of the Thyroid
The thyroid gland is a highly vascularized organ
located anteriorly in the neck, deep to the platysma, sternothyroid and
sternohyoid muscles, and extending from the 5th cervical (C5) to the 1st
thoracic (T1) vertebrae. The gland consists of two lobes (left and right)
connected by a thin, median isthmus overlying the 2nd to 4th tracheal rings,
typically forming an "H" or "U" shape. Occasionally the isthmus is absent
and the thyroid exists as two distinct lobes. Embryologically, the thyroid
gland develops as a thickening in the pharyngeal floor that elongates inferiorly
as the thyroglossal duct, dividing into two lobes as it descends through the
Beneath the visceral layer of the pretracheal,
deep cervical fascia, the thyroid gland is surrounded by a true inner capsule,
which is thin and adheres closely to the gland. The capsule sends
projections into the thyroid forming septae and dividing it into lobes and
lobules. Dense connective tissue attachments secure the capsule of the
thyroid to both the cricoid cartilage and the superior tracheal rings.
The lobules of the gland are composed of
follicles, the structural unit of the thyroid. Each follicle is lined by a
simple layer of epithelium surrounding a colloid-filled core. This colloid
contains iodothyroglobulin, the precursor to thyroid hormones.
Blood Supply and Nerves
Because the thyroid gland is a hormone secreting
organ, it is highly vascularized. It receives its blood supply from the
inferior thyroid arteries. These arteries lie between the fibrous
capsule and the pretracheal layer of deep cervical fascia.
The superior thyroid artery is the first branch
of the external carotid artery and supplies the top half of the thyroid gland.
It divides into anterior and posterior branches supplying respective sides of
the thyroid. On the anterior side, the right and left branches anastomose
with each other. On the posterior side, the right and left branches
anastomose with their respective inferior thyroid arteries.
The inferior thyroid artery supplies the lower
half of the thyroid and is the major branch of the thyrocervical trunk, which
comes off the subclavian artery. It too divides into several branches,
supplying the inferior portion of the thyroid and anastomosing posteriorly with
the superior thyroid branches.
There are three main veins that drain the venous
plexus on the anterior surface of the thyroid. They include the
superior, middle, and
inferior thyroid veins, and each drains its respective portion of the
thyroid. The superior and middle thyroid veins drain into the internal
jugular veins, whereas the inferior thyroid vein drains into the brachiocephalic
veins, behind the manubrium of the sternum.
Lymphatic drainage of the thyroid gland is quite
extensive and flows multidirectionally. Immediate drainage flows first to
the periglandular nodes, then to the prelaryngeal (Delphian),
paratracheal nodes along the recurrent laryngeal nerve, and then to
mediastinal lymph nodes.
The principal innervation of the thyroid gland
is derived from the
superior, middle, and inferior cervical sympathetic ganglia of the
autonomic nervous system and parasympathetic fibers from the vagus
nerves. These nerves reach the thyroid gland by coursing with the
blood vessels (superior and inferior thyroid periarterial plexuses).
||Superior and inferior
||Superior, middle, and
inferior thyroid veins
prelaryngeal, pretracheal, and paratracheal lymph nodes
middle, and inferior sympathetic ganglia
Cephalad to the superior pole of
the thyroid gland, the external branch of the superior laryngeal nerve
runs alongside the superior thyroid artery before turning medially to supply the
cricothyroid muscle. High ligation of the superior thyroid artery during
thyroidectomy places this nerve at risk of inadvertent injury, which would
produce dysphonia by altering pitch regulation.
The cricothyroid artery
is a potentially bothersome branch of the superior thyroid artery, which runs
cephalad to the upper pole of the thyroid gland and runs toward the midline on
the cricothyroid ligament. This vessel can be lacerated during emergent
The inferior thyroid artery is closely
associated with the recurrent laryngeal nerve. This nerve can be
found after it emerges from the superior thoracic outlet, in a triangle bounded
laterally by the common carotid artery, medially by the trachea, and superiorly
by the thyroid lobe. The relationship of the recurrent laryngeal nerve and
the inferior thyroid artery is highly variable in that the nerve can lie deep to
the artery, superficial to the artery, or between the branches of the artery,
and be different on either side of the neck. Consideration of this nerve
and its branches must be given during dissection and thyroidectomy.