As the major fraction of the total thyroxine is bound to transport proteins (TBG, prealbumin, and albumin), the determination of total thyroxine only provides correct information when the thyroxine‑binding capacity in serum is normal.
The free thyroid hormones are in equilibrium with the hormones bound to the carrier proteins. A change in the TBG concentration can lead to elevated or lowered total T4 concentrations being measured although the free T4 concentration is in the euthyroid range.
The performance of a T‑uptake or TBC assay provides a measure of the available thyroxine‑binding sites.
