Biomarker Reference · FT4
Free Thyroxine (FT4) is a clinical biomarker used in thyroid-endocrine assessment. Free thyroxine is the unbound, biologically available fraction of the prohormone T4 secreted by the thyroid gland. It is measured in 5 laboratory tests on BiomarkersLabs.com.
Free thyroxine is the unbound, biologically available fraction of the prohormone T4 secreted by the thyroid gland. Most T4 circulates bound to thyroxine-binding globulin, transthyretin, and albumin; only the free fraction enters cells for peripheral conversion to active T3. Dried blood spot free T4 measures the same physiological entity as serum free T4 but uses immunoassay adapted to the matrix.
Free T4 quantifies thyroid hormone availability and is essential for classifying thyroid dysfunction when TSH is abnormal — low in primary hypothyroidism, high in hyperthyroidism, and discordant in central thyroid disease or non-thyroidal illness. Practitioners order FT4 alongside TSH to guide levothyroxine dosing and investigate subclinical or secondary disorders.
Reference ranges vary by laboratory and assay platform. Interpret in context of the issuing laboratory's stated range.
5 tests on BiomarkersLabs.com include this biomarker.
Free Thyroxine reference range in this test: Approximately 0.8–1.8 ng/dL; dried blood spot free T4 is calculated or directly measured by LC-MS and may not match immunoassay-based serum results.
Free Thyroxine reference range in this test: Approximately 0.8–1.8 ng/dL (10–23 pmol/L); assay-specific.
Free Thyroxine reference range in this test: Approximately 0.8–1.8 ng/dL; method- and platform-specific.
Free Thyroxine reference range in this test: Platform-dependent; typical serum ranges ~0.8–1.8 ng/dL.
Free Thyroxine reference range in this test: Method-specific for dried blood spot; approximate serum equivalents 12–22 pmol/L.
Related Biomarkers
Available exclusively to verified licensed healthcare practitioners. CLIA-certified laboratory results delivered to your secure portal.
Create Free Account