Normal FT4 Range for 3-Month-Old Infants Using ECLIA Method
The normal range of FT4 for a 3-month-old baby using the ECLIA method is NOT 0.8-2.1 ng/dL as suggested in your question. Based on pediatric reference data using ECLIA methodology, the appropriate reference range for infants in this age group is narrower and lower than adult values.
Age-Specific Reference Ranges Using ECLIA
For children aged 4-6 years using ECLIA (ECLusys), the FT4 reference range is 1.12-1.67 ng/dL, which is significantly narrower than the 0.8-2.1 ng/dL range mentioned in your question 1
The reference range becomes progressively lower with increasing age in children, with 7-8 year olds showing 1.07-1.61 ng/dL, and older children showing even lower ranges (0.96-1.60 ng/dL for 9-10 years) 1
For infants younger than 4 years, the reference range would likely be even higher than the 4-6 year range, as thyroid hormone levels are typically highest in early infancy and decline with age 1
Critical Methodological Considerations
Each laboratory must establish its own reference intervals using the specific immunoassay platform employed, as reference intervals vary significantly between different manufacturer assays 2
The ECLIA method (electrochemiluminescence immunoassay) produces different reference ranges compared to other methodologies, making it essential to use age-appropriate and method-specific reference values 1
Using adult reference ranges for pediatric patients can lead to misdiagnosis of hypothyroidism or oversight of mild subclinical hypothyroidism, emphasizing the critical importance of age-appropriate reference values 1
Why the 0.8-2.1 ng/dL Range Is Problematic
The range of 0.8-2.1 ng/dL appears to be either an adult reference range or a range from a different assay methodology, not appropriate for a 3-month-old infant using ECLIA 1
Serum FT3, FT4, and TSH levels in children differ substantially from those in adults, requiring evaluation using chronological age-appropriate reference values 1
The lower limit of 0.8 ng/dL is below the pediatric reference ranges established for ECLIA methodology in children as young as 4 years (1.12 ng/dL), suggesting this range is not appropriate for infants 1
Proper Clinical Approach
FT4 should always be interpreted in conjunction with TSH levels for comprehensive thyroid function assessment, as normal FT4 with abnormal TSH indicates subclinical thyroid dysfunction 2
Contact your specific laboratory to obtain the validated reference range for FT4 in 3-month-old infants using their particular ECLIA platform (such as Vidas or ECLusys) 2
If your laboratory has not established pediatric reference ranges for infants under 4 years, they should do so or provide guidance based on published pediatric endocrinology literature specific to the ECLIA methodology 1