<p>Circulating thyroid hormones are greater than 99% bound to plasma proteins, including thyroxine-binding globulin (TBG), thyroxine-binding prealbumin (TBPA) and albumin (TBA). The higher affinity of both TBG and TBPA for T4 partially explains the higher serum levels, slower metabolic clearance and longer half-life of T4 compared to T3. Protein-bound thyroid hormones exist in reverse equilibrium with small amounts of free hormone where only unbound hormone is metabolically active.<sup class="text-reference-group"><a class="reference-popover-link" data-content="American Association of Clinical Endocrinologists (AACE): Clinical Practice Guidelines for Hypothyroidism in Adults" href="#reference-F4636">15</a></sup></p>