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Test Code FREE T4 Thyroxine Free

Clinical System Name

Thyroxine Free




This method measures free T4 by competitive immunoassay. Free T4's with results of 0.6 or 0.7 ng/dL are sent out reflexively for Free T4 by equilibrium dialysis. 

Sample Requirements

Specimen: Whole blood

Container(s): Gold SST, Red, Gold Microtainer

Preferred Vol: 1.0 mL

Minimum Vol: 0.5 mL



Processing Instructions

Reject due to: gross lipemia

Spin: Y

Aliquot: Y

Temp: 2-8 C

Storage location: CPA refrigerator,  FrigD rack. Serum should be removed from separator for storage.

Note: Minimum volume of 1.0 mL serum may be shared with TSH, T3, T4, free T3, and free T4


Off-site collection: Spin blood. Transfer serum to plastic aliquot container. Refrigerate.


Specimen Type Temperature Time
gel separator with serum Refrigerated 48 h
separated serum Room temp 2 h
  Refrigerated 7 d
  Frozen -20 C 4 w



STAT Performed TAT
N M-Sa, day shift 1-3 d


Performing Laboratory

Seattle Children's Laboratory    


Department:  Chemistry

Location: Chem West

Phone Number:  (206) 987-2617




Reference Range



Birth - 3 days  2.0 - 5.0 
3 days - 1 month 0.9 - 2.2
1 month - 18 years 0.8 - 2.0
18 years - adult 0.8 - 2.2



Method: Chemiluminescence by Vitros 3600, third generation assay

Analytical Volume: 0.200 mL serum

Limitations: Turbidity may affect results. Bilirubin 20 mg/dL causes positive interference of 20%.  Bilirubin less than 0.4 mg/dL is ok.

CPT Codes