to 25 mcg increments every 4 to 6 weeks, as needed, until the patient is euthyroid, based on serum TSH or free-T4 levels 2.

From the Research

Switching to Levothyroxine

To switch a 30-year-old female with hypothyroidism from Armour thyroid 60 mg daily and liothyronine 10 mcg twice daily to levothyroxine, consider the following steps:

• Calculate the equivalent levothyroxine dose: The exact equivalent dose is not directly stated in the provided studies, but it is known that Armour thyroid contains both T4 and T3, and the conversion can be complex 3, 4.
• Start with a low dose: Many hypothyroid patients start levothyroxine treatment at a low dose, especially if they have residual thyroid function or significant comorbidities 4.
• Monitor TSH and free T4 levels: Regular monitoring of TSH and free T4 levels is crucial to adjust the levothyroxine dose and achieve euthyroidism 3, 5.
• Consider combination therapy: Some patients may require combination therapy with levothyroxine and liothyronine, especially if they have persistent symptoms or metabolic abnormalities despite normalization of serum thyrotropin level 5, 6.

Factors to Consider

When switching to levothyroxine, consider the following factors:

• Residual thyroid function: Patients with residual thyroid function may require lower doses of levothyroxine 4.
• Body weight: Levothyroxine dose may need to be adjusted based on body weight or lean body mass 3.
• Comorbidities: Patients with significant comorbidities, such as cardiac disease, may require lower doses of levothyroxine 4.
• Genetic factors: Some patients may have genetic factors that affect their response to levothyroxine, such as polymorphisms in the DIO2 gene 5, 7.

Dose Adjustment

To adjust the levothyroxine dose, consider the following:

• Start with a low dose and gradually increase as needed 4.
• Monitor TSH and free T4 levels regularly to adjust the dose 3, 5.
• Consider combination therapy with liothyronine if patients have persistent symptoms or metabolic abnormalities despite normalization of serum thyrotropin level 5, 6.
• Be aware of the potential for resistance to exogenous thyroxine, characterized by elevated TSH and decreased T3/T4 ratio 7.