Normal Free T4 Range for a 2-Month-Old Infant
The normal range for free T4 in a 2-month-old infant is approximately 1.3-2.8 ng/dL. 1
Reference Ranges for Free T4 in Infants
- The normal free T4 reference interval for infants aged 1-12 months is 1.3-2.8 ng/dL, as determined by liquid chromatography tandem mass spectrometry (LC/MS/MS) 1
- In neonates and young infants, free T4 levels tend to be higher than in older children and adults, reflecting the physiological adaptation to extrauterine life 1
- For comparison, the mean free T4 level in healthy term infants has been reported as 4.24 ± 0.23 ng/dL in the immediate newborn period 2
Age-Related Changes in Free T4 Levels
- Free T4 levels are typically higher in younger infants (14-21 days) compared to slightly older infants (22-30 days), showing a gradual decline with age 3
- By 1-2 months of age, free T4 levels begin to stabilize but remain higher than adult reference ranges 1
- As children grow, free T4 reference intervals become more consistent, ranging between 1.3 and 2.4 ng/dL for children 1 to 18 years old 1
Clinical Significance of Free T4 Measurement
- Free T4 measurement is considered an accurate indicator of thyroid function in infants 2
- When evaluating thyroid function in infants, both free T4 and TSH should be measured together for comprehensive assessment 4
- All infants with suspected congenital hypothyroidism should have free T4 levels measured, with values below 0.8 ng/dL raising concern for hypothyroidism 2
- For infants with mildly elevated TSH but normal free T4 levels, monitoring rather than immediate treatment may be appropriate 5
Factors Affecting Free T4 Interpretation
- Measurement methods significantly impact reference ranges - LC/MS/MS provides more specific quantification compared to immunoassay platforms 1
- Immunoassay platforms may show wider reference intervals (0.48-2.78 ng/dL) compared to mass spectrometry methods 1
- Illness, particularly respiratory distress syndrome, can affect free T4 levels in infants 2
- Iron deficiency may influence thyroid function, as iron is crucial for thyroid hormone metabolism 6
Monitoring Considerations
- When monitoring thyroid function in infants, it's important to use age-appropriate reference ranges rather than adult values 1
- For infants with initial mild thyroid function abnormalities, the initial serum level of free T4 (cutoff of approximately 1.06 ng/dL) may help discriminate between transient abnormalities and true congenital hypothyroidism 5
- Free T4 levels should be interpreted in the context of TSH values and clinical presentation 4