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UW Health SMPH
American Family Children's Hospital

UW Hospital and Clinics Lab Test Directory

Test Name: TSH Reflex Free T4

Test Code(s): TSHT4

CPT Code(s): 84443, 84439

Test Description: Test Component:


Free T4 will be performed if the TSH is <0.36 uIU/mL or >4.20 uIU/mL.

Methodology: Chemiluminescent Immunoassay

Days Performed: Daily, 24 hours.

Turnaround Time: Stat: 1 hour.
Routine: 4 hours.

Specimen Requirements

Specimen: Blood

Collection Container: Preferred: Green cap with yellow ring (PST)
Also Acceptable: Green top, Red cap with yellow ring (SST), Red top

Collection Volume: Preferred: 1 mL
Pediatrics: 0.5 mL

Sample Analyzed: Plasma or serum

Volume Required: Preferred: 0.5 mL
Pediatrics: 0.2 mL

Specimen Processing: Centrifuge.  If a complete barrier has not formed, transfer cell-free plasma/serum to plastic vial.  Refrigerate.

Specimen Transport: Transport specimen to laboratory. Transport with coolant pack if coming from outreach location.

Stability: Refrigerated: 7 days
Frozen: Okay


Reference Interval:



Age 8 days and up: 0.36 - 4.20 uIU/mL


FT4 (if performed):


1 year and up: 0.70-1.45 ng/dL

Test Limitations:



In a small cluster of patients, the monoclonal antibody in the reagent failed to detect the TSH molecule causing a falsely low TSH value which may lead to inappropriate diagnosis or treatment of patients. Clinical signs and symptom, patient history and other thyroid tests should be evaluated if the TSH value is discordant.




Results may be affected if patient has developed Human Anti-Murine Antibodies (HAMA) due to exposure to murine based products.

Thyroid autoantibodies in human serum may interfere and cause falsely elevated Free T4 results.

Thyroid Stimulating Hormone

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