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Beaumont Laboratory

Free Thyroxine (FT4)

FT4, Free T4, Free Thyroid Index, T4, Free, Free Thyroxine (FT4), Antrim #17150, EPIC: LAB5120, SOFT: FRT4

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/Drawsite 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. (Min: 0.5 mL)

Preparation for Courier Transport

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

Rejection Criteria

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

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

Farmington Hills Chemistry Laboratory.
Grosse Pointe Chemistry Laboratory.
Royal Oak Automated Chemistry Laboratory.
Troy 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: 0.6 - 2.0 ng/dL.
10 days - adult: 0.8 - 1.8 ng/dL.

Pregnancy:
1st trimester: 0.8 - 1.8 ng/dL.
2nd trimester: 0.6 - 1.2 ng/dL.
3rd trimester: 0.6 - 1.2 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 Code

84439.

Test Codes

Antrim #17150, EPIC: LAB5120, SOFT: FRT4

Last Updated

01/26/2018

Microtainer® and Vacutainer® are registered trademarks of Becton, Dickinson and Company.
UroVysion® is a registered trademark of Abbott Laboratories. ThinPrep® is a registered trademark of Hologic, Incorporated.

This directory currently reflects information only for specimens collected and/or processed at the Farmington Hills, Grosse Pointe, Royal Oak, and Troy campuses.