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Test Code FT4B Free Thyroxine (T4) 

Important Note

This test may be affected by biotin interference. Patients on high dose biotin (5mg/day or more) should not have samples drawn until at least 24 hours after the last biotin administration.

Specimen and Container/Tube

Specimen Type: Plasma

Preferred Specimen: Li-Heparin Mint Green Top

Other Acceptable specimens: Serum/Gold Top, Serum/Red Top

Specimen Stability

Storage Temperature

Stability

Room Temperature

5 days

Refrigerated

7 days

Frozen

30 days

Collection Instructions

Collect specimen per standard laboratory collection procedures. 

 

Specimen Handling and Transport Instructions

UCMC Onsite Instructions: 

  • Collect specimen per standard collection procedure and send to laboratory immediately. 

 

UCMC Offsite and UC MedLab Outreach Instructions: 

  • Samples should be centrifuged within 2 hours of collection.
  • Centrifuge specimens at >2500 x g for 10 minutes at room temperature.
  • Aliquot sample into plastic transport tube containing a minimum of 0.2 mL of specimen.
  • Samples must be sent refrigerated. 

Rejection Criteria

  • Common specimen rejected situations include: incorrect specimen type, insufficient volume, missing or incomplete specimen identifiers, incorrect specimen transport, or specimens outside stability limits. 
  • Specimens that are clotted or grossly hemolyzed.
  • Specimens that were transported unspun/whole blood and draw time is greater than 6 hours

Reference Values

Test

Age

Sex

Reference Range

FT4

0 – 1 month

All

1.24 – 3.89 ng/dL

FT4

1 month – 1 year

All

1.09 – 1.71 ng/dL

FT4

1 year – 18 years

All

1.01 – 1.63 ng/dL

FT4

>18 years

All

0.9 0- 1.70 ng/dL

Critical Values

Not applicable 

Turnaround Time

Turnaround times are relative to the time the specimen is received in the test laboratory. 

 

STAT Turnaround Time: Not available as STAT test

Routine Turnaround Time: 4 hours

Testing Schedule:  24/7

Clinical Indications

Free T4 is a useful tool in clinical routine diagnostics for the assessment of the thyroid status. It should be measured together with TSH if thyroid disorders are suspected and is also suitable for monitoring thyrosuppressive therapy.

Test Components

Free Thyroxine

CPT Codes

84439

LOINC Codes

 3024-7

Methodology

Electrochemiluminescence immunoassay (ECLIA) (Roche Diagnostics)

Synonyms/Keyword Search

ft4, FT4, T4F

Additional Information

  • Results should always be assessed in conjunction with patient's medical history, clinical examination and other findings.
  • Any influence that might affect the binding behavior of the binding proteins can alter the result of the fT4 tests (e.g. drugs, NTIs (Non-Thyroid-Illness) or Hyperthroxinemia)).
  • The test cannot be used in patients receiving treatment with lipid-lowering agents containing D-T4. If the thyroid function is to be check in such patients, the therapy should first be discontinued for 4-6 weeks to allow the physiological state to become re-established.
  • Autoantibodies to thyroid hormones can interfere with the assay.
  • In in vitro studies the drugs Furosemide and Levothyroxine caused elevated fT4 findings at the daily therapeutic dosage level.
  • In rare cases, interference due to extremely high titers of antibodies to analyte-specific antibodies, streptavidin or ruthenium can occur. These effects are minimized by the suitable test designs.

 

 

Last Review Date

6/9/2025

Volume

Preferred Volume: 0.5 mL

Minimum Volume: 0.2 mL