Summary of thyroid gland Gross anatomy histology embryology and clinical anatomy
Thyroid gland, the largest
endocrine gland of our body which is located in front of trachea at the lower
part of neck.
It lobes are extends from the oblique line of
thyroid cartilage to the 5th-6th tracheal cartilage ring , isthmus extends from
2nd to 4th tracheal ring
Vertebral level : it lies
opposite to the C5, C6, C7 and T1 vertebrae
Parts
of thyroid gland
Thyroid gland is consists of two lobes connected by isthmus.
A small pyramidal lobe may
present occasionally which extend
upwards from the upper border of the isthmus, slightly to the left of the
midline.
Sometimes this pyramidal lobe is connected to the hyoid bone
by a strong fibromuscular band called levator glandulae thyroideae (remnant of
thyroglossal duct).
Dimensions of thyroid gland:
Thyroid gland weighs approximately 25 gms. Each lobe of thyroid gland is 5 cm
long, 3 cm wide and 3cm thick.
Capsule
of thyroid gland
Thyroid gland is enclosed by two
capsules
The inner capsule is also known
as true capsule which is formed by
condensation of fibrous stroma of the gland.
An outer capsule is known as false
capsule which is formed by the pre-tracheal layer of the deep cervical fascia.
It is thin along the posterior border of the lobes and thick on the medial
surface of the lobes.
The pre-tracheal fascia is
attached above to the body of the hyoid bone
and oblique line of the thyroid cartilage and after enclosing the thyroid gland it descends
down in front of the trachea and blends with fibrous pericardium.
On the medial surface of thyroid
lobe, this capsule thickens to form the suspensory ligament of Berry, which
attaches the thyroid lobes to the cricoid cartilage.
Gross
features and relation of thyroid gland
Each lobe of thyroid gland has
following relations:
Apex/Upper lobe: it is sandwiched between the sternothyroid and inferior constrictor muscle of pharynx.
Superior thyroid artery, branch of external carotid artery and external
laryngeal nerve diverge from each other close to upper pole. The artery is superficial
and nerve lies deep to the upper pole.
Base/Lower pole: it is related to
inferior thyroid artery and recurrent laryngeal nerve.
Lateral (superficial) surface is
related to the following muscles:
Sternohyoid, sternothyroid,
superior belly of omohyoid and anterior border of sternocleidomastoid.
Medial surface is related to:
Two tubes– trachea and oesophagus
Two muscles – inferior
constrictor and cricothyroid
Two cartilages – cricoid and
thyroid
Two nerves – external laryngeal
and recurrent laryngeal
Posterolateral surface is related
to:
Carotid sheath and its contents
(common carotid artery, internal jugular vein and vagus nerve). Ansa cervicalis
is embedded in the anterior aspect of carotid sheath.
Anterior border is thin and is
related to anterior branch of superior thyroid artery.
Posterior border is thick and
rounded and is related to parathyroid glands and anastomosis between superior
and inferior thyroid arteries.
Isthmus of thyroid gland has
following relations:
Anterior surface is related to sternothyroid and sternohyoid
muscles and anterior jugular veins.
Posterior surface is related to
2nd -4th tracheal rings.
Superior border is related to anastomosis between anterior
branches of superior thyroid arteries.
Inferior border is related to
inferior thyroid veins that leave
isthmus along this border.
Blood
supply of thyroid gland
Thyroid gland is highly vascular and is supplied by
the following arteries.
Superior thyroid artery:
It is a branch of external carotid artery, which descends
down to the upper pole of lateral lobe of thyroid gland with external laryngeal
nerve (the nerve diverges from the artery near the upper pole).
At the apex of lateral lobe, it divides into
anterior and posterior branches.
Anterior branch descends along anterior border and
anastomoses with the its fellow of opposite side along the superior border of
isthmus.
Posterior branch descends along posterior border and
anastomoses with the ascending branch of inferior thyroid artery.
It supplies upper 2/3rd of the lobe and upper ½ of
the isthmus.
Inferior thyroid artery:
It is a branch of thyrocervical trunk.
It first runs upwards, then passes behind the
carotid sheath and descends to reach the base of thyroid gland (forms a loop
with convexity upwards).
Near the base (lower pole) the artery is very close
to recurrent laryngeal nerve.
It supplies the lower 1/3rd of the lobe and lower
half of the isthmus.
Thyroideaima artery: It is not always present. It is
a branch of brachiocephalic trunk or arch of aorta. Ascends in front of trachea
to enter the isthmus.
Venous drainage of thyroid gland
It occurs mainly via 3 pairs of veins. All thyroid
veins do not accompany arteries. The three pairs of veins arise from the venous
plexus present deep to the true capsule of thyroid gland.
Superior thyroid veins emerge at the upper pole of
thyroid gland and terminates in internal jugular vein.
Middle thyroid veins are short veins, emerges at the
middle of the lobe of thyroid gland and drains in internal jugular vein.
