Evolution of thyroid hormone distributor proteins in fish

In plasma, thyroid hormone (TH) is bound to several TH distributor proteins (THDPs), constituting a TH delivery/distribution network. Extensive studies of THDPs from tetrapods has proposed an evolutionary scenario concerning structural and functional changes in THDPs, especially for transthyretin (TTR). When assessing, in an evolutionary context, the roles of THDPs as a component constituting part of the vertebrate thyroid system, the data from fish THDPs are critical. In this review the phylogenetic distributions, spatiotemporal expression patterns and binding properties of THDPs in fish are described, and the question of whether the evolutionary hypotheses proposed in tetrapod THDPs can be applied to fish THDPs is assessed. The phylogenetic distributions of THDPs are highly variable among fish groups. Analysis in this review reveals that the evolutionary hypotheses proposed in tetrapod THDPs cannot be applied to fish THDPs, and that the role of plasma lipoproteins as THDPs grows in importance in fish groups. In primitive fish, zinc is an import factor in TH binding to TTR, and high zinc content may facilitate the acquisition of high TH binding activity during the early evolution of TTR. Finally, the possible roles of THDPs in the vertebrate thyroid system are discussed.