What is the proper dosing for Synthroid (levothyroxine) in adults with primary hypothyroidism?

Levothyroxine (Synthroid) Dosing for Primary Hypothyroidism in Adults

The recommended starting daily dosage of levothyroxine for adults with primary hypothyroidism is 1.6 mcg/kg/day based on ideal body weight, with dose titration based on TSH levels every 4-6 weeks until the patient is euthyroid. 1

Starting Dose Recommendations

Standard Adult Dosing:

• Full replacement dose: 1.6 mcg/kg/day based on ideal body weight 1, 2
• Dose adjustments: Titrate by 12.5-25 mcg increments every 4-6 weeks until euthyroid 1

Special Populations Requiring Lower Starting Doses:

• Elderly patients: Start with lower dose (less than 1.6 mcg/kg/day) 1, 3
  • For patients ≥65 years: Consider 1.09 μg/kg actual body weight or 1.35 μg/kg ideal body weight 3
• Patients with cardiac disease or at risk for atrial fibrillation: Start with lower dose (25-50 mcg/day) and titrate more slowly (every 6-8 weeks) 1, 2

Administration Guidelines

• Take as a single daily dose 2
• Take on an empty stomach, 30-60 minutes before breakfast 2, 1
• Take with a full glass of water 2
• Avoid medications that interfere with absorption:
  • Calcium and iron supplements
  • Antacids
  • Soy products 1
  • Wait at least 4 hours between levothyroxine and these medications 2

Monitoring and Dose Titration

1. Initial monitoring: Check TSH and free T4 levels 4-6 weeks after starting therapy 2, 1
2. Dose adjustments: Make incremental changes of 12.5-25 mcg based on TSH results 1
3. Monitoring during titration: Check TSH and free T4 every 6-8 weeks until stable 2
4. Target TSH levels:
   • Primary hypothyroidism: Normal reference range (0.5-4.5 mIU/L) 2
   • For secondary/tertiary hypothyroidism: Use free T4 levels (aim for upper half of normal range) rather than TSH 1
5. Long-term monitoring: Once stable, check TSH annually 2

Common Pitfalls and Considerations

• Undertreatment: Studies show approximately 42% of treated hypothyroid patients have TSH >4 mIU/L, indicating inadequate replacement 4
• Overtreatment: About 12.6% of patients have TSH <0.4 mIU/L, risking atrial fibrillation and decreased bone mineral density 4, 2
• Medication interactions: Many medications can affect levothyroxine absorption or metabolism
• Peak therapeutic effect: May not be attained for 4-6 weeks after dose adjustment 1
• Dosages >200 mcg/day are seldom required; inadequate response to >300 mcg/day may indicate poor compliance, malabsorption, or drug interactions 1

Special Situations

• Pregnancy: Requires increased monitoring (every trimester) and often increased dosage (typically by 30% or more) 2
• Elderly patients: May require lower doses due to slower thyroid hormone metabolism 3
• Cardiac disease: Start with lower doses (25-50 mcg/day) and titrate more slowly 2, 1

By following these evidence-based guidelines for levothyroxine dosing in primary hypothyroidism, clinicians can optimize treatment outcomes while minimizing the risks of under- or over-replacement.