ABSTRACT
The thyroid anlage arises as a bilateral vesicular tissue in the foramen cecum of the tongue. It is visible by day 17 of fetal life as an endodermal structure in the pharynx in close association with the heart (1,2). The developing thyroid descends as part of the thyroglossal duct to the neck. After the thyroglossal duct atrophies, the thyroid anlage expands laterally. Between 9 and 12 weeks of development, follicle formation is ongoing, and colloid production can be detected starting around 12 weeks. Well-developed colloid follicles can be seen by 14 weeks of gestation (1,2).
