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T3 Uptake
Test CodeCPT Codes
84479
Preferred Specimen
Serum (gold top) tube
Minimum Volume
Other Acceptable Specimens
Transport Container
Serum (gold top) tube or Lithium Heparin Plasma (green top) tube
Transport Temperature
Specimen Stability
Reject Criteria (Eg, hemolysis? Lipemia? Thaw/Other?)
Methodology
Chemiluminescence
Setup Schedule
Daily upon receipt
Report Available
Limitations
Reference Range
23.0 - 40.5%
Clinical Significance
Thyroxine (T4) 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. The main influence on metabolic control is exerted by the free thyroid hormone fraction. Free thyroid hormone concentrations 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.