RT Book, Section
A1 Bansal, Nidhi
A1 Athanassaki, Ioanna
A2 Kline, Mark W.
SR Print(0)
ID 1182919053
T1 The Thyroid
T2 Rudolph's Pediatrics, 23e
YR 2018
FD 2018
PB McGraw-Hill Education
PP New York, NY
SN 9781259588594
LK accesspediatrics.mhmedical.com/content.aspx?aid=1182919053
RD 2024/04/19
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.