The thyroid gland, in adult vertebrates, is made up of several cell types that are derived from
all three germ layers. The most abundant cellular population in the gland, which is also the
only one of endodermal origin, is composed by the thyroid follicular cells (TFCs) that
surround the hollow thyroid follicles. The synthesis, and the controlled release into the
bloodstream of thyroid hormones occurs within the follicles and it is made possible by a
variety of proteins which are syn-