Thyroxine (T4), Free, Direct
Also known as: Free T4, Direct, Serum, Free Thyroxine, T4, Free, Direct, Serum
Use
Free T4 may be indicated when binding globulin (TBG) problems are perceived, or when conventional test results seem inconsistent with clinical observations. It is normal in subjects with high thyroxine-binding globulin hormone binding who are euthyroid (ie, free thyroxine should be normal in nonthyroidal diseases). It should be normal in familial dysalbuminemic hyperthyroxinemia.
Special Instructions
Patients should be informed to discontinue any high-dose biotin supplementation at least 72 hours before sample collection to avoid interference. Proper communication of the patient's intake of supplements is necessary for accurate assessment.
Limitations
The Free T4 test can be influenced by various medications, including contrast agents, propranolol, amiodarone, and heparin, potentially increasing FT4 levels, while carbamazepine may lead to decreased levels. The test is less reliable in patients with various nonthyroidal illnesses, as transient elevations can occur, and discrepancies are noted between different methodologies, such as direct and equilibrium dialysis techniques. Thus, test results should be interpreted with caution, particularly where reference methods vary.
Methodology
Immunoassay (ECLIA)
Biomarkers
LOINC Codes
- 3024-7
- 3024-7
Result Turnaround Time
1 day
Related Documents
For more information, please review the documents below
Specimen
Serum
Volume
1 mL
Minimum Volume
0.7 mL
Container
Red-top tube, gel-barrier tube or green-top (lithium heparin) tube
Collection Instructions
If a red-top tube or plasma is used, transfer separated serum or plasma to a plastic transport tube.
Patient Preparation
Heparin has been reported to have effects on free T4 assay; hence samples should not be collected during or soon after its administration.
Storage Instructions
Room temperature
Causes for Rejection
Citrate plasma specimen; improper labeling
Stability Requirements
| Temperature | Period |
|---|---|
| Room Temperature | 14 days |
| Refrigerated | 14 days |
| Frozen | 14 days |