How are thyroid hormones transported in the blood?

  • <1% transported freely in plasma unbound to plasma proteins:
    • Forms the active portion of the hormone
  • Mostly transported by non-covalent binding to plasma proteins:
    • Thyroid binding globulin (TBG) – predominant transport protein
    • Transthyretin
    • Albumin
  • TBG has 10x the affinity for T4 than T3:
    • Therefore T3 is more readily available and is removed from the circulation