Thyroid cascade starts with TSH. If TSH is low or high, Free T4 is triggered. If TSH is low and FT4 is low, Free T3 is triggered. If TSH is low and FT4 is normal, Free T3 is triggered. If TSH is high and FT4 is low, TPO (thyroid peroxidase antibody) is triggered. If TSH is high and FT4 is normal, TPO (thyroid peroxidase antibody) is triggered.
Test Code
ThyCasPnl
Specimen Requirements
Minimum of 2.0 mL serum; alternately 2.0 mL serum from Red Top tube
Supplies for Submission
SST/Gold Top (1); alternately Red Top (1)
Specimen Collection
For patients receiving therapy with high biotin doses (> 5 mg/day), wait for at least 8 hours following the last biotin administration before collecting samples.
Label serum separator tube (SST) with two identifiers. Acceptable identifiers include patient’s name, date of birth, social security number, requisition number, and medical record number.
Allow SST contents to clot in an upright position for at least 30 minutes (60 minutes for Red Top tube).
SST specimen - centrifuge within 2 hours of collection and refrigerate.
Red Top specimen - centrifuge within 2 hours of collection, transfer serum to a plastic transport tube labelled appropriately and as Red Top serum, and refrigerate.
Storage Requirements
Refrigerated. Specimens are stable if if stored at room temperature for 8 days, refrigerated for 14 days, or frozen for 24 months.
Turn Around Time
1 day
Limitations
If reflex tests involve Free T3 or Free T4, and if the sample contains a biotin concentration of > 10 ng/mL, there is the potential of biotin interference effects, and hence, falsely elevated results.