Anatomy of Thyroid Gland

Anatomy of thyroid gland.

The thyroid, the largest endocrine gland, is a highly vascular organ located in the lower part of the front and sides of the neck. It is reddish brown in colour and consists of two conical pear-shaped lobes joined together by an isthmus. The thyroid gland is larger in females than in males and further increases in size during menstruation and pregnancy.

The thyroid gland is situated below the larynx, against the second, third, and fourth tracheal rings. It lies against vertebrae C5, C6, C7, and T1. Each lobe extends from fourth or fifth tracheal ring to the middle of the thyroid cartilage. Each lobe measures about 5 cm x 2.5 cm x 2.5 cm, and the isthmus measures 1.2 cm x 1.2 cm. The isthmus extends from the second to the fourth tracheal ring. In adulthood, it typically weighs between 15 and 25 grams.

Capsules of Thyroid.

1. The true capsule is the outer layer of connective tissue that surrounds the gland and sends septa into the thyroid tissue. It contains a dense network of capillaries. During surgery, the thyroid is removed along with the true capsule to prevent bleeding.
2. The false capsule is a thin layer of loose areolar tissue derived from the pretracheal layer of the deep cervical fascia. It surrounds the larynx, trachea, and thyroid, and is thin along the posterior border of the lobes but thick on the inner surface of the gland, forming a suspensory ligament of Berry that connects the lobe to the cricoid cartilage.

Thyroid lobes.

The thyroid lobes possess an apex that is directed upwards and slightly laterally, restricted superiorly by the attachment of the sternothyroid to the oblique line of the thyroid cartilage. The apex is related to the superior thyroid artery and external branch of the superior laryngeal nerve. The base of the lobes is at the level of the 4th or 5th tracheal ring and is related to the inferior thyroid artery and recurrent laryngeal nerve.

Anatomical variations.

1. Pyramidal lobe. The pyramidal lobe is a conical protrusion that typically arises from the left side of the isthmus and may extend upward to the upper border of the thyroid cartilage or even the hyoid bone. If present during surgery, it should be removed along with the main lobe to prevent leaving behind functioning thyroid tissue. It is found in 12–81% of cases. On occasion, a fibrous or fibro-muscular band, referred to as the levator glandulae thyroidae, descends from the body of the hyoid bone to the isthmus or the pyramidal lobe.
2. The tubercle of Zuckerkandl. It is a lateral or posterior projection of the thyroid lobe. The recurrent laryngeal nerve runs medially to it, and the superior parathyroid gland is attached to its superior aspect. If present during surgery, it should be removed along with the main lobe to prevent leaving behind functioning thyroid tissue. It is present in 60% of surgical dissections.

Ligaments.

1. Posterior suspensory ligament or berry’s ligament. The suspensory ligament of Berry, also known as the posterior suspensory ligament, is a condensation of pretracheal fascia that firmly attaches to the posteromedial surface of the thyroid lobe with the cricoid cartilage and trachea (First and sometimes the second tracheal ring). This connection allows the thyroid gland to move up and down during swallowing. The recurrent laryngeal nerve (RLN) or its branches may be located lateral, medial, or pass through the suspensory ligament before entering the larynx, so surgical procedures in this area require careful attention to avoid damage to the nerve and to avoid bleeding from the inferior thyroid artery branches. Additionally, the ligament may retain some thyroid tissue after thyroidectomy, potentially affecting postoperative thyroglobulin levels and radionuclide uptake.
2. Anterior suspensory ligament. The anterior suspensory ligament, on the other hand, is also a condensation of the pretracheal fascia, attaching the upper front portion of the thyroid lobe and the isthmus to the laryngotracheal complex. It serves to separate the thyroid gland from the strap muscles and contains medium-sized blood vessels.

Superior laryngeal nerve.

The laryngeal nerves are crucial for the functioning of the larynx. There are two main branches of the laryngeal nerves: the superior laryngeal nerve and the recurrent laryngeal nerve.

• The superior laryngeal nerve arises from the vagus nerve at the level of the hyoid bone and has two branches: the external laryngeal nerve and the internal laryngeal nerve. The external laryngeal nerve is thin and accompanies the superior thyroid artery and vein(s). It ends by supplying the cricothyroid muscle. During thyroid surgery, it’s crucial to be extremely careful while ligating the superior thyroid artery and vein(s) and should be ligated as close to the upper pole of the thyroid as possible to avoid injury to the nerve. Injury to the nerve causes a subtle change in vocal pitch and a decreased vocal range.
• The internal laryngeal nerve is thick and supplies the laryngeal cavity above the level of the vocal folds.

Joll’s Triangle. It is used for the identification of external branch of superior laryngeal nerve at the upper pole. Nerve lies in the floor of the triangle formed by the cricothyroid muscle which it supplies. Its boundaries are:

• • Superior border is formed by the attachment of the strap muscles to the thyroid cartilage
  • Medial border is formed by the midline.
  • Lateral border is formed by the upper pole of thyroid gland and superior thyroid vessels.

