The thyroid gland develops as a pouch in the midline of the ventral aspect of the pharynx between the first and second pharyngeal pouches, towards the end of the fourth week of foetal development (Fig. 15.2). The pouch rapidly becomes bilobular, connected by a narrow hollow neck, the thyroglossal duct. The duct gradually becomes a solid stalk which then atrophies, but its pharyngeal connection remains as a pit, the foramen caecum, towards the back of the tongue in adults. The parathyroid glands develop from the third and fourth pharyngeal pouches, then move towards the posterolateral aspect of the thyroid where they are situated in adult life.
