Starting Dose of Levothyroxine for Adults with Hypothyroidism
The recommended starting daily dosage of levothyroxine for adults with primary hypothyroidism is 1.6 mcg/kg/day for most patients, while older patients (>70 years) or those with cardiac disease should start with a lower dose of 25-50 mcg/day. 1, 2
Dosing Guidelines Based on Patient Characteristics
Standard Adult Dosing
- Full replacement dose: 1.6 mcg/kg/day 1
- Typically 75-100 mcg/day for women
- Typically 100-150 mcg/day for men
Special Populations Requiring Lower Starting Doses
- Elderly patients (>70 years): Start with 25-50 mcg/day 2, 3
- Patients with cardiac disease or risk factors: Start with less than 1.6 mcg/kg/day 1
- Frail patients with multiple comorbidities: Start with 25-50 mcg/day 2
Titration Protocol
- For standard adult patients: Increase by 12.5-25 mcg increments every 4-6 weeks until euthyroid 1
- For elderly or cardiac patients: More conservative titration every 6-8 weeks 1
- Monitor TSH every 6-8 weeks while titrating dose 2
- Target TSH within the reference range (0.45-4.5 mIU/L) 4
- For reproductive-age women, consider targeting TSH in mid-normal range (1.0-2.5 mIU/L) 4
Clinical Considerations
Safety
Research has shown that starting with a full replacement dose (1.6 mcg/kg/day) in cardiac asymptomatic patients is safe and may be more convenient and cost-effective than a low starting dose regimen 5. However, this approach should be avoided in elderly patients or those with cardiac disease.
Dose Variations by Etiology
The optimal levothyroxine dose varies based on the cause of hypothyroidism:
- Patients with atrophic thyroiditis typically require lower doses (approximately 1.26 mcg/kg/day)
- Patients with Hashimoto's thyroiditis or post-radioiodine hypothyroidism require moderate doses (approximately 1.5-1.6 mcg/kg/day)
- Patients with central hypothyroidism or post-thyroidectomy for thyroid cancer require higher doses (1.8-2.1 mcg/kg/day) 6
Age-Related Considerations
Levothyroxine requirements decrease with age, likely reflecting the progressive decrease in thyroxine degradation rate that occurs with aging 3. This is why elderly patients need lower starting doses and often require lower maintenance doses.
Common Pitfalls to Avoid
- Overtreatment: Can lead to subclinical hyperthyroidism, bone mineral density loss, and increased risk of atrial fibrillation 4
- Undertreatment: May result in persistent hypothyroid symptoms and increased cardiovascular risk 4
- Medication interactions: Certain foods (soy products), medications (calcium, iron supplements), and conditions (malabsorption) can affect levothyroxine absorption 1
- Ignoring age-related changes: Failing to reduce doses in elderly patients can lead to adverse effects 3
Monitoring
- Check TSH and free T4 levels every 6-8 weeks during dose adjustments 2, 4
- Once stable, monitor every 6-12 months 2
- For central hypothyroidism, use free T4 levels (not TSH) to guide therapy, targeting the upper half of normal range 1
By following these guidelines, levothyroxine replacement therapy should be effective, well-tolerated, and associated with minimal complications when properly monitored.