Thyroxine-Binding Globulin Deficiency

Thyroxine-binding globulin (TBG) is one of three major transport proteins, which are primarily responsible for binding to and transporting thyroid hormones to the necessary tissues.  The other two serum transport proteins include transthyretin and human serum albumin.  While there are higher amounts of albumin in serum, TBG has a greater affinity to thyroxine (T4). Abnormalities in the functionality and amount of TBG can cause variations in the total amount of T4 in the serum, but not in the amount of bioactive free T4.  Since the amount of free T4 circulating in the serum remains the same, deficiency in thyroxine-binding globulin often does not lead to adverse metabolic effects seen in an individual with abnormal thyroid hormone levels. However, it can cause errors in the interpretation of thyroid hormone labs, which can ultimately lead to inappropriate treatment.  This article illustrates the etiology, diagnosis, and management of thyroxine-binding globulin deficiency.