Major thyroid hormone transport protein in serum. Expressed by the liver and secreted in plasma. Genetic variants in SERPINA7 influence the serum levels of thyroxine-binding globulin and define the thyroxine-binding globulin quantitative trait locus (TBGQTL) [MIM:300932]. Individuals with low or high serum levels of thyroxine-binding globulin show, respectively, reduced or elevated protein-bound iodine but are euthyroid and do not manifest major metabolic alterations (PubMed:1294376, PubMed:1515456, PubMed:1901689, PubMed:1906047, PubMed:2155256, PubMed:2501669). Two qualitative TBG variants occur in particular populations. TBG-A is found in 40% of Australian aborigines, it has reduced affinity for thyroxine and triiodothyroxine and increased susceptibility to inactivation by heat or acid (PubMed:2495303). TBG-S ('s' for slow shift on isoelectic focusing) is found in blacks, Eskimos, Melanesians, Polynesians and Indonesians, but not in Caucasians; TBG-S is slightly more thermolabile (PubMed:2115061). Belongs to the serpin family.