Thyroid Anatomy « Dr-Lokku

Dr-Lokku » Your first category

Thyroid Anatomy

(7 posts)

Started 12 months ago by ranga0007
Latest reply from pinastro

Tags:

thyroid

ranga0007

offline
Dr-lokku
The thyroid is a brownish-red and highly vascular gland located anteriorly in the lower neck, extending from the level of the fifth cervical vertebra down to the first thoracic. The gland varies from an H to a U shape and is formed by 2 elongated lateral lobes with superior and inferior poles connected by a median isthmus (with an average height of 12-15 mm) overlying the second to fourth tracheal rings. The isthmus is encountered during routine tracheotomy and must be retracted (superiorly or inferiorly) or divided. Occasionally, the isthmus is absent, and the gland exists as 2 distinct lobes. Each lobe is 50-60 mm long, with the superior poles diverging laterally at the level of the oblique lines on the laminae of the thyroid cartilage. The lower poles diverge laterally at the level of the fifth tracheal cartilage. Thyroid weight varies but averages 25-30 g in adults (slightly heavier in women). The gland enlarges during menstruation and pregnancy.

A conical pyramidal lobe often ascends from the isthmus or the adjacent part of either lobe (more often the left) toward the hyoid bone, to which it may be attached by a fibrous or fibromuscular band, the levator of the thyroid gland. Remnants of the thyroglossal duct may persist as accessory nodules or cysts of thyroid tissue between the isthmus and the foramen caecum of the tongue base. The development of the thyroid is beyond the scope of this article but is discussed elsewhere in this journal.

Usually, 2 pairs of parathyroid glands lie in proximity to the thyroid gland. Anatomy and development of these glands are also discussed in the eMedicine article Embryology of the Thyroid and Parathyroids.
Attachments
1. development.jpg (32.1 KB, 0 downloads) 12 months old
2. anatomy01_thyroid.jpg (43.5 KB, 0 downloads) 12 months old
Posted 12 months ago #
ranga0007

offline
Dr-lokku
Structure

Under the middle layer of deep cervical fascia, the thyroid has an inner true capsule, which is thin and adheres closely to the gland. Extensions of this capsule within the substance of the gland form numerous septae, which divide it into lobes and lobules. The lobules are composed of follicles, the structural units of the gland, consisting of a layer of simple epithelium enclosing a colloid-filled cavity. This colloid (pink on hematoxylin and eosin [H&E] stain) contains an iodinated glycoprotein, iodothyroglobulin, a precursor of thyroid hormones. Follicles vary in size, depending upon the degree of distention, and they are surrounded by dense plexuses of fenestrated capillaries, lymphatic vessels, and sympathetic nerves.

Epithelial cells are of 2 types: principal cells (ie, follicular) and parafollicular cells (ie, C, clear, light cells). Principal cells are responsible for formation of the colloid (iodothyroglobulin), whereas parafollicular cells produce the hormone calcitonin, a protein central to calcium homeostasis. Parafollicular cells lie adjacent to the follicles within the basal lamina.
Attachments
1. thyroid_anatomy.jpg (32.4 KB, 0 downloads) 12 months old
Posted 12 months ago #
ranga0007

offline
Dr-lokku
Vascular Anatomy and Relation with Laryngeal Innervation

The arterial supply to the thyroid gland comes from the superior and inferior thyroid arteries and, occasionally, the thyroidea ima. These arteries have abundant collateral anastomoses with each other, both ipsilaterally and contralaterally. The thyroid ima is a single vessel, which originates, when present, from the aortic arch or the innominate artery and enters the thyroid gland at the inferior border of the isthmus.

Superior thyroid artery and superior laryngeal nerve

The superior thyroid artery is the first anterior branch of the external carotid artery. In rare cases, it may arise from the common carotid artery just before its bifurcation. The superior thyroid artery descends laterally to the larynx under the cover of the omohyoid and sternohyoid muscles. The artery runs superficially on the anterior border of the lateral lobe, sending a branch deep into the gland before curving toward the isthmus where it anastomoses with the contralateral artery
Attachments
1. Bld_supply.jpg (34.8 KB, 0 downloads) 12 months old
Posted 12 months ago #
ranga0007

offline
Dr-lokku

Inferior thyroid artery and recurrent laryngeal nerve

The inferior thyroid artery arises from the thyrocervical trunk, a branch of the subclavian artery. It ascends vertically and then curves medially to enter the tracheoesophageal groove in a plane posterior to the carotid sheath. Most of its branches penetrate the posterior aspect of the lateral lobe (see Image 3). The inferior thyroid artery has a variable branching pattern and is closely associated with the recurrent laryngeal nerve. The latter also ascends in the tracheoesophageal groove and enters the larynx between the inferior cornu of the thyroid cartilage and the arch of the cricoid. The recurrent laryngeal 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 between the nerve and the inferior thyroid artery is highly variable, as demonstrated by the classic work of Reed, who in 1943 described 28 variations in this relationship. The nerve can be found deep to the inferior thyroid artery (40%), superficially (20%), or between branches of the artery (35%).1 Significantly, the relationship between nerve and artery on one side of the neck is similar to that found on the other side in only 17% of the population. Furthermore, at the level of the inferior thyroid artery, branches of the recurrent laryngeal nerve that are extralaryngeal may be present (5%). Preservation of all those branches is important during thyroidectomy.

Posted 12 months ago #
ranga0007

offline
Dr-lokku

Innervattion

Principal innervation of the thyroid gland derives from the autonomic nervous system. Parasympathetic fibers come from the vagus nerves, and sympathetic fibers are distributed from the superior, middle, and inferior ganglia of the sympathetic trunk. These small nerves enter the gland along with the blood vessels. Autonomic nervous regulation of the glandular secretion is not clearly understood, but most of the effect is postulated to be on blood vessels, hence the perfusion rates of the glands.

Posted 12 months ago #
pinastro

offline
Key Master

Nice Doctor, this could have been a blog ...why put it on a discussion forum ??

but nice job..keep the engine running..people are reading these forums for sure

Posted 11 months ago #
pinastro

offline
Key Master

nice work

Posted 11 months ago #