Anatomy of the thyroid gland

Question 1

A 40-year-old man presents with a swelling on the anterior part of his neck. On examination, a single, firm, rounded mass can be felt on the left side of the laryngotracheal region. It moves up and down with swallowing. Ultrasound reveals a solid nodule in the left lobe of his thyroid gland. Subsequently, a needle biopsy indicates that malignant changes have taken place in the cells.

• Preliminary Diagnosis: Tumor of the left lobe of the thyroid

1) Why does the mass move up and down when swallowing?

2) What can explain the difficulty breathing?

3) What structures would be endangered by subtotal or total thyroidectomy?

4) Why is the nature of the patient’s voice of interest postoperatively?

Question 2

Why does the mass move up and down when swallowing?

Answer 2

Due to the ligament of berry (found in the medial side of the lobe, where the pre-tracheal fascia becomes thicker, connecting the lobe to the cricoid cartilage)

Question 3

What is the thyroid gland?

Answer 3

• Found in the anterior triangle of the neck
• Pyramid shape with an apex, base, 3 surfaces and 2 borders
• Formed by 2 lobes (right & left), connected by an isthmus, that extends from C5-T1
• It is not visible under normal conditions, but it can be felt during swallowing
• The isthmus connects the right and left lobes at the C2, C3, & C4

Question 4

What is the vertebral relation of the thyroid gland?

Answer 4

• The thyroid cartilage is from C4-C5
• The thyroid gland is from C5-T1

Question 5

Describe the development of the thyroid gland

Answer 5

1) An ectodermal thickening will form a foramen cecum of the tongue on the floor of the pharynx

2) It descends into the neck forming a thyroglossal duct, which keeps it connected to the pharynx for a short time

3) the thyroglossal duct continues down to bifurcate into two ducts which go the right and left lobes of the thyroid gland

4) The glands then start functioning at the 4th month of IUL

Question 6

What is the foramen cecum?

Answer 6

It is a small depression between the anterior 2/3 and posterior 1/3 of the tongue

Question 6

What is the thyroglossal duct?

Answer 6

A minute epithelial tube/fibrous cord that integrates in adults

Question 7

What abnormal things can occur at the thyroglossal duct?

Answer 7

Formation of a pyramidal lobe (remnant of the thyroglossal duct)

1) Thyroglossal duct cysts

• This might develop at any point during the descending, which are seen near or within the hyoid born forming a swelling in the anterior part of the neck

2) Ectopic thyroid tissue: Not located between C5-T1

3) No isthmus but with a pyramidal lobe

Question 8

What are the different locations where an ectopic thyroid tissue can implant?

Answer 8

1) Lingual thyroid

2) Sublingual thyroid

3) Prelaryngeal/pretracheal thyroid

4) Intratracheal thyroid

5) Intrathoracic thyroid

Question 9

What are the different structural variations of the thyroid gland?

Answer 9

1) Pyramidal lobe: Thyroid tissue extension from the isthmus of the thyroid gland, usually occurs at the left side

2) Levator glandular thyroid: A fibro-muscular extension from the hyoid bone to the isthmus/pyramidal lobe

Question 10

Describe the coverings of the thyroid gland

Answer 10

1) True capsule (Fibrous “connective tissue of the gland itself”):

• The venous plexus is inside it
• It is the peripheral condensation of the connective tissue of the gland

2) False capsule (Fascial “pretracheal fascia covering the true capsule”):

• From the pretracheal layer
• It is thin on the posterior aspect
• Forms a suspensory ligament of berry, connecting the lobe to the cricoid cartilage
• Contains the parathyroid gland along its posterior lobe
• FYI: there is a space between the true/false capsule

Question 11

On which aspect will the thyroid bulge if it enlarges?

Answer 11

It often occurs posteriorly as the CT around the posterior aspect is thin, which might compress the trachea, and esophagus, causing symptoms like hoarseness (due to the presence of the recurrent laryngeal nerve) and difficulty swallowing

Question 12

Where is the capillary plexus present in the thyroid gland?

