FT4 Normal Reference Range for Infants Aged 8-13 Days
For infants aged 8-13 days, the FT4 reference range is approximately 1.19-2.59 ng/dL (15.3-33.4 pmol/L), though this must be interpreted using your specific laboratory's assay-specific reference range, as values vary significantly between different immunoassay platforms. 1, 2
Critical Considerations for Interpretation
Assay-Specific Variability
- Reference ranges vary substantially between different immunoassay manufacturers and platforms, making it essential to use the reference range established by your specific laboratory using their exact analyzer and reagent system. 1, 2
- The American College of Medical Genetics emphasizes that using published reference ranges from different assay systems can lead to misdiagnosis of thyroid disorders in neonates. 1
Age-Specific Dynamics in the First Two Weeks
- FT4 levels are highest in the first 24-48 hours after birth (approximately 15.3-43.6 pmol/L or 1.19-3.39 ng/dL), then decline progressively over the subsequent days. 3
- By days 8-13 of life, FT4 levels have stabilized to lower values compared to the immediate postnatal period. 4, 3
- For the broader age range of 14-21 days, one study using the Beckman Coulter DxI 800 analyzer reported FT4 reference intervals of 11.59-21.00 pmol/L (0.90-1.63 ng/dL). 4
Platform-Specific Reference Ranges
Siemens Atellica® IM Analyzer:
- For neonates aged 2-4 days: 1.19-2.59 ng/dL (15.3-33.4 pmol/L) 5
- For neonates aged 5-7 days: 1.21-2.29 ng/dL (15.6-29.5 pmol/L) 5
- For neonates aged 8-30 days: 1.02-2.01 ng/dL (13.2-25.9 pmol/L) 5
Beckman Coulter DxI 800:
- For infants aged 14-21 days: 0.90-1.63 ng/dL (11.59-21.00 pmol/L) 4
- For infants aged 22-30 days: 0.79-1.52 ng/dL (10.14-19.60 pmol/L) 4
Essential Clinical Context
Mandatory Concurrent TSH Measurement
- FT4 must always be interpreted in conjunction with TSH levels for comprehensive thyroid function assessment, as isolated FT4 measurement is insufficient for diagnosing congenital hypothyroidism. 1, 2
- In congenital hypothyroidism of thyroidal origin, TSH is elevated with low FT4, while central hypothyroidism shows low FT4 without clearly elevated TSH. 1
Factors Affecting Thyroid Function Tests
- Recent iodine exposure from CT contrast can transiently impact thyroid function tests and should be considered when interpreting results. 2
- Non-thyroidal illness can cause abnormal thyroid function tests without actual thyroid dysfunction. 2
- Certain medications including amiodarone, dopamine, glucocorticoids, and dobutamine can affect thyroid hormone levels. 2
Common Pitfalls to Avoid
- Never apply adult reference ranges or reference ranges from a different assay platform to neonatal samples, as this leads to misdiagnosis. 6, 1
- Failing to account for the rapid physiological changes in thyroid function during the first week of life can result in inappropriate classification of normal infants as abnormal. 3
- Using reference ranges that don't match your laboratory's specific analyzer and reagent system is a critical error that can lead to missed diagnoses or unnecessary treatment. 1, 2