Familial Euthyroid Hyperthyroxinemia Resulting from Increased Thyroxine Binding to Thyroxine-Binding Prealbumin

MORE than 99 per cent of the thyroxine (T4) in blood is normally bound to specific plasma T4-binding proteins, including T4-binding globulin (TBG), T4-binding prealbumin (TBPA), and albumin.1 However, it is generally agreed that the concentration of unbound or free T4 determines the level of biologically available T4. A free T4 index, which is calculated by multiplying the total serum T4 by the 3,5,3′-triiodothyronine (T3) resin uptake (an assessment of available TBG binding sites for T4 in serum), usually reflects biologically available T4 . . .