Inferior thyroid veins emerge at the lower border of
isthmus, descend in front of trachea (right and left vein communicate and form
plexus in front of trachea) to drain into left brachiocephalic vein.
Vein of Kocher: Sometimes a fourth vein (Vein of
Kocher) emerges between the middle and inferior thyroid veins which drains into
internal jugular vein.
Lymphatic drainage of thyroid gland
Lymphatics from the upper part of thyroid gland
drain into pre-laryngeal and upper deep cervical lymph nodes. From the lower
part of thyroid gland, the lymphatics drain into pretracheal, paratracheal and lower
deep cervical lymph nodes.
Development of thyroid gland
It is the
first of the body's endocrine glands to develop, on approximately the 24th day
of intra uterine life . it is originates from two source : the primitive
pharynx and the neural crest.
Stroma and follicular cells are developed from
primitive pharynx and parafollicular cell develops from neural crest cells.
The thyroid gland forms as a proliferation of
endodermal epithelial cells on the median surface of the developing pharyngeal
floor. The site of this development lies between 2 structures which developed
tongue, the tuberculum impar and the copula, and it is known as the foramen
cecum.
This embryonic swelling develops from the first
pharyngeal arch and occurs midline on the floor of the developing pharynx,
eventually helping form the tongue as the two lateral lingual swellings
overgrow it.
The foramen cecum begins rostral to the copula, also
known as the hypobranchial eminence. This median embryologic swelling consists
of mesoderm that arises from the second pharyngeal pouch (although the third and
fourth pouches are also involved). The thyroid gland, therefore, originates
from between the first and second pouches.
The initial thyroid precursor, the thyroid
primordium, starts as a simple midline thickening and develops to form the
thyroid diverticulum. This structure is initially hollow, although it later
solidifies and becomes bilobed. The stem usually has a lumen, the thyroglossal
duct, that does not descend into the lateral lobes. The two lobes are located on either side of the
midline and are connected via an isthmus
Histology of Thyroid gland
1.
Thyroid follicle is lined by simple cuboidal
epithelium
2.
Thyroid follicles are filled with colloid.
3.
Thyroid follicle is formed by Follicular cells and parafollicular cells
Clinical anatomy of
thyroid gland
Why
during deglutition or swallowing, thyroid swellings rise and fall.
Once the thyroid gland is encased, the
pretracheal fascia connects to the hyoid bone's body and the thyroid
cartilage's oblique lines. It thickens on the thyroid lobe's medial surface to
produce the Berry suspensory ligament, which joins the thyroid gland's lobe to
the cricoid cartilage. Consequently, during deglutition, when the larynx and
pharynx are lifted due to the contraction of the pharynx's longitudinal muscles
linked to the posterior border of the thyroid cartilage, the thyroid gland also
moves up and down. This aids in separating the enlargement of the thyroid from
other neck swellings.
Why
thyroid gland is removed along with its true capsule?
The venous plexus is situated deep
into the true capsule in the case of the thyroid gland. Thus, during a
thyroidectomy, the thyroid gland and true capsule are removed in order to
prevent bleeding. However, in the case of the prostate gland, the venous plexus
is between the genuine and false capsules; as a result, when the prostate gland
is surgically removed (prostatectomy), both capsules are left behind.
Why
superior thyroid artery is ligated close to thyroid gland?
The superior thyroid artery is
located in close proximity to the proximal portion of the external laryngeal
nerve, and it diverges close to the lateral lobe apex (the nerve is deep to the
apex, while the artery is superficial). To prevent damage to the external
laryngeal nerve, the superior thyroid artery during a thyroidectomy should be
clamped near the apex.
Why
Inferior thyroid artery is ligated away from the thyroid gland.
Near the base or lower pole of
the thyroid gland, the inferior thyroid artery is located extremely close to
the recurrent laryngeal nerve. To prevent damage to the recurrent laryngeal
nerve, the thyroidectomy site should be ligated away from the thyroid gland's inferior
pole.
Goitre
"Goitre" is the term
used to describe thyroid gland hypertrophy. Iodine shortage in the diet (lower
levels of T3 and T4) causes endemic goiter. These goiters exhibit
hypothyroidism symptoms and indicators.Excess T3 and T4 secretion causes toxic
goiters, also known as thyrotoxicosis, which manifests clinically as elevated
BMR, tremors, and tachycardia. Thyroid enlargement often grows either downward
or backward. Individuals with big goitres exhibit the typical signs and
symptoms of
Dysphonia, or voice hoarseness
Dysnoea, or trouble breathing
Dysphagia, or trouble swallowing
Retinopathy and Lymphoedema
Thyroid hypofunction in adults is
referred to as myxedema, and in newborns and children as cretinism.
Partial
Thyroidectomy
In partial thyroidectomy the
posterior part of thyroid lobes are left behind to avoid removal of parathyroid
glands which otherwise may result in tetany.
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