Test Overview
Test Methodology

Chemiluminescent Immunoassay

Test Usage

Evaluation of thyroid function independent of binding protein status. Free T4 levels correlate with T4 secretion and metabolism. In hypothyroidism, FT4 is depressed and conversely, FT4 is raised in hyperthyroidism. Evaluating free T4 levels is useful when altered levels of total T4 occur due to changes in T4 binding proteins, especially TBG.

Reference Range *

0.76 - 1.70 ng/dL

* Reference ranges may change over time. Please refer to the original patient report when evaluating results.

Test Limitations

Specimen may be stored at room temperature up to 8 hours. [12/05]

No thyroid test is perfectly predictive of status of patients on thyroid medication. The test has less specificity when there is decreased binding of thyroxine by proteins.

Test Details
Days Set Up
Daily, 24 hours
Analytic Time

4 hours

Soft Order Code
MiChart Code
T4, Free
  • Free T4 (Thyroxine)
  • Free Thyroxine
  • Synthroid (Levothyroxine)
  • Thyroxine, Free
  • Unbound T4
  • FT4
  • FREE T4
Chemical Pathology
Specimen Requirements
Collection Instructions

Collect specimen in a red top or SST tube. Centrifuge, aliquot serum into a plastic vial and refrigerate up to 48 hours or freeze for longer storage.

Alternate Specimen
Red Top Tube
Normal Volume
0.5 mL serum
Minimum Volume
0.25 mL serum
Rejection Criteria
Anticoagulants not acceptable.
CPT Code
Fee Code