Thyroid Embryology

The thyroid gland is the first of the body’s endocrine glands to develop, arising as an out-pouching of the primitive foregut around the third week of gestation (on approximately the 24th day). It originates at the base of the tongue at the foramen cecum.

• The thyroid originates from two main structures:

• The primitive pharynx and the neural crest.

The thyroid gland forms as a proliferation of endodermal epithelial cells on the median surface of the developing pharyngeal floor:

• The site of this development lies between 2 key structures, the tuberculum impar and the copula, and is known as the foramen cecum.

  • The thyroid gland initially arises caudal to the tuberculum impar, which is also known as the median tongue bud:

    • This embryonic swelling develops from the first pharyngeal arch and occurs midline on the floor of the developing pharynx, eventually helping form the tongue as the two lateral lingual swellings overgrow it.

• The foramen cecum begins rostral to the copula, also known as the hypobranchial eminence:

  • This median embryologic swelling consists of mesoderm that arises from the second pharyngeal pouch (although the third and fourth pouches are also involved):

    • The thyroid gland, therefore, originates from between the first and second pouches.

The paired lateral anlages originate from the fourth branchial pouch and fuse with the median thyroid anlage at approximately the fifth week of gestation:

• The lateral anlages are neuroectodermal (neural crest) in origen (ultimobranchial bodies) and produce the calcitonin producing parafolicullar or C cells, which thus come to lie in the superior posterior region of the gland.

  • The rudimentary lateral thyroid anlage develops from neural crest cells, while the median thyroid anlage, which forms the bulk of the gland, arises from the primitive pharynx.

The initial thyroid precursor, the thyroid primordium, starts as a simple midline thickening (endoderm) and develops to form the thyroid diverticulum.

• This structure is initially hollow, although it later solidifies and becomes bilobed.
• The stem usually has a lumen, the thyroglossal duct, that does not descend into the lateral lobes.
• The two lobes are located on either side of the midline and are connected via an isthmus

The thyroid follicular cells develop from the median thyroid anlage:

• The become apparent around the 8th week of gestation and start producing colloid around the 11th week of gestation.