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Test ID: FRT4 T4 (Thyroxine), Free, Serum

Reporting Name

T4 (Thyroxine), Free, S

Useful For

Evaluating suspected thyroid function disorders using free thyroxine measured together with thyroid-stimulating hormone

Clinical Information

Free thyroxine (FT4) comprises a small fraction of total thyroxine. FT4 is available to the tissues and is, therefore, the metabolically active fraction.


Elevations in FT4 cause hyperthyroidism, while decreases cause hypothyroidism.


Elevated values suggest hyperthyroidism or exogenous thyroxine.


Decreased values suggest hypothyroidism.


Free thyroxine (FT4) works well to correct total T4 values for thyroxine-binding globulin alterations but may give misleading values when abnormal binding proteins are present or the patient has other major illnesses (euthyroid sick syndrome).

Testing Algorithm

For more information see: Thyroid Function Ordering Algorithm

Report Available

Same day/1 to 2 days

Day(s) Performed

Monday through Sunday

Clinical Reference

1. Melmed S, Polonsky KS, Larsen PR, Kronenberg H. Williams Textbook of Endocrinology. 12th ed. Saunders Company; 2011:348-414

2. Rifai N, Chiu RWK, Young I, Burnham CAD, Wittwer CT, eds. Tietz Textbook of Laboratory Medicine. 7th ed. Elsevier; 2023

Method Name

Electrochemiluminescence Immunoassay

Specimen Type


Specimen Required

Patient Preparation: For 12 hours before specimen collection do not take multivitamins or dietary supplements containing biotin (vitamin B7), which is commonly found in hair, skin, and nail supplements and multivitamins.

Collection Container/Tube:

Preferred: Serum gel

Acceptable: Red top

Submission Container/Tube: Plastic vial

Specimen Volume: 1 mL

Collection Instructions: Centrifuge and aliquot serum into a plastic vial.

Specimen Minimum Volume

0.5 mL

Specimen Stability Information

Specimen Type Temperature Time Special Container
Serum Refrigerated (preferred) 14 days
  Frozen  30 days
  Ambient  72 hours

Reference Values


0-5 days: 0.9-2.5 ng/dL

6 days-2 months: 0.9-2.2 ng/dL

3-11 months: 0.9-2.0 ng/dL

1-5 years: 1.0-1.8 ng/dL

6-10 years: 1.0-1.7 ng/dL

11-19 years: 1.0-1.6 ng/dL


Adult (≥20 years): 0.9-1.7 ng/dL


For SI unit Reference Values, see

Test Classification

This test has been cleared, approved, or is exempt by the US Food and Drug Administration and is used per manufacturer's instructions. Performance characteristics were verified by Mayo Clinic in a manner consistent with CLIA requirements.

CPT Code Information


LOINC Code Information

Test ID Test Order Name Order LOINC Value
FRT4 T4 (Thyroxine), Free, S 83122-2


Result ID Test Result Name Result LOINC Value
FRT4 T4 (Thyroxine), Free, S 83122-2
Mayo Clinic Laboratories | Endocrinology Catalog Additional Information: