Biomarker education
Free T4: What It Measures & Why People Track It
Also called: Free thyroxine, FT4
Free T4 measures the unbound, active portion of thyroxine, a thyroid hormone.
General biomarker education — not personalized medical advice. Learn more
Result on your report
IllustrativeFree T4
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Range: Low – High
LowReference intervalHigh
What it generally measures
Labs report the fraction of thyroxine that is not bound to carrier proteins in the blood.
Common units on reports: ng/dL
Why people track it over time
Free T4 is generally reviewed together with TSH rather than by itself, since the two provide complementary information about thyroid-related lab context.
Changes over time do not by themselves establish a diagnosis. Learn more
Questions you might ask a healthcare professional
Questions to bring
IllustrativeAsk how Free T4 and TSH relate to each other on this report.
Confirm medication timing instructions before drawing conclusions from a single value.
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