Most thyroid hormone in the blood is bound to TBG. Changes in TBG levels — from pregnancy, estrogen therapy, liver disease, or inherited TBG excess or deficiency — alter total thyroid hormone values without changing thyroid function itself. Measuring TBG helps distinguish true thyroid disease from binding-protein artifacts.
This test is used when total thyroid hormone levels do not match TSH or clinical picture, when inherited TBG abnormalities are suspected (often seen in family screening), and during evaluation of unusual thyroid laboratory patterns. Sample type is serum.
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