Transport of Iodothyronines

Abstract

In human serum T4, T3, and rT3 are bound to three proteins, which in the order of their affinity for T4 (or T3 or rT3) are thyroxine binding globulin (TBG), thyroxine binding prealbumin (TBPA) and albumin. 3,3′-T2, 3′,5′-T2 and 3′-T1are bound mainly to albumin (Figs. 2.1 and 2.2) whereas tetraiodothyroacetic acid (TETRAC) binds avidly to TBPA in addition to albumin. There is at present little information on the nature of serum binding of 3,5-T2, 3-T1, or triiodothyroacetic acid (TRIAC). Table 2.1 presents some physical, chemical, and iodothyronine-binding characteristics of circulating iodothyronine-binding proteins in man. These proteins also bind thyroid hormones (T4 and T3) in the serum of subhuman primates and in horses. However, in calves, sheep, pigs, goats, dogs, and rabbits, albumin and TBG or a protein that migrates in the vicinity of TBG during electrophoresis are present, but there is little or no TBPA. On the other hand, pigeons, chickens, and kangaroos have a TBPA-like protein in addition to albumin, but they do not have TBG. Guinea pigs and rats appear to have neither TBG nor TBPA, and in these species thyroid hormones circulate bound to albumin only. (Farer et al. 1962; Refetoff et al 1970.)