Answer 12

Deep to the true capsule, to avoid hemorrhage during surgery the thyroid is removed along with the true capsule

• Outermost to innermost: false capsule > true capsule > venous plexus

Question 13

What are the parts of the thyroid gland?

Answer 13

we have:

1) Apex

• Extends to the oblique line of the thyroid cartilage, sandwiched between the inferior constrictor medially and the sternothyroid laterally
• Due to it, a thyroid enlargement cannot move upwards

2) Base

• Located between the 5th and 6th trachea cartilage

3) 3 Surfaces

• Medial
• Anterolateral
• Posterolateral

4) 2 borders

• Anterior
• Posterior

Question 14

What are the constrictor muscles?

Answer 14

There are 3 constrictor muscles in the pharynx (superior, middle, inferior) which push the food downwards

• The inferior constrictor muscle is posterior to the thyroid gland
• The sternothyroid muscle is anterior to the thyroid gland

Question 15

What are the branches of the vagus nerve?

Answer 15

Vagus:

1) Superior laryngeal nerve:

1a) Internal laryngeal nerve (goes inside the larynx to supply the mucosa above the vocal cord, cough when food is stuck above the vocal cords)

1b) External laryngeal nerve (supplies the cricothyroid, cough if food is stuck below the vocal cord)

2) Recurrent laryngeal nerve

Question 16

What is the function of the cricothyroid muscle?

Answer 16

Produces high pitch sound

Question 17

What are the relations of the medial surface of the thyroid gland?

Answer 17

• 2 Tubes, 2 Muscles, 2 Nerves

1) Tubes: Larynx/Trachea, Pharynx/Esophagus

2) Nerves: External laryngeal & Recurrent laryngeal

3) Muscles: Inferior constrictor muscles & Cricothyroid muscle

Question 18

What are the anterolateral surface relations of the thyroid gland?

Answer 18

1) Sternohyoid (most superficial one)

2) superior belly of Omohyoid (Laterally)

3) Sternothyroid (Deep)

• FYI Only the sternohyoid and sternothyroid are related to the isthmus

Question 19

What are the posterolateral surface relations of the thyroid gland?

Answer 19

Carotid sheath and its content

Question 20

What is the relation of the anterior border of the thyroid gland?

Answer 20

The anterior descending branch of the superior thyroid artery, a branch of the external carotid

Question 21

What is the posterior border of the thyroid gland related to?

Answer 21

The anastomoses between the superior thyroid artery (branch of the external carotid) and inferior thyroid artery (branch of the thyrocervical trunk), the parathyroid gland, and the lower part of the thoracic duct

Question 22

What is the anterior surface of the isthmus related to?

Answer 22

The sternothyroid and sternohyoid muscles

Question 23

What is the posterior surface of the isthmus related to?

Answer 23

2nd, 3rd, & 4th tracheal ring

Question 24

What is the superior border of the isthmus related to?

Answer 24

Pyramidal lobe and the levator glanduli thyroidae

Question 25

What is the inferior border of the isthmus related to?

Answer 25

The inferior thyroid veins

• The inferior thyroid vein drains into the brachiocephalic

Question 26

What is the venous drainage of the thyroid gland?

Answer 26

1) Superior thyroid vein & Middle thyroid vein both of which drain into the internal jugular vein

2) The inferior thyroid vein which drains into the left brachiocephalic vein

3) Sometimes there could be an extra vein called “Kochar” between the middle and inferior veins and drains into the internal jugular vein

Question 27

What is the blood supply of the thyroid gland?

Answer 27

1) Superior thyroid artery

• A branch of the external carotid artery, which divides into anterior and posterior branches
• It supplies the upper 3rd of the lobes and the upper half of the isthmus

2) Inferior thyroid artery

• A branch of the thyrocervical trunk, supplying the lower 2/3 of the lobes and the lower half of the isthmus (arises from the subclavian in 15% of the cases, the rest is from the thyrocervical trunk)

3) In 3% of individuals they have an artery called thyroidea ima artery, which could originate either from the brachiocephalic trunk or the arch of the aorta

Question 28

Where should we ligate the thyroid arteries?

