What is this test?
FT3 measures the free part of triiodothyronine, an active thyroid hormone that affects metabolism.
Why might it be ordered?
- To supplement the evaluation of suspected hyperthyroidism.
- For follow-up in certain cases where TSH and FT4 do not provide sufficient information.
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 thyroid meds and biotin. For tracking, use similar conditions and sampling 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?
FT3 is interpreted with TSH and FT4. Severe systemic illness can change FT3 even when the thyroid itself is not the primary problem. 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.