- <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