TY - CHAP M1 - Book, Section TI - The Thyroid A1 - Bansal, Nidhi A1 - Athanassaki, Ioanna A2 - Kline, Mark W. PY - 2018 T2 - Rudolph's Pediatrics, 23e AB - The thyroid gland originates from a median anlage and paired lateral anlagen during the third week of gestation, at a site that persists as the foramen cecum at the base of the tongue in adults. The median anlage is derived from the primitive pharyngeal floor and the lateral anlagen from the fourth pharyngeal pouches. The medial thyroid anlage descends in the neck anterior to structures that form the hyoid bone and larynx. During its descent, the anlage remains connected to the foramen cecum via an epithelial-lined tube known as the thyroglossal duct. The lateral anlagen fuse with the median anlage at approximately the fifth week of gestation. It was previously believed that the median anlage is the source of origin of the follicular cells and the lateral anlagen is the source of calcitonin-producing parafollicular cells. This notion has been challenged by the demonstration of calcitonin-producing cells in ectopic lingual thyroid and the histologic observation of thyroid follicular cells differentiated within the lateral anlagen. Thyroid follicles are initially apparent by 8 weeks, and colloid formation begins by the 11th week of gestation. The thyroid forms bilateral lobes connected by an isthmus in the middle, typically just below the cricoid cartilage. In about 50% of individuals, there is a pyramidal lobe in the midline that represents the most caudal end of the thyroglossal duct. Persistence of the thyroglossal duct results in formation of a thyroglossal cyst. Lack of descent leads to a lingual thyroid. SN - PB - McGraw-Hill Education CY - New York, NY Y2 - 2024/04/19 UR - accesspediatrics.mhmedical.com/content.aspx?aid=1182919053 ER -