Thyroid

Staci Bryson, MD

Larissa V. Furtado, MD

Key Facts

Embryology

Endodermal thyroid diverticulum forms on floor of pharynx/base of tongue opposite the 1st and 2nd pharyngeal pouches during weeks 3 and 4
Bilobed, solid thyroid diverticulum forms in close proximity to aortic sac and mirrors its caudal descent during weeks 4 and 5
Stalk of thyroid diverticulum forms thyroglossal duct, which breaks down by end of week 5
By week 5, thyroid has distinct left and right lobes connected by isthmus
Thyroid continues to descend in neck to its final position just inferior to cricoid cartilage by week 7
Ultimobranchial body (4th pharyngeal pouch) fuses with thyroid and gives rise to parafollicular “C” cells (neural crest derived)
Vascular mesoderm invades endoderm of thyroid diverticulum to form plates
- Plates form cells grouped around a lumen (follicle unit) by week 10
Thyroid gland grows rapidly during first 16-18 weeks of gestation
Colloid production begins between weeks 10 and 12; achieves mature appearance by week 20
Thyroxine secreted by week 18

Microscopic Anatomy

Thin fibrous capsule delimits thyroid
Thyroid follicles are made up of a single layer of cuboidal to low columnar epithelium with round, basally oriented nuclei around a central lumen containing colloid
Parafollicular or “C” cells have pale cytoplasm and oval nuclei; not readily identified in hematoxylin and eosin-stained sections