What are the typical dosages of levothyroxine (T4) and liothyronine (T3) for patients with hypothyroidism?

Thyroid Hormone Replacement Dosages for Hypothyroidism

For patients with hypothyroidism, levothyroxine (T4) should be started at 1.6 mcg/kg/day for adults under 70 without cardiac disease, while liothyronine (T3) is typically dosed at 25-75 mcg daily for mild hypothyroidism. 1, 2

Levothyroxine (T4) Dosing Guidelines

Adult Dosing

• Standard adult starting dose: 1.6 mcg/kg/day 1, 3
• Elderly patients or those with cardiac conditions: 25-50 mcg/day, with more gradual titration 3
• Patients at risk for atrial fibrillation: Lower starting dose (less than 1.6 mcg/kg/day) 3

Titration Protocol

• Increase by 12.5-25 mcg increments every 4-6 weeks until euthyroid 3
• For cardiac patients or elderly: Titrate more slowly, every 6-8 weeks 3
• Target TSH ranges:
  • Low-risk patients: 0.5-2.0 mIU/L
  • Intermediate to high-risk patients: 0.1-0.5 mIU/L
  • Patients with persistent disease: <0.1 mIU/L
  • Elderly patients: 1.0-4.0 mIU/L 1

Pediatric Dosing

• Age-specific dosing per kg body weight:
  • 0-3 months: 10-15 mcg/kg/day
  • 3-6 months: 8-10 mcg/kg/day
  • 6-12 months: 6-8 mcg/kg/day
  • 1-5 years: 5-6 mcg/kg/day
  • 6-12 years: 4-5 mcg/kg/day

  • 12 years but growth incomplete: 2-3 mcg/kg/day

  • Growth and puberty complete: 1.6 mcg/kg/day 3

Liothyronine (T3) Dosing Guidelines

Adult Dosing

• Mild hypothyroidism: Start at 25 mcg daily 2

  • Increase by up to 25 mcg every 1-2 weeks
  • Usual maintenance dose: 25-75 mcg daily

• Myxedema:

  • Starting dose: 5 mcg daily
  • Increase by 5-10 mcg daily every 1-2 weeks
  • When 25 mcg daily is reached, may increase by 5-25 mcg every 1-2 weeks
  • Usual maintenance dose: 50-100 mcg daily 2

Pediatric Dosing for T3

• Congenital hypothyroidism:
  • Start at 5 mcg daily
  • Increase by 5 mcg every 3-4 days until desired response
  • Infants (few months old): May require only 20 mcg daily for maintenance
  • At 1 year: May require 50 mcg daily
  • Above 3 years: May require full adult dosage 2

Combination Therapy (T4+T3)

For patients who remain symptomatic on T4 monotherapy:

• Reduce LT4 dose by 25 mcg/day
• Add 2.5-7.5 mcg liothyronine once or twice daily 4
• Recommended LT4/LT3 ratio: 13:1 to 20:1 5
• Goals of combination therapy: Achieve physiological ratio of free T3/free T4 and non-suppression of TSH 5, 6

Important Clinical Considerations

Monitoring

• Regular monitoring of thyroid function, including TSH and free T4, is essential 1
• For patients on T3 therapy or combination therapy, monitor both FT4 and FT3 levels 7
• Morning laboratory testing (around 8 am) is recommended for accurate assessment 1

Special Populations

• Pregnant patients: Adjust dose to maintain trimester-specific TSH reference range and monitor TSH every 4 weeks until stable 1
• Central hypothyroidism: Cortisol replacement should be initiated 1 week prior to starting levothyroxine to prevent adrenal crisis 1
  • Use serum free-T4 level to titrate dosing (not TSH) 3, 7
  • Target free-T4 level in the upper half of the normal range 3

Common Pitfalls

1. Overtreatment risks:

   • Atrial fibrillation (occurs in 10-25% of hyperthyroid patients)
   • Osteoporosis, particularly in elderly patients 1

2. Administration issues:

   • Take on empty stomach, 30-60 minutes before food
   • Avoid taking with foods that decrease absorption (e.g., soybean-based products) 3
   • Certain medications can interfere with absorption (calcium, iron supplements, antacids)

3. Inadequate response:

   • Dosages greater than 200 mcg/day are seldom required
   • Poor response to >300 mcg/day may indicate poor compliance, malabsorption, or drug interactions 3

The peak therapeutic effect of levothyroxine may not be attained for 4-6 weeks after dosage adjustment, so patience is required when evaluating response to therapy 3.