A study on the structure of the thyroid gland of 288 vertebrate species, conducted both on 99 dissections and analysis of the literature, leads to propose the following interpretations: i) the presence of this gland is a synapomorphy of Vertebrata; ii) a compact gland is a synapomorphy of Gnathostomes; iii) a diffuse gland is a possible synapomorphy of Halecostomes; iv) several independent cases of concentration in a few lobes in several groups, like Scaridae, Scombridae and some Carangimorphariae occurred and even a compact gland in Osteoglossomorphes. Moreover, the scattering of thyroidian islets in other organs (cephalic kidney, choroid,\ldots) has occurred several times in teleost lineages and a compact thyroid gland embedded in a blood sinus appears to be a probable synapomorphy of Tetraodontiformes and Lophiiformes.