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Test Code FRT4 T4 (Thyroxine), Free, Serum

Reporting Name

T4 (Thyroxine), Free, S

Useful For

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

Testing Algorithm

See Thyroid Function Ordering Algorithm in Special Instructions.

Performing Laboratory

Mayo Medical Laboratories in Rochester

Specimen Type


Specimen Required

Patient Preparation: Twelve hours before this test do not take multivitamins or dietary supplements containing biotin or vitamin B7 which are 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

Specimen Minimum Volume

0.5 mL

Specimen Stability Information

Specimen Type Temperature Time
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

Day(s) and Time(s) Performed

Monday through Sunday; Continuously

Test Classification

This test has been cleared or approved by the U.S. 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 3024-7


Result ID Test Result Name Result LOINC Value
FRT4 T4 (Thyroxine), Free, S 3024-7

Clinical Information

Free thyroxine (FT4) comprises a small fraction of total thyroxine. The 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).

Clinical Reference

Melmed S, Polonsky KS, Larsen PR, et al: Williams Textbook of Endocrinology. 12th edition. Elsevier Saunders Company, 2011, pp 348-414

Analytic Time

Same day/1 day

Reject Due To


Mild OK; Gross OK


Mild OK; Gross OK


Mild OK; Gross OK




Method Name

Electrochemiluminescence Immunoassay