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Test Code FT4 T4 (Thyroxine) Free, Plasma or Serum

Additional Codes

Epic Test ID


Performing Laboratory

Bronson Laboratory Services-Chemistry

Reference Values

0.8-1.7 ng/dL

*Note: reference range revised from package insert range of  0.9-1.7 on 12/1/18 following request from endocrinology, review of literature, internal patient data and medical director approval.


Chemiluminescent Immunoassay

This test is recommended over total T4 because the free T4
represents the physiologically active hormone rather than the
quantity of carrier proteins.

Specimen Requirements & Patient Preparation

Patient Preparation: For 12 hours before this test do not take multivitamins or dietary supplements containing biotin (vitamin B7), which is commonly found in hair, skin, and nail supplements and multivitamins.   Biotin can interfere with many immunoassay tests.

Submit only 1 of the following specimens:


Specimen Type: Serum
Container/Tube: Gold or Red

Specimen Volume: 5 mL

Specimen Minimum Volume: 1 mL



Specimen Type: Plasma
Container/Tube: Mint or Green
Specimen Volume: 5 mL

Specimen Minimum Volume: 1 mL

Specimen Transport Temperature


Specimen Stability

Serum*, Refrigerated - 7 days

Serum*, Frozen - 1 month

*Note: Heparinized plasma in gel separator tubes is less stable than serum and is only suitable for add-on testing up to 2 days following sample collection.

Day(s) Test Set Up

Monday through Sunday

Test Classification and CPT Coding