Thyroid-Binding Globulin (Thyroxine-Binding Globulin, TBG)
Produced by: The liver
Definition / Purpose: Thyroid-binding globulin is a circulating protein that binds to T3 and T4 and carries them in the bloodstream.
Why it is worthless: It is sometimes used with another worthless test, T3 Uptake to find the cause of low or elevated thyroid hormone. It is not used often because it just is not a useful test. It can be modified by other factors including liver disease, fluctuating estrogen levels, birth control pills, cortisol levels, and testosterone levels.
Lab Reference Range: 1.1-2.1 mg/dL
When to order:  Pretty much never.
Pearls: TBG is one of three transport proteins that are responsible for carrying T4 and T3 in the blood stream.