The thyroid gland develops from the endodermal lining of the floor of the primitive pharynx
The thyroid gland begins to develop during the fourth week of gestation from a primordium originating as an endodermal thickening of the floor of the primitive pharynx. The primordium grows caudally and forms a ductlike invagination known as the thyroglossal duct. The thyroglossal duct descends through the tissue of the neck to its final destination in front of the trachea, where it divides into two lobes. During this downward migration, the thyroglossal ductundergoes atrophy, leaving an embryologic remnant, the pyramidal lobe of the thyroid, which is present in about 40% of the population. About the ninth week of gestation, endodermal cells differentiate into plates of follicular cells that become arranged into follicles. By week 14, well-developed follicles lined by the follicular cells contain colloid in their lumen. During week 7, epithelial cells lining the invagination of the fourth branchial pouches(sometimes called the fifth branchial pouches) known as the ultimobranchial bodies start their migration toward the developing thyroid gland and become incorporated into the lateral lobes. After fusing with the thyroid, ultimobranchial body cells disperse among the follicles, giving rise to parafollicular cells that become incorporated into the follicular epithelium.