Thyroxine (T4), Thyroxine-binding Globulin (TBG), and T4−TBG Index Labcorp test 004457

This test measures the total amount of thyroxine (T4) in your blood and the protein that carries it, called thyroxine-binding globulin (TBG). By comparing these levels, the test calculates an index that estimates how much thyroid hormone is actually available for your body to use.