FT4 Reference Range for Neonates Under 1 Month
The provided evidence does not contain specific FT4 reference ranges for children under 1 month of age expressed in ng/dL, but the available neonatal data can be converted and applied to this age group.
Available Neonatal Reference Ranges
For neonates aged 3-7 days (first week of life), the FT4 reference range is 20.5-37.1 pmol/L, and for neonates aged 13-15 days (second week), the range is 15.3-26.5 pmol/L 1. Converting these values to ng/dL (using the conversion factor: 1 ng/dL = 12.87 pmol/L):
- Days 3-7: 1.59-2.88 ng/dL
- Days 13-15: 1.19-2.06 ng/dL
For infants aged 14-30 days, another study provides reference ranges of 11.59-21.00 pmol/L (days 14-21) and 10.14-19.60 pmol/L (days 22-30), which convert to approximately 0.90-1.63 ng/dL and 0.79-1.52 ng/dL respectively 2.
Critical Interpretation Points
The lower limit of the FT4 reference range in the first week of life (1.59 ng/dL or 20.5 pmol/L) is substantially higher than adult reference ranges, which is crucial for diagnosing central congenital hypothyroidism 1.
FT4 levels decline progressively during the first month of life, with higher values in the first week compared to weeks 2-4 1, 2.
For infants 1-12 months old measured by tandem mass spectrometry, the reference range is 1.3-2.8 ng/dL 3.
Essential Clinical Caveats
Reference ranges vary significantly between different immunoassay platforms and manufacturers 4, 3. The ranges provided above are assay-specific (Cobas/Roche for 1, Beckman Coulter DxI 800 for 2).
Always use the reference range established by your specific laboratory using their exact analyzer and reagent system 5, 4. Using inappropriate reference ranges can lead to misdiagnosis with serious consequences including unnecessary treatment or missed diagnoses 5.
FT4 must always be interpreted in conjunction with TSH levels for comprehensive assessment 4, 6. In congenital hypothyroidism of thyroidal origin, TSH is elevated with low FT4, while central hypothyroidism shows low FT4 without clearly elevated TSH 1.
Age in days matters significantly—a value normal at day 3 may be abnormal at day 14 1, 2.