What is the starting dose of thyroxine (T4) for an 8-year-old child with hypothyroidism?

Levothyroxine Dosing for an 8-Year-Old Child with Hypothyroidism

For an 8-year-old child with hypothyroidism, initiate levothyroxine at 4-5 mcg/kg/day, which typically translates to approximately 100-125 mcg/day for most children in this age group. 1

Age-Specific Dosing Guidelines

The FDA-approved dosing for pediatric hypothyroidism follows a weight-based algorithm that decreases with age 1:

• Children aged 6-12 years require 4-5 mcg/kg/day 1
• This represents a lower dose per kilogram compared to younger children, reflecting decreased metabolic demands as children approach puberty 1
• For context, younger children (1-5 years) require 5-6 mcg/kg/day, while infants need substantially higher doses (10-15 mcg/kg/day) 1, 2

Practical Dosing Approach

Calculate the initial dose based on the child's actual body weight:

• For a typical 8-year-old weighing 25-30 kg, the starting dose would be 100-150 mcg/day 1
• Round to the nearest tablet strength available (typically 25,50,75,88,100,112,125 mcg) 1
• Full replacement therapy should be instituted immediately in children, as delays can impair intellectual and physical development 1, 2

Special Considerations for School-Age Children

Hyperactivity can occur when initiating therapy in older children 1:

• If hyperactivity is a concern, start with one-fourth of the full replacement dose 1
• Increase weekly by one-fourth increments until reaching the full recommended dose 1
• This gradual approach is only necessary if behavioral issues are anticipated—otherwise, start at full replacement dose 1

Monitoring Protocol

Check TSH and free T4 levels every 6-8 weeks during dose titration 3, 4:

• Target TSH within the normal reference range (typically 0.5-4.5 mIU/L) 3
• Maintain free T4 in the upper half of the normal range 1, 2
• Once stable, monitor every 6-12 months or with symptom changes 3, 4

Dose Adjustments

Adjust levothyroxine in 12.5-25 mcg increments based on laboratory results 3, 1:

• Larger adjustments may lead to overtreatment, particularly in children with underlying conditions 3
• The dose per kilogram will naturally decrease as the child grows, but absolute dose typically increases 1
• Growth and puberty status affect dosing requirements—children with incomplete growth and puberty require 2-3 mcg/kg/day 1

Critical Pitfalls to Avoid

Undertreatment risks permanent neurodevelopmental consequences 1, 2:

• Inadequate thyroid hormone during childhood impairs intellectual development and linear growth 2
• Delays in diagnosis and treatment have deleterious effects on cognitive and physical development 1, 2

Overtreatment causes iatrogenic hyperthyroidism 3, 5:

• Excessive dosing increases risk for behavioral problems, poor sleep, and accelerated bone maturation 5
• Approximately 36.5% of children started at the higher end of dosing ranges (>12.5 mcg/kg/day) require dose reduction for hyperthyroxinemia 5

Never administer levothyroxine with foods that impair absorption 1:

• Soybean infant formula significantly decreases levothyroxine absorption 1
• Administer at least 4 hours apart from interfering substances 1

Administration Technique

For children who cannot swallow tablets 1:

• Crush the tablet immediately before administration 1
• Suspend in 5-10 mL (1-2 teaspoons) of water 1
• Administer by spoon or dropper 1
• Do not store the suspension—prepare fresh each time 1