Free Thyroxine (FT4)
Thyroid Gland and Thyroid Hormones
Free Thyroxine (FT4)
FT4 is the unbound circulating thyroxine concentration. Interpretation depends on assay performance and the clinical setting.
From synthesis to tissue action
Thyroid function involves iodide handling, hormone formation in thyroglobulin, secretion, protein binding and local conversion. Transporters, deiodinases and receptor isoforms shape tissue exposure. These steps explain why circulating total hormone and intracellular thyroid action are different measurements.
A distinction that matters
Binding disturbances or analytical interference can produce misleading results.
Interpreting the axis
TSH and free thyroid hormones are interpreted together with symptoms and assay conditions. Illness, medicines and energy deficiency can alter the pattern. A change during heavy training should not automatically be treated as primary thyroid disease.
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