What is the initial dosing for levothyroxine (thyroid hormone replacement) therapy?

Initial Levothyroxine Dosing Guidelines

For adult patients with primary hypothyroidism, the recommended starting dose of levothyroxine is 1.6 mcg/kg/day for patients under 70 years without cardiac disease, while older patients or those with cardiac disease should start at lower doses of 25-50 mcg/day with gradual titration. 1, 2, 3

Age and Comorbidity-Based Dosing

• For patients under 70 years without cardiac disease or other comorbidities, start with full replacement dose of 1.6 mcg/kg/day based on ideal body weight 1, 2
• For patients over 70 years and/or those with cardiac disease or multiple comorbidities, start with a lower dose of 25-50 mcg/day 1, 2, 3
• For patients at risk of atrial fibrillation or with underlying cardiac disease, use a lower starting dose and titrate more slowly to avoid exacerbation of cardiac symptoms 3

Dose Titration Protocol

• Monitor TSH every 6-8 weeks while titrating hormone replacement to goal of TSH within the reference range 1, 2
• Increase dosage by 12.5-25 mcg increments every 4-6 weeks until the patient is euthyroid 2, 3
• Free T4 can be used to help interpret ongoing abnormal TSH levels on therapy, as TSH may take longer to normalize 1
• Once adequately treated, repeat testing every 6-12 months or as indicated for a change in symptoms 1, 2

Special Populations

Subclinical Hypothyroidism

• For asymptomatic patients with TSH between 4.5-10 mIU/L, treatment is controversial 1
• For patients with TSH >10 mIU/L, levothyroxine therapy is generally recommended, even if asymptomatic 1

Pregnant Patients

• Pregnant women with elevated TSH should be treated with levothyroxine to restore TSH to the reference range 1
• Monitor TSH every 6-8 weeks during pregnancy and adjust dose as needed, as requirements often increase during pregnancy 1

Thyroid Cancer Patients

• For patients with thyroid cancer, TSH suppression may be indicated 1
• Maintain TSH levels <0.1 mIU/L for patients with persistent disease or high risk of recurrence 1

Common Pitfalls and Caveats

• Avoid relying on a single abnormal TSH value for diagnosis or treatment decisions, as TSH levels can vary and may normalize without treatment 1
• Be aware that levothyroxine absorption can be reduced by certain medications and foods (iron, calcium supplements, etc.) 3, 4
• Overtreatment risks include thyrotoxicosis symptoms, osteoporotic fractures, and atrial fibrillation, especially in elderly patients 4
• For severe hypothyroidism with myxedema, hospital admission is required with endocrinology consultation 1
• If central hypothyroidism is suspected (low TSH with low free T4), hydrocortisone should be given before starting thyroid hormone 1

Monitoring for Treatment Success

• Primary hypothyroidism: Target TSH within the normal reference range 1, 3
• Secondary/tertiary hypothyroidism: Target free T4 in the upper half of normal range (TSH is not reliable) 3
• Development of a low TSH on therapy suggests overtreatment or recovery of thyroid function; dose should be reduced or discontinued with close follow-up 1
• The peak therapeutic effect of levothyroxine may not be attained for 4-6 weeks due to its long half-life 3, 4

Following these evidence-based dosing guidelines will help optimize treatment outcomes while minimizing risks associated with both under- and over-replacement of thyroid hormone.