Thyroid and Parathyroid

Innervation of the gland is by sympathetic fibers from the superior and middle
cervical sympathetic ganglia. The fibers enter with the blood vessels and are
vasomotor in action. Parasympathetic fibers are derived from the vagus nerve and
. reach the gland via branches of the laryngeal nerves
Microscopically, the thyroid is divided into lobules that contain 20 to 40 follicles.
There are roughly 3 × 106follicles in the adult male thyroid gland. The follicles are
spherical and average 30 mm in diameter. Each follicle is lined by cuboidal epithelial
cells and contains a central store of colloid secreted from the epithelial cells under the
influence of the pituitary hormone, thyroid stimulating hormone (TSH). The second
group of thyroid secretory cells are the C cells or parafollicular cells, which contain
and secrete the hormone calcitonin. They are found as individual cells or clumped in
small groups in the interfollicular stroma, abutting between follicular cells. They are
located in the upper poles of the thyroid lobes, reflecting their origin as
neuroectodermal cells derived from the ultimobranchial bodies, and are part of the
. amine containing precursor uptake decarboxylase (APUD) series described by Pearse
Laryngeal Nerves
It is important to note the close relationship of the thyroid gland to the recurrent
laryngeal nerves and the possible variations in the course of the recurrent nerves. The
recurrent laryngeal nerves supply the intrinsic muscles of the larynx, and damage to
one of them leads to ipsilateral vocal cord paralysis. Similarly, the external branch of
the superior laryngeal nerve, which innervates the cricothyroid muscle, also is at risk
during thyroid surgery. Damage of either nerve may result in a disability of
. phonation
Identification of the nerves, rather than attempting to avoid them, should be standard
practice for the surgeon. The recurrent laryngeal nerves originate from the vagus
nerves. On the right side, the recurrent nerve originates where the vagus nerve crosses
the first part of the subclavian artery; the nerve loops under the subclavian artery and
ascends slightly obliquely to enter the larynx at the level of the cricoid cartilage and
posterior to the cricothyroid muscle. The left recurrent nerve branches from the vagus
as it crosses the aortic arch and loops posteriorly around the ligamentum arteriosus
before it ascends medially in the tracheoesophageal groove to enter the larynx
opposite the contralateral nerve. The variable course taken by the recurrent nerves is
demonstrated in Fig. 36-7. The right recurrent nerve is in the tracheoesophageal
groove in 64 percent of people, compared to 77 percent on the left. The nerve is
lateral to the trachea on the right in 28 percent of people and in 17 percent on the left.
In a minority of people the nerve is anterolateral to the trachea (right 8 percent, left 6
percent), exposing it to accidental division during subtotal lobectomy. A
misconception is that the recurrent laryngeal nerves run behind the inferior thyroid
artery, but this is true in only 53 percent of people on the right and 69 percent on the
left. In others the nerve's course is anterior to the artery (right 37 percent, left 24
percent), or between branches of the artery (right 7 percent, left 6 percent). Failure to
. identify the course of the nerves can lead to accidental damage
The recurrent laryngeal nerves are not always recurrent; in about 1 percent of people
one of the nerves is nonrecurrent. This occurs almost exclusively on the right in
association with a vascular anomaly of the right subclavian artery; rarely, it occurs on
the left with dextrocardia or situs inversus. In these situations, the nerve arises from