Overview
The thyroid gland
is an endocrine gland located in the anterior neck that wraps around the
superior tracheal rings and cricoid cartilage. It has a rich blood supply so it
can release the hormones it produces to increase metabolism. Embryological
development of the thyroid begins at the floor of the pharynx before it passes
down the thyroglossal duct.
Gross Anatomy
The thyroid gland
lies between the C5 and T1 vertebrae and consists of two pear shaped lateral
lobes, connected by an isthmus that crosses anteriorly over the second and
third tracheal cartilages. A pyramidal lobe that projects superiorly from the
isthmus is often present. The thyroid gland is in the visceral compartment of
the neck along with the trachea, oesophagus and pharynx, contained within the
pretracheal fascia. This fascia tethers the thyroid gland to the trachea and
larynx, which explains why the thyroid gland moves with the larynx on
swallowing. These organs lie deep to the sternothyroid, omohyoid and
sternohyoid muscles. The carotid sheath lies posterolaterally to the lobes of
the thyroid gland. The isthmus is located anteriorly to the trachea and
posteriorly to the pretracheal fascia.
During development
the thyroid gland forms at the floor of the pharynx near the tongues' base. The
thyroid gland then migrates down the thyroglossal duct to its adult location by
seven weeks, the duct usually disappears during development but remnants of the
thyroglossal duct can remain as a cyst or fistula. The foramen cecum on the
tongue marks the origin of the thyroglossal duct. Additional functional thyroid
tissue can remain anyway along the path of the thyroids' migration during
development, including a lingual or pyramidal lobe.
The thyroid gland
consists of follicles filled with colloid which take up iodine to make the
thyroid hormones thyroxine and triiodothyronine, which increase metabolic
activity in most of the body. Follicular cells arranged in a single layer surround
follicles and secret thyroid hormones. The parafollicular cells of the thyroid
gland secrete thyrocalcitonin which acts to lower serum calcium levels.
The superior and
inferior thyroid arteries are major arteries that supply the thyroid gland. The
superior thyroid artery originates as the first branch of the external carotids
and descends to the superior pole of the lateral lobe where it divides into
anterior and posterior glandular branches. The inferior thyroid artery is a
branch from the thyrocervical trunk, which is the first branch of the
subclavian artery. The artery ascends to the inferior pole of the lateral lobe
where it supplies the posterior, inferior parts of the thyroid gland. In around
10% of individuals a thyroid ima artery that arises from the brachiocephalic
trunk or aortic arch ascends to supply the anterior surface and isthmus of the
thyroid gland.
Three veins form a
venous plexus which drains the thyroid gland. The area of the thyroid supplied
by the superior thyroid artery is primarily drained by the superior thyroid
vein and the remaining area is drain by the middle and inferior thyroid veins.
Both the superior and middle thyroid veins drain into the internal jugular vein
and the inferior thyroid vein drains into the brachiocephalic vein.
Innervation to the
thyroid is from the superior, middle and inferior cervical sympathetic ganglion
via branches of the sympathetic trunk. These nerves do not stimulate the
endocrine secretion of the thyroid gland, this is regulated by hormones from
the anterior pituitary gland. The lymphatic drainage of thyroid gland is mostly
into deep cervical nodes and some drainage into paratracheal nodes.
Clinical Anatomy
Thyroid goitre - A goitre is a diffuse enlargement of the thyroid gland that has many causes including iodine
deficiency and immunological disease. A common cause of an irregular goitre is
a multinodular goitre consisting of hypertrophic areas and colloid cysts. These
can be medically managed or surgically excised if the goitre is large enough to
cause serious problems including respiratory obstruction. The sternothyroid
muscle limits upward expansion of the thyroid gland, therefore it is common for
the thyroid gland to expand posterior to the sternum when a goitre forms. These
retrosternal goitres can compress veins as well as the trachea.
Thyroglossal
cysts - These are fibrous cysts that can
develop from remnants of any thyroid tissue left along the thyroglossal duct
during development. These can be excised by surgery.
Recurrent
laryngeal nerve trauma - The two
recurrent laryngeal nerves arise from the vagus nerves and descend into the
chest where the right nerve hooks around the subclavian artery, and the left
nerve hooks around the aortic arch. They then ascend up the neck between the
oesophagus and trachea before passing posterior to the thyroid gland to
innervate the larynx. Due to their close relation to the thyroid gland, great
care must be taken to avoid damage to them during thyroid surgery.
Quick Anatomy
Key Facts
Developmental
precursor |
Arterial
supply |
Venous
drainage |
Nervous
supply |
Lymphatic
drainage |
Neural
crest cells and primitive pharynx. |
Superior
thyroid artery, a branch of the external carotid. Inferior thyroid artery, a
branch of the thyrocervical trunk. |
The
superior and middle thyroid veins drain into the internal jugular vein. The
inferior thyroid vein drains into the brachiocephalic vein. |
Superior,
middle and inferior cervical sympathetic ganglia. |
Mainly
deep cervical nodes. Some into paratracheal nodes. |
Aide-Memoire
Summary
The thyroid has two
lobes connected by an isthmus and consists of follicles which produce thyroid
hormones. During development the thyroid gland migrates from the floor of the
pharynx to its adult location. Cysts and fistulas can remain along this
developmental tract. Care is taken during thyroid surgery to avoid damage to
the recurrent laryngeal nerves. Furthermore during tracheostomies and thyroid
surgery caution must be taken to avoid the ima artery which is present in 10%
of individuals, anterior to the thyroid gland.