Quantitative determination of free thyroxine from a dried blood spot using chemiluminescence immunoassay (CLIA) on the LIAISON platform.
1 analyte
3–5 working days
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.
Free T4 (thyroxine) levels help classify thyroid function status:
| Classification | fT4 Range | Clinical Significance |
|---|---|---|
| Normal | 12–22 pmol/L (0.93–1.7 ng/dL) | Within standard reference range |
| Low | <12 pmol/L | Suggests hypothyroidism — interpret alongside TSH |
| High | >22 pmol/L | Suggests hyperthyroidism or overmedication |
Free T4 should always be interpreted alongside TSH for a complete thyroid picture. Pregnancy, oral contraceptives, and illness can affect levels.
Free T4 testing is recommended for:
T4 management depends on the underlying cause. The following evidence-based strategies may support thyroid function:
If hypothyroid:
If hyperthyroid:
General thyroid support:
Re-test Free T4 6–8 weeks after any medication adjustment.
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.
From enquiry to results in a few simple steps — no clinic visit required.
This test is available to healthcare professionals, wellness brands, clinics, and research institutions worldwide. We currently serve partners in:
Whether you need testing services for your patients, white-label kits for your brand, or method transfer to your own laboratory — get in touch to discuss how we can work together.
Free T4 is the unbound, biologically active form of the thyroid hormone thyroxine. It's produced by the thyroid gland and regulates metabolism, energy, and growth.
TSH tells you if the thyroid axis is disrupted; fT4 tells you by how much. Together they distinguish between subclinical and overt hypothyroidism or hyperthyroidism.
Finger-prick DBS card.
Results are typically delivered within 3 to 5 working days from the time your sample arrives at our laboratory, measured by CLIA.
This pattern indicates primary hypothyroidism — the thyroid gland is underproducing hormone and the pituitary is compensating by producing more TSH. Your clinician will interpret alongside symptoms and clinical history.
Masdiag's free T4 test is available worldwide through our partner network. We currently serve healthcare professionals, wellness brands, and clinics in Europe, the United Kingdom, Asia, Australia, New Zealand, and the United States. Contact us to discuss testing services, white-label kits, or method transfer to your laboratory.
Whether you need testing services, method transfer, or white-label kit development — we'd love to hear from you.