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T4 Free – Direct Dialysis, Serum

Test ID: T4FREE S
Orderable by Ontario ND
Test
Specimen
Performance / Interpretation
Interface / Setup

Test

Aliases/Synonyms

Thyroxine

Method

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.

Specimens

Serum

Clinical Utility

For the differential diagnosis of euthyroid hyperthyroxinemia from hyperthyroidism or for the differential diagnosis of euthyroid hypothyroxinemia from hypothyroidism.

Test Version

25-Nov-2025

Specimen

Specimens

Serum

Collection Containers

Gold top (clot activator & gel)

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 4 days
Refrigerated 14 days
Frozen 35 days

Rejection Criteria

Specimen Heparin Plasma; EDTA Plasma
Lipemia Present

Test Version

25-Nov-2025

Performance / Interpretation

Method

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.

Turnaround Time

10 days

Results

Name Units Reference Range Conversion Factor
T4 Free - Direct Dialysis ng/dL
  • Adult: 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

Test Version

25-Nov-2025

Interface / Setup

HL7 Interface Codes

Order Code Result Codes Units
T4FREE S 64612T4, FREE, DIRECT DIALYSIS ng/dL

Test Version

25-Nov-2025