What is the recommended dosage of thyroxine (levothyroxine) per kilogram of body weight in children and adolescents?

Thyroxine Dosage Guidelines for Children and Adolescents

The recommended dosage of levothyroxine (thyroxine) for children varies by age, with newborns requiring 10-15 mcg/kg/day, infants 6-10 mcg/kg/day, young children 4-6 mcg/kg/day, and adolescents 2-3 mcg/kg/day, with dosing decreasing as age increases. 1

Age-Specific Dosing Recommendations

The FDA-approved dosing guidelines for levothyroxine in pediatric hypothyroidism are as follows:

• 0-3 months: 10-15 mcg/kg/day 1
• 3-6 months: 8-10 mcg/kg/day 1
• 6-12 months: 6-8 mcg/kg/day 1
• 1-5 years: 5-6 mcg/kg/day 1
• 6-12 years: 4-5 mcg/kg/day 1
• >12 years but growth and puberty incomplete: 2-3 mcg/kg/day 1
• Growth and puberty complete: 1.7 mcg/kg/day 1

Treatment Initiation and Monitoring

• For congenital hypothyroidism, therapy should be initiated at full replacement doses as soon as possible to prevent deleterious effects on intellectual and physical development 1, 2
• Initial dosing of 10-15 mcg/kg/day in newborns has been shown to normalize cognitive development when started within 2 weeks of age 2
• Higher initial doses (12-17 mcg/kg/day or approximately 50 mcg/day for average weight newborns) have been shown to normalize TSH levels by 2 weeks of therapy 3
• For infants with very low (<5 mcg/dL) or undetectable serum T4 concentrations, a higher initial dose of 50 mcg/day is recommended 1

Dose Adjustments and Monitoring

• Dose should be adjusted based on clinical response and laboratory parameters 1
• The goal of therapy should be to maintain total thyroxine or free thyroxine in the upper half of the reference range during the first 3 years of life 2
• TSH should be normalized to ensure optimal thyroid hormone dosage and compliance 2
• In older children with chronic or severe hypothyroidism, a more gradual approach may be warranted, starting with 25 mcg/day with increments of 25 mcg every 2-4 weeks 1
• To minimize hyperactivity in older children, consider starting with one-fourth of the recommended full replacement dose and increasing weekly by one-fourth until the full dose is reached 1

Special Considerations

• Children weighing more than 40 kg should be dosed as adults 4
• For adolescents 15 years and older, adult dosing guidelines generally apply 5
• For children at risk for cardiac failure, a lower starting dose (e.g., 25 mcg/day) should be considered with increases in 4-6 weeks based on clinical and laboratory response 1
• Some studies suggest that lower initial doses (7.5-8.0 mcg/kg/day) may be adequate for most infants with congenital hypothyroidism, with early assessment of thyroid function to identify those who need dose increases 6

Administration

• Levothyroxine tablets may be crushed and suspended in 5-10 mL of water for infants and children who cannot swallow tablets 1
• The suspension should be administered immediately and not stored 1
• Foods that decrease absorption of levothyroxine, such as soybean infant formula, should not be used for administering the medication 1

Pitfalls and Caveats

• Undertreatment can lead to poor intellectual and physical development, while overtreatment can cause symptoms of hyperthyroidism 1, 2
• Early studies suggested that physiologic requirements of thyroxine may be lower than previously recommended dosages (2.5-5 mcg/kg/day for children 4-17 years), but more recent evidence supports the higher dosing schedules outlined above 7, 1
• Patients with more severe hypothyroidism at diagnosis may require higher doses and more frequent monitoring 6
• TSH is not elevated in central hypothyroidism, so clinical judgment remains important despite normal screening results 2