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Free Thyroxine (FT4)

FT₄, Free T₄, Free Thyroid Index, T₄, Free, Free Thyroxine (FT₄)

Test Codes

EPIC: LAB5120, Beaker: FRT4, Antrim: 17150

Department

Chemistry

Specimen Collection Criteria

Collect: One Gold-top SST tube. (Minimum Whole Blood: 4.0 mL)

Contact the Laboratory for the acceptability of other tube types.
See Minimum Pediatric Specimen Requirements for Microtainer^® collection.

Note: Patient Preparation (Farmington Hills ONLY): Twenty-four (24) hours before this test, the patient should not take multivitamins or dietary supplements containing biotin (vitamin B7) which is commonly found in hair, skin and nail supplements and multivitamins.

Physician Office/Draw Specimen Preparation

Let SST specimens clot for 30-60 minutes then immediately centrifuge to separate serum from cells. Refrigerate (2-8°C or 36-46°F) the centrifuged SST tube within two hours of collection. (Minimum: 0.5 mL)

Preparation for Courier Transport

Transport: Centrifuged SST tube, refrigerated (2-8°C or 36-46°F). (Minimum: 0.5 mL)

Rejection Criteria

• Grossly hemolyzed specimens.
• Red-top tubes with serum not separated from cells within two hours of collection. 

In-Lab Processing

Let SST specimens clot 30-60 minutes. Centrifuge SST tubes or Microtainers^® to separate serum from cells. Deliver immediately to the appropriate testing station.

Storage

Specimen Stability for Testing:

Centrifuged SST Tubes and Microtainers^® with Separator Gel
Room Temperature (20-26°C or 68-78.8°F): 2-4 hours
Refrigerated (2-8°C or 36-46°F): 7 days
Frozen (-20°C/-4°F or below): Unacceptable

Red-top Tubes and Microtainers^® without Separator Gel
Room Temperature (20-26°C or 68-78.8°F): 2-4 hours
Refrigerated (2-8°C or 36-46°F): Unacceptable
Frozen (-20°C/-4°F or below): Unacceptable

Serum Specimens (Pour-Overs)
Room Temperature (20-26°C or 68-78.8°F): 2-4 hours
Refrigerated (2-8°C or 36-46°F): 7 days
Frozen (-20°C/-4°F or below): 7 days

Specimen Storage in Department Prior to Disposal:

Refrigerated (2-8°C or 36-46°F): 7 days

Laboratory

Dearborn Chemistry Laboratory
Farmington Hills Chemistry Laboratory 
Grosse Pointe Chemistry Laboratory 
Royal Oak Automated Chemistry Laboratory 
Troy Chemistry Laboratory
Taylor Chemistry Laboratory
Trenton Chemistry Laboratory
Wayne Chemistry Laboratory 

Performed

Sunday – Saturday,  24 hours a day. 
Results available within 24 hours of receipt in the Chemistry Laboratory.

Reference Range

0-9 days: 0.5-1.7 ng/dL.
10 days - adult: 0.7-1.5 ng/dL.

Pregnancy:
1^st trimester: 0.7-1.5 ng/dL.
2^nd trimester: 0.5-1.0 ng/dL.
3^rd trimester: 0.5-1.0 ng/dL.

Test Methodology

Chemiluminescent Immunoassay.

Interpretation

Clinical Condition	FT4
Euthyroid	0.8 to 1.8 ng/dL
Hypothyroid	less than 0.8 ng/dL
Hyperthyroid	greater than 1.8 ng/dL

Heterophilic antibodies in human serum can react with reagent immunoglobulins, interfering with in vitro immunoassays. Patients routinely exposed to animals or animal serum products can be prone to this interference and anomalous high values can be observed. Additional information may be required for diagnosis.

The anticonvulsant drug phenytoin may interfere with Free T₄ levels due to competition for TBG (thyroxine-binding globulin) binding sites.

Free T₄ levels may be decreased in patients with non-thyroidal conditions and in patients taking carbamazepine.

Clinical Utility

Free T₄ levels give a more accurate picture of the thyroid status in patients with abnormal T₄-binding globulin levels such as those who are pregnant or those who are receiving treatment with estrogens, androgens, dilantin, or salicylates.

CPT Codes

84439

Contacts

Last Updated

10/25/2023