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T3 Uptake
MessageTest performed by York Hospital Laboratory.
Test Code
T3U
CPT Codes
84479
Preferred Specimen
0.2 mL serum collected in a serum separator tube (SST)
Other Acceptable Specimens
0.2 mL serum collected in a no additive (red-top) tube, transferred to a plastic screw-cap vial 0.2 mL plasma collected in a lithium heparin (green-top) gel separator tube
Transport Temperature
Refrigerated
Specimen Stability
Room temperature: 4 hours Refrigerated: 7 days Frozen: 4 weeks
Methodology
Chemiluminescence
Setup Schedule
Daily
Report Available
Next Day
Reference Range
23.5 - 40.5 %
Clinical Significance
Thyroxine (R4) and triiodothyronine (T3) are transported in serum bound to protein, principally to thyroxine-binding globulin (TBG). The concentration of unoccupied thyroid hormone binding sites, for example, residual binding capacity, is inversely related to the total thyroid hormone concentrations. Free thyroid hormone can be measured directly, while an estimate can be made by using a binding capacity value, as measured by a T3 uptake or a T4 uptake test, to correct the total T3 or T4 concentration and obtain a free thyroid index (FT3I or FT4I). This is particularly important in conditions not associated with thyroid disease, where concentrations of the thyroid hormones or their binding proteins are altered, for example, in pregnancy.
