1. Thyroid gland
Position: It lies in the front of the neck in relation to the larynx, pharynx trachea and esophagus.
Shape: The gland consists of right and left lobes connected by a narrow isthmus.
- It is very vascular gland, surrounded by a sheath derived from the pretracheal fascia.
- The fascia attaches the gland to the larynx and trachea, so the gland moves up and down with the larynx during deglutition.

2. Fascial layers of Neck

4. Thyroid gland (cont.)
- Each lobe is pear-shaped with its apex directed upward reaching the oblique line on the lamina of the thyroid cartilage; its base lies below at the level of the fifth tracheal ring.
- The isthmus extends across the midline in front of the second, third, and fourth tracheal rings.
- A pyramidal lobe usually presents extending upwards from the isthmus, to the left of the midline.
- A fibromuscular band connects the pyramidal lobe to the hyoid bone called levator glandulae thyroideae.

7. Relations of the lobes 1. Anterolaterally (superficial):
- It is covered by the skin, superficial fascia and deep fascia.
- It is overlapped by the sternothyoid, sternohyoid, omohyoid and anterior border of sternomastoid muscles.

8. Relations of the lobes (cont.)
2. Medially:
- It is related to the larynx and pharynx above.
- It is related to the trachea and esophagus below.
- Cricothyroid muscle and its nerve supply, the external laryngeal nerve.
- It is related to the recurrent laryngeal nerve (very important) in the groove between the trachea and esophagus.

9. Relations of the lobes 3. Posterolaterally:
- It is related to the carotid sheath with the common carotid artery, internal jugular vein, and vagus nerve.
- The posterior border of the lobe is related posteriorly to the superior and inferior parathyroid glands and the anastomosis between the superior and inferior thyroid arteries.

10. Relations of the isthmus:
Anteriorly: The sternothyroids, sternohyoids, anterior jugular veins fascia and skin.
Posteriorly: The second, third, and fourth rings of the trachea.
The terminal branches of the superior thyroid arteries anastomose along its upper border.

11. Blood supply: A. Arterial supply:
1. Superior thyroid artery:
- It is a branch of the external carotid artery.
- It descends to the upper pole of each lobe, accompanied by the external laryngeal nerve.

12. A. Arterial supply (cont.):
2. Inferior thyroid artery:
- It is a branch of the thyrocervical trunk.
- It ascends behind the gland to the level of the cricoid cartilage.
- It then turns medially and downward, to reach the posterior border of the gland.
- the recurrent laryngeal nerve crosses either in front of or behind the artery or may passes between its branches.
3. Thyroidea ima artery:
- It is an occasional branch may be present.
- If present it may arise from the brachiocephalic artery or the arch of aorta.
- It ascends in front of the trachea to the isthmus.

14. B. Venous & Lymph drainage
1. Superior thyroid vein:
- It drains into the internal jugular vein.
2. Middle thyroid vein:
- It drains also into internal jugular vein.
3. Inferior thyroid vein:
- It receives its tributaries from the isthmus and the lower poles of the gland.
- The inferior thyroid veins of the two sides unite with each other and drain into the left brachiocephalic vein.
Lymph drainage
The lymph from the thyroid gland drains mainly into the deep cervical lymph nodes.
A few lymph vessels descend to the paratrachial nodes.

16. Clinical notes 1- Movement of the thyroid gland on swallowing
The gland moves up and down with larynx during swallowing because it is enveloped in the pretracheal fascia.
2- Thyroid gland and the airway
In enlargement of thyroid gland it compresses the trachea because of its intimate relation.
3- Retrosternal goiter
In enlargement of the thyroid gland it descends downwards behind the sternum (compressing the trachea or veins).
This is because the attachment of the sternothyroids muscles to the thyroid cartilage prevents its ascent.
4- Thyroid arteries and important nerves
The superior thyroid artery is related to the external laryngeal nerve.
The terminal branches of the inferior thyroid artery are related to the recurrent laryngeal nerve.
In thyroidectomy operations damage to either nerve may happen.
5- Injury of external laryngeal nerve results in paralysis of Cricothyroid muscle and this leads to hoarseness of voice and inability to tense voice.

