Thyroid diverticulum arises from floor of primitive pharynx and descends into neck. - Connected to tongue by thyroglossal duct, which normally disappears but may persist as cysts or the pyramidal lobe of thyroid. - Foramen cecum is normal remnant of thyroglossal duct.

Most common ectopic thyroid tissue site is the tongue (lingual thyroid). Removal may result in hypothyroidism if it is the only thyroid tissue present.

Thyroglossal duct cyst presents as an anterior midline neck mass that moves with swallowing or protrusion of the tongue (vs persistent cervical sinus leading to brachial cleft cyst in lateral neck).

Thyroid follicular cells are derived from endoderm. Parafollicular cells (aka, C cells, produce calcitonin) are derived from neural crest.