Chicken and zebrafish are two model species regularly used to study the role of thyroid hormones in vertebrate development. Similar to mammals, chickens have one thyroid hormone receptor
α(TR α) and one TR βgene, giving rise to three TR isoforms: TR α, TR β2, and TR β0, the latter with a very short amino-terminal domain. Zebrafish also have one TR βgene, providing two TR β1 variants. The zebrafish TR αgene has been duplicated, and at least three TR αisoforms are expressed: TR αA1-2 and TR αB are very similar, while TR αA1 has a longer carboxy-terminal ligand-binding domain. All these TR isoforms appear to be functional, ligand-binding receptors. As in other vertebrates, the different chicken and zebrafish TR isoforms have a divergent spatiotemporal expression pattern, suggesting that they also have distinct functions. Several isoforms are expressed from the very first stages of embryonic development and early chicken and zebrafish embryos respond to thyroid hormone treatment with changes in gene expression. Future studies in knockdown and mutant animals should allow us to link the different TR isoforms to specific processes in embryonic development.