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HelpTest Code LAB127 T4 Free
Performing Laboratory
St. Louis Children's Hospital Laboratory - Chemistry
Specimen Requirements
Patient Preparation: Samples should not be taken from patients receiving therapy with high biotin doses (i.e., >5 mg/day) until at least 8 hours following the last biotin administration
Collection Container/Tube:
Preferred: Mint green-top Lithium Heparin (MNTGRN LiHep) tube
Acceptable: Green samplette/microtainer, plain red-top tube, Gold SST gel tube, or Lavender-top (EDTA) tube
Submission Container/Tube: If specimen is coming from outside facility and collected in a microtainer, plain red-top tube, or lavender-top tube aliquot processed sample into a plastic vial for transport (labeling with specimen type).
Specimen Collection Volume: 2.0 mL (minimum 0.6 mL)
Special Instructions: Send to lab promptly
Sample Rejection
Mislabled or unlabeled specimens
Logistics
Test Availability: Monday-Sunday
Cut-off time: None, performed as received
Turnaround Time: Same Day; 2 hours from receipt in performing lab
Reference Values
Reference Interval:
| <4 days: | 1.00-3.00 ng/dL |
| 4 days-30 days: | 0.80-2.20 ng/dL |
| ≥ 30 days: | 0.90-1.70 ng/dL |
| Free T4 concentrations rise acutely to as high as 5.00 ng/dL on the first day of life following TSH surge and then decrease gradually to adult levels by one month of age. | |
Critical Value: none established
Methodology
Electrochemiluminescence Immunoassay (ECLIA) competition principle
Limitations
- This test cannot be used in patients receiving treatment with lipid-lowering agents containing D-T4. If thyroid function is to be checked in such patients, the therapy should first be discontinued for 4-6 weeks to allow the physiological state to become re-established.
- Furosemide and Levothyroxine cause elevated FT4 at daily therapeutic dosage levels.
- In rare cases, interference due to extremely high titers of antibodies to streptavidin or ruthenium can occur.
Additional Information
FOR SLCH CORE LABORATORY USE ONLY
Lab Processing Instructions: Centrifuge to separate. If specimen is collected in microtainer, plain red-top tube, or lavender-top tube aliquot sample into a Roche false-bottom tube for testing (labeling aliquot with specimen type).
Processed Volume: 0.5 mL (minimum 0.3 mL) plasma/serum
Analyte Stability:
| Specimen Type | Temperature | Time |
|---|---|---|
| Whole Blood (unprocessed) | Ambient | 12 hours |
| Plasma/Serum (processed) | Refrigerated (preferred) | 7 days |
| Ambient | 24 hours | |
| Frozen | 30 days | |
| Freeze /thaw only one cycle | ||
Test Classification and CPT Code
84439
LOINC
3024-7
Last Reviewed
06/2025