Normal Free Thyroxine (FT4) Values for 3-Month-Old Infants Using ECLIA Method
The normal FT4 reference range for a 3-month-old infant using the ECLIA (electrochemiluminescence immunoassay) method is not specifically established in the available evidence, but pediatric reference ranges for young children (4-6 years) show FT4 values of 1.12-1.67 ng/dL (14.4-21.5 pmol/L) using the ECLusys ECLIA method 1.
Age-Specific Considerations for Infant FT4 Values
Infants have significantly different thyroid hormone levels compared to older children and adults, making age-appropriate reference ranges critical for accurate interpretation 1.
The youngest age group with established ECLIA reference ranges in the available evidence is 4-6 years, with FT4 values of 1.12-1.67 ng/dL using the ECLusys ECLIA method 1.
Neonatal screening studies using dried blood spots show mean FT4 concentrations of 20-22 pg/mL on days 5-6 of life, with 97.8% of values between 10-35 pg/mL 2.
Critical Limitations and Clinical Approach
Method-specific reference ranges are essential, as FT4 values calculated from different assay methods can vary by up to 30%, making it impossible to apply reference ranges from one method to another 3.
The ECLIA method (specifically ECLusys kits) has established pediatric reference ranges starting at age 4 years, but no published reference ranges exist for infants under 4 years using this specific methodology 1.
Using adult reference ranges for pediatric patients can lead to misdiagnosis of hypothyroidism or oversight of subclinical thyroid dysfunction 1.
Practical Recommendations for Clinical Interpretation
Contact your laboratory directly to obtain their method-specific reference range for 3-month-old infants, as each laboratory should establish age-appropriate reference ranges for their specific assay platform 1.
If laboratory-specific infant ranges are unavailable, TSH measurement is more reliable than FT4 alone for screening thyroid dysfunction in infants, with combined TSH and FT4 measurement providing optimal diagnostic accuracy 2.
Neonatal hypothyroidism is definitively diagnosed when FT4 is markedly suppressed (typically <6.2 pg/mL in dried blood spots) with TSH >99 μU/mL 2.
Common Pitfalls to Avoid
Never apply adult FT4 reference ranges (0.7-1.8 ng/dL or 9-23 pmol/L) to infants, as this will result in diagnostic errors 1, 3.
Do not assume reference ranges are transferable between different ECLIA platforms or manufacturers, even when using the same analytical principle 3.
Avoid interpreting FT4 values in isolation—always correlate with TSH and clinical presentation, as FT4 alone has lower diagnostic sensitivity for hypothyroidism compared to TSH 2, 4.