What is this test?
FT4 measures the free part of thyroxine, a major hormone produced by the thyroid gland.
Why might it be ordered?
- To assess thyroid function for symptoms or abnormal TSH.
- To monitor treatment for underactive or overactive thyroid function.
What sample is needed?
A healthcare professional collects a venous blood sample. Depending on the method, the laboratory uses serum or plasma.
How should you prepare?
Report when you take your thyroid medicine and whether you take biotin. For follow-up, use similar collection conditions each time. Tell the laboratory about all medicines, hormone treatments, vitamins and supplements you take. Do not stop or change treatment unless your clinician tells you to. Follow any instructions about fasting and the time of collection.
How should you understand the result?
FT4 is most commonly interpreted with TSH. Pregnancy, acute illness, medications and the laboratory method can affect the value. Your result is interpreted using the reference interval on the laboratory report and in the context of your symptoms, medical history and other test findings.
Sources and editorial status
This patient-facing text is an editorial draft. It should be approved by a laboratory physician before official publication.