Individual test · Labcorp 004457

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

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. This is particularly useful for checking thyroid function in situations that affect blood proteins, such as pregnancy or hormone therapy, where standard tests might be misleading. It helps determine if abnormal hormone levels are due to a thyroid problem or simply a change in carrier proteins. This test does not diagnose the specific underlying cause of thyroid disease or replace the need for measuring thyroid-stimulating hormone (TSH).