Thyroxine (T4), Thyroxine-binding Globulin (TBG), and T4−TBG Index

Casandra

Casandra Test Code LC28803Version 1 (DRAFT)

Performing Lab

Lab LabcorpLab Test ID 004457CPT Codes 84436, 84442

Thyroxine (T4), Thyroxine-binding Globulin (TBG), and T4−TBG Index

Also known as: TBG, Thyroxine (T4), and T4−TBG Index

Clinical Use

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Use

This test is used to evaluate thyroid function by measuring the levels of thyroxine (T4), thyroxine-binding globulin (TBG), and calculating the T4−TBG index. It assists in diagnosing conditions related to abnormal thyroid hormone binding or production, such as hypothyroidism or hyperthyroidism. Additionally, measuring TBG helps determine the binding capacity of serum proteins to thyroxine, providing a more accurate interpretation of T4 levels in cases where the binding protein levels are abnormal.

Special Instructions

Patients should be asked about biotin supplementation as high doses can interfere with the test results. It is recommended that patients discontinue biotin supplements at least 72 hours before specimen collection to avoid potential interference with the test results.

Limitations

The test may exhibit interference when samples are collected from individuals taking high doses of biotin. Such interference could lead to inaccurate results, misrepresenting the levels of thyroxine and its binding globulin. It does not directly measure free thyroxine, which may be affected by abnormal binding protein levels. Additionally, the minimum volume requirement does not allow for repeat testing in case of sample deterioration or test failure.

Test Details