thyroxine-binding globulin; Serpin A7; T4-binding globulin (SERPINA7, TBG)

Function

• TBG is one of the 3 major plasma thyroid hormone-binding proteins (prealbumin & albumin are the other 2
• the majority of thyroid hormone is bound by TBG (70% of T4 & 75-80% of T3)
• each TBG molecule binds one molecule of T3 or T4
• prealbumin binds 10% of T4 & 10% of T3

Structure

belongs to the serpin family

Compartment

secreted

Expression

• expressed by the liver (hepatocytes) & secreted into plasma

Pathology

• defects in SERPINA7 are a cause of thyroxine-binding globulin deficiency

Polymorphism

• two qualitative TBG variants occur in particular populations
  • TBG-A is found in 40% of Australian aborigines
    • it has reduced affinity for thyroxine & triiodothyroxine & increased susceptibility to inactivation by heat or acid
  • TBG-S ('s' for slow shift on isoelectic focusing) is found in blacks, Eskimos, Melanesians, Polynesians & Indonesians
    • not found in Caucasians
    • TBG-S is slightly more thermolabile

More general terms

• alpha 1 globulin
• glycoprotein

Additional terms

• protein disulfide isomerase (cellular thyroid hormone binding protein, prolyl 4-hydroxylase subunit beta, P4HB, ERBA2L, PDI, PDIA1, PO4DB)
• thyroxine-binding globulin in serum/plasma

Database

• Entrez gene: http://www.ncbi.nlm.nih.gov/sites/entrez?db=gene&cmd=Retrieve&dopt=Graphics&list_uids=6906
• Kegg: http://www.genome.jp/dbget-bin/www_bget?hsa:6906
• OMIM: https://mirror.omim.org/entry/314200
• UniProt: http://www.uniprot.org/uniprot/P05543.html