Free T3 (Free Triiodothyronine, FT3)
Definition
Measures the level of T3 hormone in the blood. Free T3 is the active form that enters tissues; bound T3 is bound to proteins. Tests can measure total T3 (both) or free T3.
Purpose
Investigation of suspected hyperthyroidism. Detection of pituitary gland dysfunction. Monitoring the effectiveness of thyroid treatment. Follow-up of abnormal TSH or T4 results.
Results
Interpreted together with TSH and T4:
| T3 | TSH | Free T4 | Interpretation |
|---|---|---|---|
| Normal | Normal | Normal | Does not suggest thyroid disease |
| High | Low | High | Thyroid-origin hyperthyroidism |
| Low | High | Low | Thyroid-origin hypothyroidism |
Normal Values
Laboratory-specific, see the reference range on your report.
Risks and Limitations
The T3 test is not commonly used for hypothyroidism screening, because hypothyroidism is detected earlier with other thyroid tests. A normal T3 does not rule out thyroid disease. Also, total T3 is considered more accurate than free T3, which is the reverse of free T4 being preferred over total T4.