Normal Reference Range of FT4 in Children Under 1 Year
The normal reference range for free thyroxine (FT4) in children under 1 year varies significantly by age and assay method, with infants aged 14-21 days showing ranges of 11.59-21.00 pmol/L (approximately 0.9-1.6 ng/dL), declining to 10.14-19.60 pmol/L (approximately 0.8-1.5 ng/dL) by days 22-30, and stabilizing to 1.3-2.8 ng/dL for infants 1-12 months old. 1, 2
Age-Specific Reference Intervals
First Month of Life (Days 3-30)
The reference ranges change dramatically during the first weeks of life:
- Days 3-7: FT4 ranges from 20.5-37.1 pmol/L (approximately 1.6-2.9 ng/dL) 3
- Days 13-15: FT4 ranges from 15.3-26.5 pmol/L (approximately 1.2-2.1 ng/dL) 3
- Days 14-21: FT4 ranges from 11.59-21.00 pmol/L (approximately 0.9-1.6 ng/dL) 1
- Days 22-30: FT4 ranges from 10.14-19.60 pmol/L (approximately 0.8-1.5 ng/dL) 1
Infants 1-12 Months
For infants aged 1-12 months, the FT4 reference interval is 1.3-2.8 ng/dL, which is notably higher than older children and adults. 2
Critical Considerations for Interpretation
Assay-Specific Variations
Each laboratory must establish its own reference intervals using the specific immunoassay platform employed, as reference intervals vary significantly between different manufacturer assays. 4 Studies comparing different methodologies show substantial variation:
- Immunoassay platforms show wider reference ranges (0.48-2.78 ng/dL for males, 0.85-2.09 ng/dL for females) compared to mass spectrometry methods 2
- Tandem mass spectrometry (LC/MS/MS) provides tighter, more specific reference intervals than traditional immunoassays 2
Age-Dependent Decline
FT4 levels show a rapid decline during the first two weeks of life, with mean FT4 significantly higher in the younger age group (days 14-21) compared to days 22-30. 1 This physiological decline continues throughout the first year, with reference ranges showing wide spread immediately after birth that rapidly decreases within the first 2 years of life 5
Gestational Age Impact
Very preterm neonates (26-31 weeks gestational age) have significantly lower FT4 levels that decline postnatally to values well below those of term neonates, with many having FT4 levels within the hypothyroid range despite normal TSH. 6 This transient hypothyroxinemia occurs in the majority of very preterm newborns and requires different reference ranges than term infants 6
Clinical Application Algorithm
When interpreting FT4 in infants under 1 year:
Determine exact postnatal age in days (not just weeks or months), as reference ranges change rapidly in the first month 1, 3
Verify the specific assay platform used by your laboratory, as published reference ranges only apply if using identical analyzer and reagent systems 4
Consider gestational age at birth: Preterm infants (<33 weeks) require measurement during the second week of life with lower expected values 6
Always interpret FT4 in conjunction with TSH levels for comprehensive thyroid function assessment 4, 7
Common Pitfalls
Using adult reference ranges will misclassify most healthy infants as hypothyroid, as infant FT4 levels are substantially higher than adult values 2, 5
Ignoring assay-specific differences leads to misdiagnosis, as immunoassay platforms show significantly different ranges than mass spectrometry methods 2
Failing to account for rapid postnatal changes in the first month results in inappropriate interpretation, particularly when comparing day 3-7 values to day 14+ values 1, 3
Overlooking gestational age in preterm infants causes unnecessary treatment, as their physiologically lower FT4 levels with normal TSH represent transient hypothyroxinemia rather than true hypothyroidism 6