T4 Free – Direct Dialysis, Serum
Orderable by Ontario ND
Test Overview
Clinical Utility
For the differential diagnosis of euthyroid hyperthyroxinemia from hyperthyroidism or for the differential diagnosis of euthyroid hypothyroxinemia from hypothyroidism.
Method
Liquid chromatography-mass spectrometry (LC-MS) and Equilibrium Dialysis
Method Description
In the direct equilibrium dialysis (ED) method, patient serum is dialyzed against a buffer for 16-18 hours. ED separates the free T4 molecules from the protein bound T4 molecules. The smaller free T4 molecules can cross the dialysis membrane, but the larger bound T4 molecules cannot. After dialysis, free T4 can be measured using either immunoassay or mass spectrometry. We currently use a sensitive radioimmunoassay that has an analytical sensitivity of 0.2 ng/dL. Results are independent of T4-binding protein concentrations and are unaffected by the presence of molecular variants of these proteins or by circulating thyroid autoantibodies.
Aliases/Synonyms
Thyroxine
Specimen
Specimen Type
SerumContainers
Collection Containers
Gold top (clot activator & gel)
Volume
Sample Volume
1.0 mL
Minimum Volume
0.5 mL
Patient Preparation
Fasting preferred
Collection & Handling
Collection Instructions
Provide date of birth for results interpretation.
Handling Information
Store and send cold or frozen.
Stability
| Ambient | Refrigerated | Frozen |
|---|---|---|
| 4 days | 14 days | 35 days |
Rejection Criteria
| Criteria | Specification |
|---|---|
| Specimen | Heparin Plasma; EDTA Plasma |
| Lipemia | Present |
Performance & Interpretation
Turnaround Time
10 days
Results
-
T4 Free - Direct Dialysisng/dLAdult: 0.9 - 2.2
Prematures, 25 - 30 Weeks
Birth - 7 Days: 0.5 - 3.3
Prematures, 31 - 36 Weeks
Birth - 7 Days: 1.3 - 4.7
Cord Blood, >37 Weeks:1.2-2.2
Birth - 4 Days: 2.2-5.3
2 Months - 2 Years: 1.2 - 2.5
3 Years - 17 Years :1.0-2.4
Pregnancy
First Trimester: 0.9 - 2.0
Second Trimester: 0.8 - 1.5
Third Trimester: 0.8-1.7
Referral Location
Out-of-Country
Interface & Setup
HL7 Interface Codes
| Order Code | Result Name | Result Codes | Units |
|---|---|---|---|
| T4FREE S | T4, FREE, DIRECT DIALYSIS | 64612 | ng/dL |
Test Version
Last Updated
2025-11-25