The thyroid gland originates from endodermal cells on the midline pharyngeal floor between the 24th day of gestation. It initially develops caudal to the tuberculum impar from the 1st pharyngeal arch on the floor of the developing pharynx. The thyroid primordium starts as a simple midline thickening that forms the thyroid diverticulum. During descent, the thyroid gland forms its mature bilobed shape connected by an isthmus as it reaches its final location in the neck by the 7th gestational week. Parafollicular C cells are derived from the ultimobranchial body which fuses with the thyroid gland and disseminates its cells.

1. Thyroid EmbryologyThyroid Embryology
Stephanie JohnsonStephanie Johnson
PGY 2PGY 2

2. Initial embryologyInitial embryology
Thyroid:Thyroid:
 11stst
endocrine glandendocrine gland
 Day ~24th gestationDay ~24th gestation
 Proliferation ofProliferation of endodermalendodermal cells oncells on
midline pharyngeal floormidline pharyngeal floor
 Between 2 key structures:Between 2 key structures:
– tuberculum impartuberculum impar
– CopulaCopula
……called “called “foramen cecumforamen cecum””

4. Initial embryologyInitial embryology
Thyroid:Thyroid:
 Initially develops caudal to theInitially develops caudal to the
“tuberculum impar”“tuberculum impar” (median tongue bud)(median tongue bud)
 From the 1From the 1stst
pharyngeal archpharyngeal arch
 Midline on floor of developing pharynxMidline on floor of developing pharynx
 Eventually helps form the tongue as the 2Eventually helps form the tongue as the 2
lateral lingual swellings overgrow itlateral lingual swellings overgrow it

5. Initial embryologyInitial embryology
 Foramen cecum begins ant to the copula,Foramen cecum begins ant to the copula,
aka ‘aka ‘hypobranchial eminencehypobranchial eminence ’’
 Median embryologic swelling = mesodermMedian embryologic swelling = mesoderm
that arises from thethat arises from the secondsecond pharyngealpharyngeal
pouch (although the third and fourthpouch (although the third and fourth
pouches are also involved)pouches are also involved)
 SummarySummary - thyroid gland originates from- thyroid gland originates from
b/wb/w 11stst
and 2and 2ndnd
pharyngeal pouchespharyngeal pouches

6. Initial embryologyInitial embryology
““thyroid primordium”thyroid primordium”
 initial thyroid precursorinitial thyroid precursor
 starts as a simple midline thickeningstarts as a simple midline thickening
 forms thyroid diverticulumforms thyroid diverticulum
 initially hollowinitially hollow
 later solidifies and becomes bilobedlater solidifies and becomes bilobed

8. Descent of thyroid glandDescent of thyroid gland
 Initial descent starts ant to pharyngeal gutInitial descent starts ant to pharyngeal gut
 Thyroid still connected to the tongue via theThyroid still connected to the tongue via the
thyroglossal ductthyroglossal duct
 Later - tubular duct solidifies then obliteratesLater - tubular duct solidifies then obliterates
entirely (during gestational weeksentirely (during gestational weeks 7-107-10))
 Foramen cecumForamen cecum = opening of the= opening of the
thyroglossal duct into the tonguethyroglossal duct into the tongue

9. Descent of thyroid glandDescent of thyroid gland
Pyramidal lobe:Pyramidal lobe:
 in up to 50% peoplein up to 50% people
 persistence of the inferior end of thepersistence of the inferior end of the
thyroglossal duct (fx to obliterate)thyroglossal duct (fx to obliterate)
 may be attached to the hyoid bonemay be attached to the hyoid bone
(~thryoglossal duct cyst) OR may be(~thryoglossal duct cyst) OR may be
incorporatedincorporated intointo a thyroglossal ducta thyroglossal duct
cystcyst

10. Descent of thyroid glandDescent of thyroid gland
 Further descent – ant (ventral) to theFurther descent – ant (ventral) to the
hyoid bone and laryngeal cartilagehyoid bone and laryngeal cartilage
 DuringDuring descent – thyroid forms itsdescent – thyroid forms its
mature shape: median isthmusmature shape: median isthmus
connecting 2 lateral lobesconnecting 2 lateral lobes
 Descent COMPLETE at 7Descent COMPLETE at 7thth
gestationalgestational
weekweek

11. Parafollicular cellsParafollicular cells
 AkaAka C cellsC cells
 Secrete calcitonin (Ca regulation)Secrete calcitonin (Ca regulation)
 FromFrom ultimobranchial bodyultimobranchial body (last(last
structure derived from the branchialstructure derived from the branchial
pouches)pouches)
 Ultimobranchial body – fromUltimobranchial body – from 55thth
pharyngeal pouchpharyngeal pouch

12. Parafollicular cellsParafollicular cells
 Migrating cells from the neural crest regionMigrating cells from the neural crest region
infiltrateinfiltrate  ultimobranchial body (UBB)ultimobranchial body (UBB)
 This struct (neural crest + UBB) =This struct (neural crest + UBB) =
incorporatedincorporated  thyroid glandthyroid gland
 UBBUBB fusesfuses with the thyroid gland andwith the thyroid gland and
disseminatesdisseminates its cells into itits cells into it
 SummarySummary: the C cells of the thyroid are of: the C cells of the thyroid are of
neural crestneural crest origin.origin.

13. Clinical correlationsClinical correlations
1)1) Thyroglossal duct cystThyroglossal duct cyst
 duct fx to atrophyduct fx to atrophy
 can form a sinuscan form a sinus
 Rx – “Sistrunk” procedure (includes resection part ofRx – “Sistrunk” procedure (includes resection part of
hyoid bone)hyoid bone)
2) Ectopic thyroid2) Ectopic thyroid
 can occur anywhere along the path of descentcan occur anywhere along the path of descent
 most common at the base of the tongue ‘most common at the base of the tongue ‘linguallingual
thyroidthyroid’’

15. Clinical correlationsClinical correlations
3)3) Accessory thyroidAccessory thyroid
 from remnants of thyroglossal ductfrom remnants of thyroglossal duct
 may be functional but insufficient formay be functional but insufficient for
normal function of thyroidnormal function of thyroid