Free thyroxine — thyroid hormone
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High-quality testing run in a CLIA-certified and CAP-accredited lab in Santa Clara, California.
About Free T4
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About Free T4
Overview and History
Free T4 (free thyroxine) is an iodinated amino acid hormone produced by thyroid follicular cells and released into the bloodstream. Biochemically it is one of the thyroid hormones derived from tyrosine and iodine; most circulating T4 is bound to carrier proteins while a small free fraction is biologically available to tissues.
Free T4 is a primary substrate for peripheral conversion to the more active form, free T3. Clinically, assays that measure the free fraction were developed to better reflect tissue-available hormone and reduce confounding from variations in binding proteins or circulating carriers.
Interpreting Highs and Lows
Higher-than-expected Free T4 levels can reflect increased thyroid hormone release, altered binding protein levels, reduced peripheral clearance, or effects of certain medications or supplements; interpretation benefits from correlating with TSH, free T3, thyroid antibodies, and clinical context. Lower-than-expected Free T4 levels may indicate reduced thyroid hormone production, impaired peripheral conversion, changes in protein binding, non-thyroidal illness, or influences from medications and nutritional status. Always interpret Free T4 together with other endocrine and systemic markers to identify patterns rather than isolated values.
Featured Test
Free T4 is included in the Foundation Zoomer’s Hormone Endocrine domain to evaluate thyroid hormone availability and its influence on metabolism, energy, and organ systems. Paired with TSH, free T3, antibodies, and metabolic markers, Free T4 helps map endocrine patterns that inform personalized, systems-based care planning.
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Mary Beth Augustine
DCN, RDN, FAND- Lead Clinical Educator
Suzanne Barker
MS, RDN, LD, CTTS, CGP- Clinical Educator
Suzette Garcia
MSc- MHNE- Clinical Educator
Brooke Mader Ganev
MS, CNS, LDN, FDN- Clinical Educator
Adair Anderson
MS, RDN, LDN- Clinical Educator
Lillie Luu Nguyen
DCN, CNS, LDN- Clinical Educator
Jason Barker
ND- Clinical Educator
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ND- Clinical Educator
Dana Filatova
DCN, CNS, LDN- Clinical Educator
Jessy Dhanjal
ND- Clinical Educator
Nour Amri
MS, LDN, CNS, CKNS- Clinical Educator
