Normal Reference Ranges for FT3 and FT4
Free T3 and free T4 reference ranges vary significantly between different immunoassay platforms and manufacturers, so you must always use the laboratory-specific reference range established by your specific analyzer and reagent system to avoid misdiagnosis. 1
Standard Adult Reference Ranges
Free T4 (FT4)
- The American Association of Clinical Endocrinologists recommends using the laboratory's established reference range for FT4, typically without age adjustment in adults 1
- FT4 should always be interpreted in conjunction with TSH levels for comprehensive thyroid assessment 1
- Common reference ranges across platforms include approximately 9-19 pmol/L (or 1.2-2.4 ng/dL), though these vary by assay 2, 1
Free T3 (FT3)
- The American Association of Clinical Endocrinologists states that free T3 should be within the reference range, though specific values depend on the assay platform 2
- Typical reference ranges include approximately 3.5-6.5 pmol/L, but again, assay-specific ranges must be used 3, 4, 5
Critical Platform-Specific Considerations
Each laboratory must use the reference range established by their specific analyzer and reagent system, because FT4 reference ranges vary significantly between different immunoassay platforms and manufacturers 1. This is not optional—using manufacturer-provided ranges without local validation can lead to misdiagnosis.
Age-Related Adjustments
FT4 lower reference limits are higher in early childhood and decrease toward adulthood, while in adults, FT4 upper limits increase from 70 years onward 6. However, the American Association of Clinical Endocrinologists typically recommends using laboratory ranges without routine age adjustment in adults 1.
Gender Considerations
Some studies suggest significant differences between female and male populations, indicating that partitioning reference ranges by gender may be necessary 4. Male ranges for FT4 and FT3 tend to be slightly higher than female ranges 4, 5.
Factors That Can Affect Interpretation
- Amiodarone can alter thyroid hormone levels 1
- Non-thyroidal illness can cause abnormal thyroid function tests without actual thyroid dysfunction 1
- Iodine exposure from CT contrast can transiently impact thyroid function tests 1
- Pregnancy can cause physiologically low TSH with normal free hormone levels 2
Diagnostic Algorithm
If TSH is elevated (>4.5 mIU/L) with low free T4, it indicates overt primary hypothyroidism 2. If TSH is suppressed (<0.45 mIU/L) with elevated free T4 and/or free T3, it indicates overt hyperthyroidism 2.
Common Pitfall to Avoid
Never rely on a single reference range across all platforms—the American College of Medical Genetics emphasizes using assay-specific ranges because FT4 reference ranges vary significantly between different immunoassay platforms and manufacturers 1. Always confirm the specific reference range used by your laboratory's analyzer.