Free T4 (Thyroxine)
Also known as: Free T4, FT4, Free thyroxine
Free T4 (thyroxine) is one of the main hormones your thyroid produces, measured in its “free,” unbound form that is available for your body to use. It is often checked alongside TSH for a fuller view of thyroid function.
Why it's measured
Free T4 helps doctors interpret a TSH result and understand thyroid function more completely. The two are usually read together rather than separately.
Typical range
Reference ranges for Free T4 vary by lab and the units used. Because of this variation, your result is best understood against the range printed on your own report and in light of your TSH.
What high values can mean
A higher Free T4 is one pattern doctors consider when evaluating thyroid activity, typically alongside TSH and your symptoms.
What low values can mean
A lower Free T4 is another pattern examined together with TSH and the rest of your picture, not as a standalone diagnosis.
Questions to ask your doctor
- How does my Free T4 fit together with my TSH?
- Do these thyroid numbers need any follow-up?
- Should I be monitoring my thyroid over time?
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Try the explainerEducational information only — not medical advice, diagnosis, or treatment. Reference ranges vary by lab; always use your own lab's range and discuss results with a qualified clinician.