17. Clinical notes (cont.)
6- Injury to recurrent laryngeal nerve produces:
- Unilateral complete injury:
Voice is not greatly affected because of compensation of the other normal side.
-Bilateral complete injury:
Speech is lost (aphonia) and breathing is impaired, since the rima glottidis is partially closed.
- Unilateral partial injury:
The abductors of vocal cord are more affected resulting in adduction of vocal cords.
- Bilateral partial injury:
This leads to dyspnea (acute breathlessness) and stidor, and cricothyroidotomy or tracheostomy is necessary.
This is because of paralysis of abductors of vocal cords on both sides.
7- The parathyroid glands and thyroidectomy
In partial thyroidectomy the posterior part of the thyroid gland is left to avoid removal of the parathyroid glands which lie on the posterior surface of the thyroid gland.

19. Nerves related to Thyroid gland

23. Parathyroid glands - They are four yellowish-brown, ovoid bodies.
- They are embedded in the fascial capsule of the thyroid gland related to its posterior border.
- They receive blood supply from the superior and inferior thyroid arteries.
The two superior parathyroid glands
- These are more constant in position.
- They lie at the level of the middle of the posterior border of the thyroid gland.
The two inferior parathyroid glands:
- They lie close to the inferior poles of the thyroid gland.

24. Trachea
It is a cartilaginous and membranous tube. It conveys air to and from the lungs. It is about 10cm in length and is formed of incomplete cartilaginous rings completed posteriorly by smooth muscles.
It begins at the lower border of the cricoid cartilage opposite the 6th cervical vertebra.
It ends by dividing into two bronchi opposite the lower border of the 4th cervical vertebra.
Course: Its upper half lies in the neck and lower half in the thorax. It descends in the midline, but at its bifurcation it lies slightly to the right.

25. Trachea Relations in the neck:
Anteriorly: skin, fascia, isthmus of the thyroid gland (in front of second, third, and fourth tracheal rings), inferior thyroid veins, jugular arch, hyroidea ima artery (if present), and the left brachiocephalic vein in children. It is overlapped by the sternomastoid and sternohyoid muscles.
Posteriorly: The right and left recurrent laryngeal nerves, the esophagus and the vertebral column.
Laterally: On each side the trachea is related to a lobe of thyroid gland, carotid sheath, and inferior thyroid artery.
Blood supply: The trachea is supplied by the inferior thyroid arteries in the neck.
Lymph drainage: The lymph vessels drain into the pretracheal and paratrachial lymph nodes in the neck.
Nerve supply: The vagi, recurrent laryngeal nerves and sympathetic trunks.

27. Esophagus It is a muscular tube about 25cm (10 inches) long.
It begins as a continuation of pharynx at the level of cricoid cartilage, opposite the sixth cervical vertebrae.
It ends by joining the cardiac end of the stomach opposite the 10th thoracic vertebra.
Course: It descends in the midline then it inclines to the left and leaves the neck to the thorax.

29. Esophagus (cont.) Relations in the neck:
Anteriorly: The trachea, the recurrent laryngeal nerves in the groove between the trachea and esophagus.
Posteriorly: The prevertebral fascia, the longus colli muscle, and the vertebral column.
Laterally: On each side the trachea is related to a lobe of thyroid gland, carotid sheath, and inferior thyroid artery. The thoracic duct ascends along the left margin for a short distance.
Blood supply: The esophagus is supplied by the inferior thyroid arteries in the neck.
Lymph drainage: The lymph vessels drain into the deep cervical lymph nodes.
Nerve supply: The vagi, recurrent laryngeal nerves and sympathetic trunks.