The T4-TBG Index is a calculated marker used to estimate the amount of free (unbound) thyroxine available in the bloodstream. Thyroxine (T4) is one of the main hormones produced by the thyroid gland and plays a central role in regulating metabolism, energy production, body temperature, and growth.
In circulation, the majority of T4 is bound to transport proteins, especially thyroxine-binding globulin (TBG). Approximately 99% of T4 is bound to proteins, leaving only a small fraction available in its free, biologically active form. Changes in TBG levels—due to factors such as pregnancy, hormone therapy, liver disease, or genetic variation—can significantly affect total T4 measurements without reflecting true thyroid dysfunction.
The T4-TBG Index was developed to correct for these effects by combining measurements of total T4 with an estimate of thyroid hormone binding capacity (often measured as T3 uptake or directly through TBG levels). By adjusting total T4 for the amount of available binding protein, the index provides a better estimate of the amount of free thyroid hormone circulating in the body.
Historically, the T4-TBG Index was widely used in thyroid function testing. Today, many laboratories measure free T4 directly, which generally provides a more accurate assessment of thyroid hormone status. However, the T4-TBG Index may still appear on certain laboratory panels and can be helpful when evaluating thyroid function in situations where binding protein levels may be altered.
Abnormal results may indicate thyroid dysfunction, including conditions such as hyperthyroidism or hypothyroidism. Because the index reflects estimated free thyroid hormone activity, it is typically interpreted alongside other thyroid markers, including TSH, free T4, total T4, and T3.
T4-TBG Index
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