What is the appropriate treatment and dosage of levothyroxine (thyroxine) for a 17-day-old baby with a Thyroid-Stimulating Hormone (TSH) level of 14.21 µIU/mL, suggestive of congenital hypothyroidism?

Treatment of Congenital Hypothyroidism in a 17-Day-Old Infant with TSH 14.21 µIU/mL

Start levothyroxine immediately at 10–15 mcg/kg/day without waiting for repeat testing or free T4 results, as this TSH level at 17 days of age is abnormal and delays in treatment beyond 2 weeks of life compromise neurocognitive outcomes. 1, 2, 3

Why Immediate Treatment is Critical

• Brain development is time-sensitive: Untreated congenital hypothyroidism causes irreversible intellectual disability, and treatment must begin within the first 2 weeks of life to normalize cognitive development 2, 3
• TSH >10 µIU/mL after day 7 is definitively abnormal: Normal TSH after the first week of life is <10 µIU/mL, making your value of 14.21 µIU/mL clearly elevated and requiring immediate intervention 4
• Do not wait for confirmatory testing: While free T4 helps determine severity, the elevated TSH alone at this age warrants immediate treatment to protect neurodevelopment 2, 5

Exact Levothyroxine Dosing Protocol

Standard Starting Dose

• 10–15 mcg/kg/day is the recommended initial dose for all infants with congenital hypothyroidism 1, 2, 3
• Calculate based on current weight using the table below 1:

Infant Weight	Daily Dose Range
2.0 kg	20–30 mcg/day
2.5 kg	25–37.5 mcg/day
3.0 kg	30–45 mcg/day
3.5 kg	35–52.5 mcg/day
4.0 kg	40–60 mcg/day

Administration Instructions

• Use crushed tablet, not liquid formulation: Crush the exact dose of levothyroxine tablet (25 mcg or 50 mcg strength) and mix with 1–2 mL of breast milk or formula 1
• Give once daily in the morning: Administer 30 minutes before feeding for optimal absorption 1
• Avoid soy-based formula: Soy significantly impairs levothyroxine absorption and should be avoided 1

Special Dosing Considerations

• If cardiac symptoms present (rare in neonates): Start at lower end of range (10 mcg/kg/day) and monitor closely, though cardiac failure risk is minimal in this age group 1
• Do NOT reduce dose due to hyperactivity concerns in neonates—this recommendation applies only to older children, and underdosing in the first weeks of life causes permanent neurocognitive damage 1, 2

Treatment Goals and Monitoring

Target Laboratory Values

• TSH should normalize to <5 mIU/L within 2–4 weeks of starting therapy 5, 3
• Free T4 or total T4 must be maintained in the upper half of the age-specific reference range during the first 3 years of life to optimize neurodevelopment 2, 5, 3
• Serum T4 normalizes within 1 week of starting appropriate therapy, while TSH may take 2–21 months to fully normalize despite adequate T4 levels 6, 7

Monitoring Schedule

• First follow-up at 2 weeks: Check TSH and free T4 to confirm response to therapy 1, 3
• Then every 2 weeks during dose titration: Continue frequent monitoring until TSH normalizes and free T4 is in upper half of reference range 1, 5
• Every 1–2 months during first year: Once stable, monitor every 4–6 weeks during infancy 1, 3
• Every 3–4 months after age 3 years: Reduce frequency once growth and development are stable 1

Dose Adjustment Protocol

When to Increase Dose

• If TSH remains >5 mIU/L after 4 weeks: Increase by 12.5–25 mcg (approximately 2–3 mcg/kg) 1, 3
• If free T4 is not in upper half of reference range: Increase dose even if TSH is normalizing 5, 3

When to Decrease Dose

• If TSH becomes suppressed (<0.5 mIU/L): Reduce dose by 12.5–25 mcg to avoid iatrogenic hyperthyroidism 1, 7
• If free T4 exceeds upper limit of normal: Decrease dose immediately as overtreatment may cause hyperactivity and bone age advancement 7

Critical Pitfalls to Avoid

• Never delay treatment waiting for free T4 results: TSH >10 µIU/mL at 17 days is sufficient to start therapy immediately 4, 2, 3
• Do not underdose to avoid "hyperthyroidism": The 10–15 mcg/kg/day range is safe and necessary—lower doses (5–9 mcg/kg/day) result in prolonged TSH elevation and may compromise neurodevelopment 6, 7
• Do not use TSH normalization as sole treatment goal: Free T4 in the upper half of normal range is equally important for optimal brain development 5, 3
• Do not assume transient hypothyroidism: At 17 days with TSH 14.21 µIU/mL, assume permanent congenital hypothyroidism and treat aggressively—reevaluation for transient disease can occur after age 3 years 2, 3
• Do not miss the 2-week treatment window: Every day of delay beyond 2 weeks of life increases risk of suboptimal neurocognitive outcomes 2, 3

Why Lower Doses Are Inadequate

• Studies using 5–9 mcg/kg/day showed prolonged TSH elevation lasting 2–21 months despite normal T4, which may indicate inadequate tissue thyroid hormone delivery during critical brain development 7
• Current aggressive dosing (10–15 mcg/kg/day) produces better neurocognitive outcomes than historical lower-dose regimens, with adults treated in modern era showing improved IQ compared to those treated decades ago 3
• Rapid T4 normalization within 1 week is achievable and safe with 10–14 mcg/kg/day dosing without causing iatrogenic hyperthyroidism 6