Pathology of the Thyroid Gland

The thyroid gland develops from the larger median analage and the two lateral analagen. The medial analage, which forms the major portion of the thyroid, is derived from the floor of the foregut and the two lateral analagen are derived from the endoderm of the fourth and the fifth branchial pouches as the ultimobranchial bodies. The medial analage appears by d 24 as median endodermal diverticulum from the base of the tongue in the region of foramen cecum. The diverticulum descends down from the foramen cecum into the neck along the midline attached to the thyroglossal duct. It reaches its final position anterior to the trachea by about 7 wk; it then grows laterally, and becomes bilobed [1]. Early during the fifth week, the thyroglossal duct loses its lumen and shortly afterwards breaks into fragments [2]. However, the caudal end of the thyroglossal duct may persist in some embryos, and this constitutes the pyramidal process, which is present in about 75% of mature human thyroids [3]. The lateral thyroid analage becomes attached to the posterior surface of the thyroid during the fifth week and contributes up to 30% to the thyroid weight [2]. The causes of the fusion of the lateral and medial analage are unknown [2]. It is speculated that migration of the ultimobranchial body controls the growth of the medial analage, or that the growth of the medial analage laterally and caudally inhibits expansion of the ultimobranchial body [2]. The lateral thyroid analage is thought to give rise to the calcitonin producing C cells and the solid cell nests. It is believed that the C cells are derived from the neural crest; they migrate to the ultimobranchial body and are subsequently incorporated into the thyroid [4]. However, the existence of mixed follicular and C-cell tumors raises the possibility of the common stem cell origin for both follicular and C cells as is seen in the gastrointestinal tract [1].