Fig. 1. Normal thyroid development.

A. Thyroid develops from median endodermal thickening in floor of primordial pharynx, between first and second pharyngeal arches. Thickening rapidly leads to formation of small outpouching referred to as thyroid primordium. B. Thyroid primordium is initially hollow but quickly becomes solid and divides into right and left lobes. C. Developing thyroid gland descends in neck. Thyroid gland is connected for short time to tongue by narrow tube (i.e., thyroglossal duct). By seventh week it assumes definitive shape and reaches its final site within neck.