T₄, Free, Direct Dialysis
Also known as: Direct Free T4, Free Thyroxine, Free T4 By Equilibrium Dialysis, FT4 Direct Dialysis, Thyroxine
Use
For the differential diagnosis of euthyroid hyperthyroxinemia from hyperthyroidism or for the differential diagnosis of euthyroid hypothyroxinemia from hypothyroidism. Free T4 by equilibrium dialysis is considered the most accurate measure of free T4 and yields results independent of binding protein concentrations, molecular variants, or circulating autoantibodies. ([questdiagnostics.com](https://www.questdiagnostics.com/healthcare-professionals/about-our-tests/endocrine-disorders/thyroid?utm_source=openai))
Special Instructions
Not provided.
Limitations
Unexpectedly high free T4 results may be seen in sera from heparin-treated patients due to generation of free fatty acids that displace T4 in vitro; impurities in tracer may also cause erroneous results. ([questdiagnostics.com](https://www.questdiagnostics.com/healthcare-professionals/clinical-education-center/faq/faq61?utm_source=openai))
New York Approved
Methodology
NGS
Biomarkers
LOINC Codes
- 6892-4
- 6892-4
Result Turnaround Time
3-4 days
Related Documents
For more information, please review the documents below
Specimen
Serum
Volume
1 mL
Minimum Volume
0.5 mL
Container
Red top or serum gel tube; plastic vial for submission
Collection Instructions
Centrifuge and aliquot 1 mL of serum into a plastic vial; fasting 8‑16 hours preferred but not required; specify patient age on request form
Patient Preparation
Fasting: 8‑16 hours preferred but not required
Causes for Rejection
Lipemia; heparin plasma; EDTA plasma
Stability Requirements
| Temperature | Period |
|---|---|
| Room Temperature | 4 days |
| Refrigerated | 14 days |
| Frozen | 35 days |
Other tests from different labs that may be relevant