Thyroid gland – Arterial supply, Venous supply, Nerve supply, Lymphatic drainage and Surrounding structures and capsules.
- Surrounding structures of thyroid gland.
- Arterial supply of thyroid gland.
- Venous supply of thyroid gland.
- Nerve supply of thyroid gland.
- Lymphatic drainage of thyroid gland.
- Capsules of thyroid gland.
I. What are the surrounding structures of thyroid gland.
1. Isthmus of thyroid gland :
- The thyroid gland is fashioned like a butterfly, with two wings or lobes on either side of the windpipe that are connected by an isthmus, a tissue bridge that spans the front of the windpipe.
- The majority of thyroid tumours are detected in the lobes, with only 2-9 percent identified in the isthmus.
2. Investing layer of deep cervical fascia :
- The investing layer of the neck’s deep fascia is a tough layer of fibrous tissue that wraps around the neck like a collar. It is linked to the clavicle’s top surface from below.
- On each side, it separates to surround the sternocleidomastoid and trapezius muscles.
3. Sternohyoid muscle of neck :
The sternohyoid muscle connects the hyoid bone to the sternum and is a slender, narrow muscle. It is one of the infrahyoid muscles’ paired strap muscles that depresses the hyoid bone. The ansa cervicalis is responsible for its innervation.
- Origin : Manubrium of the sternum is the source of this term.
- Insertion : Hyoid bone
4. Sternocleidomastoid muscle of neck :
The sternocleidomastoid muscle (SCM), which splits the neck into an anterior and posterior triangle, is an essential landmark. This muscle connects the sternum and clavicle to the skull.
5. Omohyoid muscle of neck :
- The omohyoid muscle is a neck infrahyoid muscle that receives fibres from the ventral rami of C1–C3 spinal neurons through the ansa cervicalis of the cervical plexus.
- The omohyoid is a flat, paired muscle made up of superior and inferior bellies connected by an intermediate tendon.
6. Pre-tracheal fascia :
The pretracheal fascia is a part of the human neck’s structure. It forms the carotid sheath by extending medially in front of the carotid vessels.
7. Parathyroid gland :
- The parathyroid glands are located in the neck, right behind the thyroid glands.
- The pale pink parathyroid glands create parathyroid hormone, which raises calcium levels in the blood.
- The parathyroid glands are little pea-sized glands found behind the butterfly-shaped thyroid gland in the neck.
8. Carotid sheath :
The carotid sheath stretches from the base of the skull to the aortic arch below. Its top end is linked to the carotid canal’s and jugular fossa’s edges.
- The internal carotid artery, internal jugular vein, and the last four cranial nerves are all located in the upper portion of the carotid sheath.
- The pharynx is medial to it, the styloid apparatus is lateral to it, the infratemporal fossa is anterior to it, and the cervical sympathetic trunk on the prevertebral fascia is posterior to it.
- The common carotid artery, internal jugular vein, and vagus nerve are all located in the bottom section of the carotid sheath.
9. Sympathetic trunk :
- A paired bundle of nerve fibres that runs from the base of the skull to the coccyx is known as the sympathetic trunks (sympathetic chain, gangliated cord).
- For the entire length of the vertebral column, the sympathetic trunk is just lateral to the vertebral bodies.
10. Recurrent laryngeal nerve :
The recurrent laryngeal nerve (RLN), also known as the inferior laryngeal nerve, is a branch of the vagus nerve (CN X) that loops around the right subclavian artery on the right and the aortic arch on the left before returning up to the tracheoesophageal groove and finally the larynx.
11. Common carotid artery :
- The main blood supply to the head and neck is provided by the Common Carotid artery, which is a big elastic artery.
- The carotid arteries are the main blood vessels that feed the brain and face with oxygen and nutrients.
- The brachiocephalic artery gives rise to the right common carotid artery (RCCA) in the neck.
12. Internal jugular vein :
The internal jugular vein is a paired venous system that gathers blood from the brain, the face, and the neck, and transports it to the right atrium. The sigmoid sinus drains into the internal jugular vein.
II. What is the arterial supply of thyroid gland.
1. Superior thyroid artery :
- The superior thyroid artery branches from the external carotid artery right below the greater cornu of the hyoid bone and travels to the thyroid gland.
- The superior thyroid artery begins beneath the anterior border of the sternocleidomastoid and runs upward and forward in the carotid triangle for a short distance, where it is covered by the skin, platysma, and fascia; it then arches downward beneath the omohyoid, sternohyoid, and sternothyroid muscles.
2. Inferior thyroid artery :
- The inferior thyroid artery is a blood vessel that runs through the neck. It originates in the thyrocervical trunk and ascends in front of the vertebral artery and the longus colli muscle.
- The carotid sheath and its contents are then turned medially, as is the sympathetic trunk, with the middle cervical ganglion resting on the vessel.
3. Thyroid ima artery :
- The thyroid ima artery (also known as the thyroidea ima artery, arteria thyroidea ima, Neubauer’s thyroid artery, or the lowest thyroid artery) is a head and neck artery.
