Free Thyroxine Testing
The laboratory test ordered to measure free thyroxine is called "Free T4" or "FT4," which directly measures the unbound, biologically active fraction of thyroxine in serum.
Test Name and Abbreviation
- The standard abbreviation is FT4 (Free T4), which refers to free thyroxine measurement 1
- This test specifically measures the unbound thyroxine hormone that is metabolically active, as opposed to total T4 which includes both bound and unbound hormone 1
Clinical Context and Interpretation
- FT4 is measured alongside TSH (thyroid-stimulating hormone) to distinguish between subclinical hypothyroidism (elevated TSH with normal FT4) and overt hypothyroidism (elevated TSH with low FT4) 1, 2
- FT4 measurement is considered the key method for evaluating thyroid function when used in combination with TSH 3
- The normal reference range for FT4 is typically 9-19 pmol/L, though this can vary slightly between laboratories 1
Diagnostic Utility
- When evaluating thyroid dysfunction, TSH should be measured first, followed by free T4 if TSH is abnormal 1, 2
- FT4 has superior diagnostic accuracy compared to total T4, particularly in patients with altered thyroid-binding protein levels 3
- In hyperthyroidism, FT4 is elevated along with suppressed TSH, while in hypothyroidism, FT4 is low with elevated TSH 1, 4
Methodological Considerations
- Equilibrium dialysis is considered the "gold standard" reference method for FT4 measurement, though immunoassays are more commonly used in clinical practice 5
- The accuracy of FT4 measurements can be affected by pre-analytical factors including sample handling, storage conditions, and labware selection 5
- FT4 remains stable in serum samples after four freeze-thaw cycles or storage at -70°C for at least three years 5