Answer 28

1) The superior thyroid artery should be ligated close to the gland as it is there where it is away from the external laryngeal nerve (which goes to the cricothyroid)

2) Both the inferior thyroid artery and the should be ligated away from the gland

Question 29

What are the lymphatics of the thyroid gland?

Answer 29

1) Upper deep cervical nodes (drains the upper part of the gland) via the prelaryngeal, pretracheal, and paratracheal nodes

2) The lower part is drained Via the lower deep cervical nodes

Question 30

What is the nerve innervation of the thyroid?

Answer 30

Superior, middle, and inferior cervical sympathetic ganglia

Question 31

What are the different techniques of thyroidectomy?

Answer 31

1) Thyroid lobectomy

2) Isthmusectomy

3) Sub-total thyroidectomy

4) Near-total thyroidectomy

5) Total thyroidectomy

Question 32

What are the possible complications of the thyroidectomy?

Answer 32

1) Injury to the external laryngeal nerve

2) Injury to the recurrent laryngeal nerve

3) Injury to the cervical sympathetic trunk or a sympathetic ganglion

Question 33

What will happen in case of injury to the external laryngeal nerve?

Answer 33

The cricothyroid muscle will be paralyzed and thus the voice will be weakened and not tensed

Question 34

What will happen in case of injury to the recurrent laryngeal nerve?

Answer 34

This may result in hoarseness of the voice

Question 35

What will happen in case of injury to the cervical sympathetic trunk or a sympathetic ganglion?

Answer 35

Horner’s syndrome

Question 36

What will happen if an adenoma occurs in the thyroid gland?

Answer 36

The other lobe might hypertrophy to compensate for the loss of function in that lobe

Question 37

Describe the morphological structure of the thyroid gland

Answer 37

1) Covered by a capsule

2) Septa extends into the gland dividing it into lobules

3) Each lobule is made of an aggregation of follicles that are spherical in structure

4) The follicular wall is lined with a single layer of cuboidal cells that secrete into the interior of the follicle (the cuboidal shape might change in various activity)

5) Each follicle is filled with a pink-staining proteinaceous material called colloid

6) The follicular cells have an apical microvilli and many secretory granules

7) Between the thyroid follicles are the parafollicular/C-cells which secrete calcitonin

Question 38

Which type of cell lines the follicular wall?

Answer 38

Cuboidal cells

Question 39

The follicles are filled with what?

Answer 39

Colloid (filled with pink-staining, proteinaceous material)

Question 40

What is found between the thyroid follicles?

Answer 40

The parafollicular/C-cells, which secretes calcitonin

Question 41

What divides the thyroid gland into lobules?

Answer 41

The septa that extend into it

Question 42

When is the colloid abundant and the follicles large, and the lining cells are flat?

Answer 42

When the gland is inactive

Question 43

When are the lining cells cuboidal?

Answer 43

When the gland is moderately active

Question 44

When is the colloid scanty, follicles are small, and the lining cells are columnar?

Answer 44

When the gland is hyperactive

Question 45

What is the parathyroid gland?

Answer 45

• Usually four, two on each side
• They lies on the posterior surface of the thyroid, close to the tracheoesophageal groove
• They might be embedded within the thyroid gland
• Their function is to regulate the calcium/phosphate levels
• They have the same blood, lymphatic and nerve supply as the thyroid

Question 46

What are the main cell types of the parathyroid?

Answer 46

1) Chief cells: They are the most abundant and produce the parathormone which controls the serum calcium level

2) Oxyphil/Eosinophil cells (unknown function)

Question 47

What is the microscopic photo of the parathyroid gland?

Answer 47

Arranged in dense cords/nests around abundant capillaries

Question 48

Describe the development of the parathyroid gland

Answer 48

• Derived from the epithelial lining of the 3rd and 4th branchial pouches
• The superior glands will arise from the 4th pouch
• The inferior glands will arise from the 3rd pouch, which descends with the thymus
• The third pouch will also give rise to the thymus, and this is why the inferior glands are found down