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Order Code CH260
Turnaround Time 24 hours
Test Includes


Specimen Requirements
Specimen Source
Serum, Plasma
Transport Container
(Specimen Container)

SST (Tiger Top)/ Lithium Heparin/ Sodium Heparin/ EDTA

Preferred Specimens
Minimum Volume (uL)
.5 mL
Collection Instructions
(Transport Temperature)
Transport Refrigerated (cold packs)
Specimen Stability
Temperature Period
Room temperature 24 hours
Refrigerated 14 days
Frozen 30 days
Test Details
chemiluminescent microparticle immunoassay (CMIA)
Clinical Significance

Used in conjunction with Total T4 to yield a calculated Free Thyroxine Index (FTI), as an aid in the assessment of thyroid function status.

The classical in vitro thyroid hormone “Uptake” assays measure the unsaturated thyroxine binding sites of serum proteins. The Alinity i T-Uptake assay measures the total binding capacity present in a sample. The thyroid hormones, thyroxine (T4) and triiodothyronine (T3), are transported in serum bound to the thyroxine binding proteins, thyroxine binding globulin (TBG), thyroxine binding prealbumin (TBPA), and albumin. TBG, TBPA, and albumin bind approximately 75%, 15% and 10% of the total circulating T4, respectively, and bind 38%, 27% and 35% of T3, respectively. In a euthyroid patient, T4 occupies approximately one third of the binding sites.2 Free, or unbound fractions of the thyroid hormones are thought to be responsible for biologic activity. The FTI has been the most widely used method to estimate free T4. Uptake assays are of greatest value when used in conjunction with a serum Total T4 assay to provide the FTI.

The Uptake assays are used to normalize the Total T4 levels for variations in serum thyroxine binding protein (TBP) concentrations. Performing an Uptake assay and subsequent calculation of the FTI is important since certain conditions such as pregnancy, estrogen therapy, infectious and chronic active hepatitis, biliary cirrhosis or congenital disorders alter the number of T4 binding sites. These variations can produce abnormal T4 values in an individual with no thyroid disease. Since the T4 values or the T-Uptake values alone can produce misleading information, an FTI can be calculated to provide a clinically useful and accurate estimate of circulating free thyroxine. To ensure maximum diagnostic accuracy of thyroid status, an FTI should be used in conjunction with clinical evaluation and other thyroid function tests such as human thyroid stimulating hormone (TSH).

Reference Ranges

0.69 – 1.41 Units