Normal Reference Ranges for Free Thyroxine (FT4) by Age
The normal reference range for FT4 varies by age, with higher values in early childhood that gradually decrease toward adulthood, and most laboratories should use age-specific reference intervals rather than a single adult range to avoid misdiagnosis.
Pediatric FT4 Reference Ranges
The reference ranges for FT4 in children are distinctly higher than in adults and decrease progressively with age:
Infants (14-30 days)
Children and Adolescents (using ECLIA method)
- Ages 4-6 years: 1.12-1.67 ng/dL 2
- Ages 7-8 years: 1.07-1.61 ng/dL 2
- Ages 9-10 years: 0.96-1.60 ng/dL 2
- Ages 11-12 years: 1.02-1.52 ng/dL 2
- Ages 13-14 years: 0.96-1.52 ng/dL 2
- Ages 15 years: 0.95-1.53 ng/dL 2
Adult FT4 Reference Ranges
For adults, most current guidelines recommend using a single reference range without age stratification, though recent evidence suggests this may lead to diagnostic errors 3.
Standard Adult Range (Non-Age-Specific)
- The American Association of Clinical Endocrinologists recommends using the laboratory's established reference range for free T4, typically without age adjustment 3
- Most laboratories use manufacturer-specific ranges that do not differentiate by age in adults 4, 5
Age-Specific Trends in Adults
Recent large-scale studies demonstrate important age-related changes:
- FT4 levels gradually but significantly decrease with age in men 5
- FT4 levels in women are lower than in men but remain relatively consistent across age groups 5
- The lower reference limits for FT4 are higher in early childhood and decrease toward adulthood 4
- Upper reference limits for FT4 increase from age 70 years onward 4
Critical Considerations and Pitfalls
Assay-Specific Variations
- Reference intervals vary significantly between different manufacturer assays (Roche, Abbott, Siemens, Beckman Coulter) 1, 4, 5
- Each laboratory should establish its own reference intervals using the specific immunoassay platform employed 6
Clinical Implications of Not Using Age-Specific Ranges
- Using adult reference ranges in children can lead to misdiagnosis of hypothyroidism or oversight of mild subclinical hypothyroidism 2
- In adults, failure to use age-specific ranges may result in overdiagnosis, particularly in older individuals 4, 7
When to Interpret FT4 Results
- FT4 should always be interpreted in conjunction with TSH levels for comprehensive thyroid function assessment 8
- Normal FT4 with abnormal TSH indicates subclinical thyroid dysfunction 3, 8
- Non-thyroidal illness can cause abnormal thyroid function tests without actual thyroid dysfunction 3
- Iodine exposure from CT contrast can transiently impact thyroid function tests 3