The Thyroid Gland

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The
larynx | Main Anatomy
Index | Neck Index
Last updated 1 April 2006
The Thyroid Gland
- The thyroid gland consists of left and right lobes that are united by a
narrow isthmus that
extends across the trachea (anterior to the 2nd
and 3rd tracheal cartilages).
- It is the largest of all endocrine glands and clasps the
anterior and lateral surfaces of the pharynx, larynx,
oesophagus and trachea like a shield.
- Each pear-shaped lobe extends inferior on each side of
the trachea, often to the level of the sixth tracheal
cartilage posteriorly, and extends along the sides of the
oesophagus.
- A conical pyramidal lobe
is present in some people, which ascends from the isthmus
of the thyroid gland, usually from the left side, toward
the hyoid bone is
present in some people.
- A thin fibrous capsule
of connective tissue surrounds the thyroid gland.
- External to this, there is a false capsule formed a
sheath of pretracheal
fascia.
Arterial Supply of the Thyroid Gland (p. 817)
- This gland receives its blood from two large arteries:
the superior and inferior thyroid arteries.
- These vessels lie between the capsule and pretracheal
fascia (false capsule).
- All the thyroid arteries anastomose with each other on or
in the substance of the thyroid gland.
- There is little anastomosis across the median plane,
however, expect for branches of the superior thyroid
artery.
The
Superior Thyroid Artery (p. 817)
- This is the first branch of the external
carotid artery.
- It descends to the superior pole of the gland, pierces
the pretracheal fascia, and then divides into two or
three branches.
The
Inferior Thyroid Artery (pp. 817, 820)
- This is a branch of the thyrocervical
trunk.
- It runs superomedially posterior to
the carotid sheath to reach the posterior
aspect of the gland.
- It divides into several branches, which pierce the
pretracheal fascia to supply the inferior pole of the
thyroid gland.
- The inferior thyroid artery has an intimate relationship
with the recurrent laryngeal nerve.
Venous Drainage of the Thyroid Gland (p. 820)
- Usually there are three pairs of veins that drain the
venous plexus on the anterior surface of the gland.
- The superior thyroid veins drain
its superior poles.
- The middle thyroid veins
drain its lateral parts.
- The inferior thyroid veins
drain its inferior poles.
- The superior and middle
thyroid veins empty into the internal
jugular veins.
- The inferior thyroid
veins drain into the brachiocephalic
veins. They often unite to form a single vein
that drains into one of the brachiocephalic veins.
Lymphatic Drainage of the Thyroid Gland (p.
820)
- Lymph vessels run in the interlobular connective tissue,
often around the arteries.
- They communicate with the capsular network of the lymph
vessels.
- From here, vessels pass to the prelaryngeal
lymph nodes and to the pretracheal
and paratracheal lymph nodes.
- Laterally, lymph vessels located along the superior
thyroid veins pass to the inferior deep
cervical lymph nodes.
- Some lymph vessels may drain into the brachiocephalic
lymph nodes or empty directly into the thoracic duct.
Nerve
Supply of the Thyroid Gland (p. 820)
- These nerves are derived from the superior, middle and
inferior cervical sympathetic
ganglia.
- They reach the thyroid gland through the cardiac and laryngeal
branches of the vagus
nerve, which runs along the arteries supplying
the gland.
- These postganglionic fibres are vasomotor and indirectly
affect the gland through its blood supply.