- It’s an anatomical variation that, when present, primarily sends blood to the thyroid gland, but also to the trachea, parathyroid gland, and, in rare circumstances, the thymus gland (as thymica accessoria).
4. Thyrocervical trunk :
The thyrocervical trunk is one of the three branches of the initial segment of the subclavian artery, and it supplies the neck viscera, the brachial plexus, neck muscles, and the scapular anastomosis with multiple branches.
III. What is the venous supply of thyroid gland.
1. Superior thyroid vein :
- By tributaries corresponding to the branches of the superior thyroid artery, the superior thyroid vein begins in the substance and on the surface of the thyroid gland and finishes in the upper section of the internal jugular vein.
- The superior laryngeal and cricothyroid veins pass through it.
2. Inferior thyroid vein :
- The inferior thyroid veins arise in the venous plexus on the thyroid gland and communicate with the middle and superior thyroid veins. They are usually two, three, or four in number.
- The inferior thyroid veins drain directly to the brachiocephalic veins, while the superior and intermediate thyroid veins function as direct tributaries to the internal jugular vein.
3. Middle thyroid vein :
After being joined by some veins from the larynx and trachea, the middle thyroid vein takes blood from the lower half of the thyroid gland and finishes in the lower part of the internal jugular vein.
4. Kocher’s vein :
- This vein is known as the Kocher vein or the posterior external jugular vein.
- The two anterior jugular veins, just above the sternum, form the jugular venous arch, which runs downward along the midline between the pretracheal and superficial layers of the cervical fascia, i.e. in the Burns gap.
IV. What is the nerve supply of thyroid gland.
1. Superior laryngeal nerve :
The superior laryngeal nerve is a vagus nerve branch. It originates in the centre of the inferior ganglion of the vagus nerve and receives a branch from the sympathetic nervous system’s superior cervical ganglion along the way.
2. Internal laryngeal nerve :
One of the two branches of the superior laryngeal nerve, the internal laryngeal nerve supplies sensory innervation of the laryngeal mucosa down to the level of the voice cords (supraglottic larynx).
3. External laryngeal nerve :
- The external laryngeal nerve is the smaller branch of the laryngeal nerve. It supplies the cricothyroid muscle by descending on the larynx beneath the sternothyroid muscle.
- The external branch activates the cricothyroid muscle, which tightens the vocal chords and raises pitch.
4. Recurrent laryngeal nerve :
The inferior laryngeal nerve (RLN) is a branch of the vagus nerve (CN X) that has a distinctive loop around the right subclavian artery on the right and the aortic arch on the left.
V. What is the lymphatic drainage of thyroid gland.
1. Pharyngeal lymph nodes :
These can be located behind the pharynx and in front of the prevertebral fascia in the retropharyngeal area.
2. Jugulodigastric lymph nodes :
- They are located above the omohyoid and form a superior group of deep cervical lymph nodes.
- In the triangle formed by the posterior belly of the digastric, facial vein, and internal jugular vein, one lymph node of this group is located below the posterior belly of the digastric between the angle of the jaw and anterior border of the sternocleidomastoid.
- It mostly drains lymph from the palatine tonsil. As a result, it is also known as a tonsil lymph node.
- It is easily perceptible behind and below the angle of the jaw when enlarged due to disease in the palatine tonsil.
3. Lymph nodes surrounding the recurrent laryngeal nerve :
The lymph nodes of the recurrent laryngeal nerve (RLN) go along the RLNs as they pass through the trachea esophageal groove. In the Japanese Classification of Esophageal Cancer, RLN lymph nodes are classed as thoracic lymph nodes and given the number 106 rec.
4. Pre-tracheal lymph nodes :
The lymph nodes of the pretrachea are located anterior to the trachea. They run either side of the midline, following the anterior jugular veins. On the right, they drain into the deep cervical lymph nodes, while on the left, they drain into the thoracic duct.
5. Jugulo-omohyoid lymph nodes :
One of the lymph nodes of the lower group of deep cervical lymph nodes is located above the omohyoid intermediate tendon, posterior to the internal jugular vein. It’s known as a jugulo-omohyoid lymph node.
6. Retrosternal/Brachiocephalic lymph nodes :
These nodes receive lymph from the thymus, pericardium, and right side of the heart, and their efferent vessels join those of the tracheal nodes to form the bronchomediastinal trunks, which are placed in the superior mediastinum in relation to the major vessels.
VI. What are the capsules of thyroid gland.
1. True capsule of thyroid gland :
This is a result of the pretracheal fascia splitting. The superior ligament of Berry, which joins the lobe to the cricoid cartilage, thickens on the medial surface of the thyroid lobe.
2. False capsule of thyroid gland :
Fibro-elastic connective tissue makes up the real capsule. The false capsule is composed comprised of the deep cervical fascia’s pre-tracheal layer. It is made up of a deep capillary plexus that extends all the way to the actual capsule. As a result, during thyroid surgery, it is critical to remove the plexus with a capsule.