SN3 SHORT NOTE 5M ★ High-yield

Describe the development of the thyroid gland. Add a note on thyroglossal cyst and ectopic thyroid.

Anatomy · 1st Professional — Pre-Clinical

The parafollicular C cells have a different origin — from the ultimobranchial body of the fourth pouch, and ultimately from the neural crest
The thyroid is the first endocrine gland to develop
It descends further than any other pharyngeal derivative, which is why its remnants can be found anywhere from the tongue to the mediastinum

Feature	Detail
Origin	Persistence of part of the thyroglossal duct
Site	Midline (or just off it); most often subhyoid; may lie anywhere from the foramen caecum to the isthmus
Age	Usually presents in childhood or early adult life
Characteristic sign	Moves upward on protruding the tongue and on swallowing
Complication	Infection; a persistent thyroglossal fistula after incision and drainage
Treatment	Sistrunk operation — excision of the cyst with the tract and the central part of the hyoid bone

Lingual thyroid — complete failure of descent; a mass at the base of the tongue causing dysphagia or a muffled voice. In up to 70% it is the only functioning thyroid tissue, so it must never be excised without a radionuclide scan to confirm that a normal gland exists
Ectopic thyroid anywhere along the tract — suprahyoid, infrahyoid, intratracheal, or in the mediastinum
Excessive descent gives a retrosternal goitre
Levator glandulae thyroideae — a fibromuscular band from the pyramidal lobe to the hyoid, a remnant of the duct
Agenesis or dysgenesis is the commonest cause of congenital hypothyroidism

Structure	Pouch	Note
Inferior parathyroid	Third (dorsal)	Descends with the thymus, so it passes the fourth-pouch gland; may end in the mediastinum
Thymus	Third (ventral)	Descends into the superior mediastinum
Superior parathyroid	Fourth (dorsal)	Descends little, so it ends up above the third-pouch gland
Parafollicular C cells	Fourth (ventral) — ultimobranchial body	Neural crest in ultimate origin

Congenital hypothyroidism — screened for by heel-prick TSH; treatment within the first two weeks prevents the irreversible mental retardation of cretinism. One of the most cost-effective screening programmes there is
A midline neck swelling in a child is a thyroglossal cyst until proved otherwise
Ectopic parathyroid tissue explains failure of neck exploration in hyperparathyroidism; the mediastinum must then be searched
Thyroglossal duct carcinoma is rare but recognised, usually papillary
DiGeorge syndrome — third and fourth pouch failure gives absent thymus and parathyroids together

All questions in Systemic Embryology