Recurrent laryngeal nerve.

The recurrent laryngeal nerve also arises from the vagus nerve. It provides sensory nerve supply to the laryngeal cavity below the level of the vocal folds and motor supply to all intrinsic muscles of the larynx except for the cricothyroid muscle.

The left recurrent laryngeal nerve originates in the thorax, while the right recurrent laryngeal nerve originates in the root of the neck in front of the right subclavian artery. After branching from the vagus nerve, the right recurrent nerve loops around the subclavian artery, while the left recurrent nerve loops around the arch of the aorta. The recurrent laryngeal nerves run upwards to reach the tracheo-oesophageal groove where they enter the larynx. These nerves might pass medial to, lateral to, or through the suspensory ligament of berry.

During surgical procedures involving the thyroid gland, it’s important to consider these relationships to avoid injury to the nerves. RLN is closer to trachea on the left than on the right side. Damage to the recurrent laryngeal nerve is also possible and vocal cords should be checked prior to surgery. Unilateral nerve injury causes hoarseness of voice which usually improves after a few weeks. Bilateral injury causes stridor.

Beahrs Triangle or Riddle’s triangle. It is used for identification of recurrent laryngeal nerve. Its boundaries are:

• • Medial border is formed by recurrent laryngeal nerve
  • Lateral border is formed by common carotid artery
  • Superior border is formed by inferior thyroid artery

Lore’s Triangle. It is used for identification of recurrent laryngeal nerve. Its boundaries are:

• • Medial border is formed by trachea / oesophagus,
  • Lateral border is formed by retracted strap muscles
  • Superior border is formed by lower pole of thyroid.

The apex of the triangle points inward at the thoracic inlet. On the right side, the RLN runs through the triangle from the lateral to the medial side, and on the left side, it runs straight up along the tracheoesophageal groove.

Non-recurrent laryngeal nerve.

The recurrent laryngeal nerve (RLN) can have an anomalous course. It does not follow a consistent path. Instead of looping around the subclavian artery on the right side or the aortic arch on the left, it runs directly medially after originating from the vagus nerve. It supplies the larynx as it runs along the inferior thyroid artery. This anomaly is more common on the right side and is often associated with an anomalous right subclavian artery, which may arise from the left side of the descending aorta and run behind the esophagus.

Arteries.

The thyroid gland receives its blood supply from the superior and inferior thyroid arteries.

1. The superior thyroid artery, which is the first branch of the external carotid artery, travels alongside the external branch of the superior laryngeal nerve to reach the upper pole of the thyroid gland. It is closely associated with the external branch of the superior laryngeal nerve.
2. The inferior thyroid artery arises from the thyrocervical trunk, which originates from the first part of the subclavian artery. This artery reaches the lower pole of the thyroid gland and is closely related to the recurrent laryngeal nerve. Upon reaching the thyroid gland, the inferior thyroid artery divides into two branches. An inferior branch that supplies the lower part of the thyroid gland and connects with the posterior branch of the superior thyroid artery and a superior branch that supplies the parathyroid glands.
3. In some cases, a small third artery known as the thyroid ima artery may arise from the brachiocephalic trunk or the arch of the aorta. This artery ascends along the anterior surface of the trachea to provide blood supply to the thyroid gland.

Veins.

The thyroid gland is drained by three sets of veins: the superior, middle, and inferior thyroid veins. The superior thyroid vein emerges at the upper pole and drains directly into the internal jugular vein. The middle thyroid vein emerges at the middle of the lobe and also drains into the internal jugular vein. The inferior thyroid veins emerge at the lower border of the isthmus and form a plexus that drains into the right and left brachiocephalic veins.

Lymphatic drainage.

The thyroid gland drains into the central compartment, including the prelaryngeal, pretracheal, and paratracheal nodes (level VI), and into the superior mediastinum (level VII), as well as into level II, III, and IV nodes. These nodes play an important role in the treatment of thyroid malignancies.

———- End of the chapter ———–

Learning resources.

• Scott-Brown. Textbook of Otorhinolaryngology Head and Neck Surgery.
• Susan Standring. Gray’s Anatomy.
• Frank H. Netter. Atlas of Human Anatomy.
• G.J. Romanes. Cunningham’s Manual of Practical Anatomy: Head and Neck and Brain.
• B.D. Chaurasiya. Human Anatomy.
• David J. Terris. William S. Duke. Textbook of Thyroid and Parathyroid Diseases Medical and Surgical Management. 
• Stell and Maran’s. Textbook of Head and Neck Surgery and Oncology.
• Hans Behrbohm. Textbook of Ear, Nose, and Throat Diseases With Head and Neck Surgery.
• P L Dhingra. Textbook of Diseases of Ear, Nose and Throat.
   

  Author:

Dr. Rahul Bagla
MBBS (MAMC, Delhi) MS ENT (UCMS, Delhi)
Fellow Rhinoplasty & Facial Plastic Surgery.
Renowned Teaching Faculty
Mail: msrahulbagla@gmail.com
India

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