Free Thyroxine Index (FTI), Serum
Use
The Free Thyroxine Index (FTI) test is useful for estimating the amount of circulating free thyroxine by using the total thyroxine and thyroid binding capacity (T-uptake). This test assists in differentiating between euthyroid, hyperthyroid, and hypothyroid conditions, especially when binding proteins are normal. It provides important diagnostic clarity in assessing thyroid function by accounting for changes in thyroid hormone carrier proteins and thyroxine levels. In most clinical situations, the direct measurement of free thyroxine has replaced the FTI due to its sensitivity.
Special Instructions
For optimal results, patients should refrain from taking multivitamins or dietary supplements containing biotin for 12 hours prior to specimen collection. Additionally, if the patient is on lipid-lowering agents containing dextrothyroxine, these should be discontinued for 4 to 6 weeks before the test to mitigate interference.
Limitations
The test cannot be performed on patients receiving treatment with lipid-lowering agents containing dextrothyroxine. Autoantibodies to thyroid hormones and binding protein anomalies, such as those seen in familial dysalbuminemic hyperthyroxinemia, can interfere with the assay and lead to incorrect values. This test is less reliable for patients with abnormal binding proteins, where direct measurement of free thyroxine is preferred.
FDA ApprovedNew York Approved
Methodology
Other
Biomarkers
LOINC Codes
- 32215-6
- 83119-8
- 74795-6
- 32215-6
Result Turnaround Time
1-2 days
Related Documents
For more information, please review the documents below
Specimen
Serum
Volume
1 mL
Minimum Volume
0.5 mL
Container
Plastic vial
Collection Instructions
Centrifuge and aliquot serum into a plastic vial.
Patient Preparation
For 12 hours before specimen collection, do not take multivitamins or dietary supplements containing biotin. Discontinue treatment with dextrothyroxine 4 to 6 weeks prior.
Causes for Rejection
Gross hemolysis, gross lipemia, and gross icterus are acceptable.
Stability Requirements
| Temperature | Period |
|---|---|
| Room Temperature | 72 hours |
| Refrigerated | 7 days |
| Frozen | 30 days |