Quantitative determination of free thyroxine from a dried blood spot using chemiluminescence immunoassay (CLIA) on the LIAISON platform.
Free T4 measurement complements TSH in the assessment of thyroid function, enabling differentiation between overt and subclinical thyroid disease and classification of the cause of thyroid dysfunction.
Clinical indications include:
| Analyte / Group | Components | Clinical Significance |
|---|---|---|
| fT4 | Free thyroxine (unbound T4) | The biologically active, unbound fraction of thyroxine; directly reflects thyroid hormone availability to tissues |
Free T4 represents approximately 0.03% of total T4 and is unaffected by changes in binding protein concentrations, making it a more reliable marker than total T4.
Thyroxine (T4) is the predominant thyroid hormone produced by the thyroid gland. In circulation, more than 99.9% is bound to transport proteins (TBG, albumin, transthyretin), with only a tiny free fraction available to enter cells and exert biological effects. It is this free fraction that is measured as fT4.
Typical adult reference range
T4 is converted to the more active T3 in peripheral tissues
Only 0.03% of total T4 is biologically active
fT4 is particularly valuable when TSH alone is insufficient for clinical decision-making. In overt hypothyroidism, both TSH is elevated and fT4 is low. In subclinical hypothyroidism, TSH is elevated but fT4 remains normal. This distinction guides treatment decisions, as overt disease typically requires immediate treatment while subclinical disease may warrant monitoring.
In pregnancy, thyroid hormone requirements increase by 25–50%, and trimester-specific reference ranges are required for accurate interpretation. Inadequate maternal thyroid hormone supply can adversely affect foetal neurodevelopment.
fT4 is measured using the LIAISON chemiluminescence immunoassay (CLIA) platform, adapted for the DBS matrix. The assay employs a competitive immunoassay format optimised for the free hormone fraction.
Typically ordered alongside TSH for a complete thyroid function assessment. DBS collection enables paired TSH/fT4 testing from a single sample card.
Whether you need testing services, method transfer, or white-label kit development — we'd love to hear from you.