See Figure 16–1. The main anlage
of the thyroid gland develops as a median endodermal downgrowth
from the first and second pharyngeal pouches. During its migration caudally,
it contacts the ultimobranchial bodies developing from the fourth
pharyngeal pouches. When it reaches the position it occupies in
the adult, with the isthmus situated just below the cricoid cartilage,
the thyroid divides into two lobes. The site from which it originated persists
as the foramen cecum at the base of the tongue. The path the gland follows
may result in thyroglossal remnants (cysts) or ectopic thyroid tissue (lingual
thyroid). A pyramidal lobe is frequently present. Agenesis of one
thyroid lobe, almost always the left, may occur.
Thyroid anatomy. *The recurrent laryngeal nerve
runs in the tracheoesophageal groove on the left and has a slightly
more oblique course on the right before it enters the larynx just
posterior to the cricothyroid muscle at the level of the cricoid
The normal thyroid weighs 15–25 g and is attached to
the trachea by loose connective tissue. It is a highly vascularized organ
that derives its blood supply principally from the superior and
inferior thyroid arteries. A thyroid ima artery may also be present.
The function of the thyroid gland is to synthesize, store, and secrete
the hormones thyroxine (T4) and triiodothyronine (T3).
Iodide is absorbed from the gastrointestinal tract and actively
trapped by the acinar cells of the thyroid gland. It is then oxidized
and combined with tyrosine in thyroglobulin to form monoiodotyrosine
(MIT) and diiodotyrosine (DIT). These are coupled to form the active
hormones T4 and T3, which initially are stored
in the colloid of the gland. Following hydrolysis of the thyroglobulin,
T4 and T3 are secreted into the plasma, becoming almost
instantaneously bound to plasma proteins. Most T3 in euthyroid
individuals, however, is produced by extrathyroidal conversion of
T4 to T3.
The function of the thyroid gland is regulated by a feedback
mechanism that involves the hypothalamus and pituitary. Thyrotropin-releasing
factor (TRF), a tripeptide amide, is formed in the hypothalamus
and stimulates the release of the thyroid-stimulating hormone (TSH)
thyrotropin, a glycoprotein, from the pituitary. Thyrotropin binds
to TSH receptors on the thyroid plasma membrane, stimulating increased
adenylyl cyclase activity; this increases cyclic adenosine monophosphate
(cAMP) production and thyroid cellular function. Thyrotropin also stimulates
the phosphoinositide pathway and—along with cAMP—stimulates
De Felice M, Di Lauro R: Thyroid development and
its disorders: genetics and molecular mechanisms. Endocr Rev 2004;25:722.
In a patient with enlargement of the thyroid ...