The adult thyroid gland in mammals is assembled from two different embryologic structures. This composite origin reflects the dual endocrine function of the gland. The thyroglobulin-producing follicular cells derive from a small group of endodermal cells of the primitive pharynx (the thyroid anlage). The calcitonin-producing parafollicular cells are neural crest–derived cells from the ultimobranchial bodies. The ultimobranchial bodies are transient embryonic structures originated from the fourth pharyngeal pouch. The thyroid anlage and the ultimobranchial bodies migrate from their original sites, reach their final position anterior to the trachea, and fuse to form the definitive thyroid gland. The thyroid follicles derive from the thyroid anlage cells, while the C cells are scattered within the interfollicular space. After this early ontogenetic phase, the thyroid begins to function at a basal level; subsequent differentiation of the hypothalamic nuclei …