Anatomía Quirúrgica De Tiroides

Surgical Anatomy
o f t h e T h y ro i d a n d
P a r a t h y ro i d G l a n d s
Tanya Fancy, MD, Daniel Gallagher III,
Joshua D. Hornig, MD, FRSC(C)*

MD,

KEYWORDS
 Anatomy  Thyroid  Parathyroid  Embryology
 Recurrent laryngeal nerve  Superior laryngeal nerve

EMBRYOLOGY OF THE THYROID GLAND

During the fourth week of development, the foramen cecum develops as an endodermalthickening in the floor of the primitive pharynx at the junction between the first
and second pharyngeal pouches, immediately dorsal to the aortic sac. The medial
thyroid primordium derives as a ventral diverticulum at the foramen cecum. During
the fourth to seventh week of gestation, this primitive thyroid tissue penetrates the
underlying mesenchymal tissue and descends anterior to the hyoidbone and the
laryngeal cartilages to reach its final adult pretracheal position. During its descent, it
is first spherical, and then enlarges and becomes bilobed as it grows caudally. The
proximal portion of the diverticulum (connecting the gland and the foramen cecum)
retracts and forms a solid fibrous stalk early in the fifth week. This thyroglossal duct
ultimately atrophies, but any portionof it may persist to become the site of a thyroglossal duct cyst. The distal portion of this duct gives rise to the pyramidal lobe and levator
superioris thyroideae in adults.1 The lateral thyroid primordia (from the fourth and fifth
pharyngeal pouches) descend to join the central component during the fifth week of
gestation.
Calcitonin-secreting parafollicular C cells arise within theultimobranchial bodies
(recognized within the lateral thyroid primordia) from neural crest cells of the fourth
pharyngeal pouch. They fuse to the medial thyroid anlage during the fifth week of
gestation. These cells are therefore restricted to a zone deep within the middle to
upper third of the lateral lobes.
The thyroid primordium initially consists of a solid mass of endodermal cells, which
laterbreak up into a network of cords with the invasion of the surrounding mesenchyme.2 The epithelial cords organize into clusters of cells with a central lumen.

Department of Otolaryngology-Head & Neck Surgery, Medical University of South Carolina, 135
Rutledge Avenue, PO Box 250550, Charleston, SC 29425, USA
* Corresponding author.
E-mail address: hornigjd@musc.edu
Otolaryngol Clin N Am 43(2010) 221–227
doi:10.1016/j.otc.2010.01.001
0030-6665/10/$ – see front matter ª 2010 Elsevier Inc. All rights reserved.

oto.theclinics.com

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Follicles begin to appear at the beginning of the second month, and most follicles have
been formed by the end of the fourth prenatal month.1 Thereafter, additional growth is
by enlargement of existing follicles. By the end ofthe third month the gland begins to
function, and is able to concentrate radioiodine and synthesize iodothyronines.3
Thyroid Ectopia

During the course of its development, the gland (or parts of it) may fail to reach its definitive adult position. Ectopic thyroid tissue can be found at any level along the pathway of
its embryological descent. The entire gland may ascend with embryonic growthand lie
close to its point of origin at the foramen cecum, giving rise to a lingual thyroid. Lingual
thyroid masses have been found in as many as 10% of autopsies, although not all are
clinically relevant.2 Alternatively, the tissue may be sublingual or prelaryngeal in location, and often may be mistaken for a thyroglossal duct cyst. It is essential to determine
the presence or absence offunctional thyroid tissue at this ectopic location before
removal. About 70% of patients with lingual thyroid glands have no thyroid tissue in
the neck.3,4 In many cases the lingual thyroid does not function normally.4
In some patients the thyroid gland may be in its normal anatomic location but accessory ectopic tissue may also be present. Although this tissue may be functional, it is
often of...