T4 Free (Direct Dialysis) and T4 Total Test
This test reports two thyroxine values from one blood sample: free T4, measured by equilibrium dialysis, and total T4, measured by chemiluminescence. Quest describes the combination as used to help distinguish certain thyroid hormone patterns from true hyperthyroidism or hypothyroidism.
About this test
Thyroxine, or T4, circulates in the blood mostly bound to carrier proteins, with a small free fraction available to tissues. This test reports both: total T4, measured by chemiluminescence, and free T4, measured by the direct equilibrium dialysis method along with chromatography/mass spectrometry.
Quest describes the pairing as used for the differential diagnosis of euthyroid hyperthyroxinemia from hyperthyroidism, and euthyroid hypothyroxinemia from hypothyroidism — situations where the total T4 value alone can be misleading because it moves with binding-protein levels while the free T4 value does not.
Test details
| Quest code | 36725 |
|---|---|
| Methodology | Equilibrium Dialysis • Chromatography/Mass Spectrometry • Chemiluminescence |
| Sample type | 2 mL serum |
| Patient preparation | Fast before your blood draw if you can; water is fine. |
| Collection | Quest patient service center |
Related tests and panels
Quest resources
Additional details
What this test looks at
The test reports two related values from one serum sample: total T4, measured by chemiluminescence, and free T4, measured by equilibrium dialysis with chromatography/mass spectrometry. Quest lists it under several names, including T4, Free and T4, Total and Thyroxine, Free and Total, all referring to this same paired measurement.
How it's often used alongside other tests
Quest describes this paired T4 measurement as used for the differential diagnosis of euthyroid hyperthyroxinemia from hyperthyroidism, or euthyroid hypothyroxinemia from hypothyroidism. Because of that role, a clinician typically reads it together with other thyroid results rather than as a stand-alone number.
Order online and a physician order is included — no doctor's visit and no insurance needed. Your sample is collected at a Quest Diagnostics patient service center.
Laboratory results should always be interpreted by a licensed clinician who knows your health history. This page is information about a test, not medical advice.
Frequently asked questions
What does this test measure?
It measures two related thyroxine values from a single serum sample: total T4, using chemiluminescence, and free T4, using the equilibrium dialysis method along with chromatography/mass spectrometry.
How is the sample collected?
The test uses a 2 mL serum sample from a routine blood draw at a Quest Diagnostics patient service center.
Do I need to prepare for this test?
Fast before your blood draw if you can; water is fine. This is the preparation Quest publishes for this test; if your clinician gives you different instructions, follow theirs.
Why are free T4 and total T4 measured together?
Quest describes the pairing as used for the differential diagnosis of euthyroid hyperthyroxinemia from hyperthyroidism, or euthyroid hypothyroxinemia from hypothyroidism — distinctions that can be harder to make from a total T4 result alone.
What does a T4 Free, Direct Dialysis and T4 Total test cost without insurance?
$315. Celio Health's price is a flat cash price and insurance is never billed: the required physician's order is included.