Thyroxine Binding Globulin (TBG) is a 58,000 MW, single-chain, polypeptide glycoprotein and belongs to the α-globulin family of human plasma proteins. TBG is significant in the regulation of plasma levels of the thyroid hormones, functioning as the primary plasma transport protein for thyroxine (T4). TBG also transports triiodothyronine (T3), but its binding affinity is approximately 10% of that for T4. Other proteins bind thyroid hormones in the plasma, such as transthyretin (prealbumin) and albumin, but TBG is the principal carrier.
Evaluations of serum TBG are useful in the determination of thyroid function: Under hyper-thyroid conditions, TBG is more saturated with thyroid hormones, while under hypo-thyroid conditions, TBG is less saturated.
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