- Origin:
- The thyroid gland originates from a midline endodermal thickening in the floor of the primitive pharynx, at the site of the foramen cecum (between the first and second pharyngeal arches, at the base of the tongue).
- Begins to form around week 4 of development.
- Descent:
- The thyroid descends in the neck via the thyroglossal duct, a narrow epithelial tube.
- Migrates anterior to the hyoid bone (behind which it forms a recurrent loop) and laryngeal cartilages to its final position anterior to the trachea by week 7.
- The thyroglossal duct typically degenerates by week 10, leaving a small pit, the foramen cecum, as a remnant.
- Distal end, itself, may give rise to the Pyramidal Lobe
- Other remnants of the duct may persist as Accessory Thyroid Glands or Thyroglossal cyst
- Removal of Retrohyoid Duct Remnants usually requires excision of a segment from the middle of the Hyoid Bone
- Failure of descent of the Thyroglossal Duct may result in the development of a Lingual Thyroid
- Final location:
- Lies anterior and lateral to the upper trachea (typically 2nd–4th tracheal rings).
- Has two lobes connected by an isthmus.
- Thyroid follicular cells:
- Derived from endoderm of the primitive pharynx.
- These cells produce thyroid hormones (T3, T4).
- Parafollicular (C) cells:
- Derived from neural crest cells that migrate into the ultimobranchial bodies (derived from the 5th pharyngeal pouch, often considered part of the 4th pouch).
- C cells secrete calcitonin.