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Test Code LAB5725 Thyroid Function Cascade

Testing Algorithm

If TSH result is abnormal, LAB5725 (T4 Free) will be performed at an additional charge. If reflexed FT4 is normal (or decreased) AND TSH is <0.3 mcLUnits/mL then a Free T3 will be performed at an addtional charge

Performing Laboratory

St. Louis Children's Hospital Laboratory - Chemistry

Specimen Requirements

Patient Preparation: Samples should not be taken from patients receiving therapy with high biotin doses (i.e., > 5 mg/day) until at least 8 hours following the last biotin administration.

Collection Container/Tube:

Preferred: Mint green-top Lithium Heparin (MNTGRN LiHep) tube

Acceptable: Plain red-top tube or Gold SST gel tube

Submission Container/Tube: If specimen is coming from outside facility and collected in a plain red-top tube aliquot processed sample into a plastic vial for transport (labeling with specimen type).

Specimen Collection Volume: 2.0 mL (minimum 1.2 mL)

Special Instructions: None

Sample Rejection

Mislabled or unlabeled specimens

Logistics

Test Availability: Monday-Sunday

Cut-off time: None, performed as received

Turnaround Time: Same Day; 2 hours from receipt in performing lab

NOTE: if Free T3 is reflexed, testing will be sent to Barnes-Jewish Hospital Laboratory at IOH

Reference Values

Reference Interval: 

TSH  
<4 days: 1.00-25.00 mclUnits/mL
4 days-30 days: 0.50-10.00 mclUnits/mL
≥ 30 days: 0.30-4.20 mclUnits/mL                                                                
TSH concentrations may range up to 100 mclUnits/mL due to surge of TSH production on the frist day of life and then fall gradually to adult levels by one month of age.

 

Free T4
<4 days: 1.00-3.00 ng/mL
4-30 days: 0.80-2.20 ng/mL
≥ 30 days: 0.90-1.70 ng/mL                                                          
Free T4 concentrations rise acutely to as high as 5.00 ng/mL on the first day of life following TSH surge and then decrease gradually to adult levels by one month of age. 

 

Free T3
<18 years: None established
≥ 18 years: 2.0-4.4 pg/mL                                                                   

Critical Value: none established

 

Methodology

Electrochemiluminescence Immunoassay (ECLIA) sandwich principle

Limitations

  • In rare cases, interference due to extremely high titers of antibodies to analyte-specific antibodies (such as HAMA), streptavidin or ruthenium can occur.

Additional Information

FOR SLCH CORE LABORATORY USE ONLY

Lab Processing Instructions: Centrifuge to separate. If specimen is collected in a plain red-top tube aliquot sample into a Roche false-bottom tube for testing (labeling aliquot with specimen type).

Processed Volume: minimum 0.6 mL plasma/serum

Analyte Stability:

Specimen Type Temperature Time
Whole Blood (unprocessed)  Ambient NA
Plasma/Serum (processed) Refrigerated (preferred) 7 days
  Ambient 24 hours
  Frozen  30 days        

Test Classification and CPT Code

84443 - TSH

84439 - Free T4 (if appropriate)

84881 - Free T3 (if appropriate)

LOINC

TSH: 11580-8

Free T4: 3024-7

Free T3: 3051-0

Last Reviewed

06/2025