Thyroid anatomy

Notes

Gross structure

The thyroid is located in the mid-line of the neck, anterior to the trachea and inferior to the larynx.

The thyroid gland comprises of two lateral lobes and a central isthmus. It is surrounded by a fibrous capsule and located at the level of vertebrae C5-T1

The pyramidal lobe, an embryological remnant from the descent of the thyroid, typically project upwards from the isthmus, however, there is a wide degree of variation between individuals.

Its primary function is to produce thyroid hormones T3 and T4 following stimulation by TSH.

Surrounding structures

The thyroid gland is surrounded by a number of important structures.

Thyroid cartilage

Its superior border is at the level of vertebrae C4. It forms a median projection, termed the adams apple. Superior to this projection is the superior thyroid notch. The cartilage has both superior and inferior horns.

Cricoid cartilage

The cricoid cartilage sits at the level of vertebrae C6. It is the only complete ring of cartilage in the trachea. 

It is composed of a posterior component termed the lamina and an anterior component termed the arch. It attaches to first tracheal ring by the cricotracheal ligament.

Medium cricothyroid ligament

This ligament connects the inferior border of the thyroid cartilage with the superior border of the cricoid cartilage.

An incision is made through the medium cricothyroid ligament to establish an emergency airway - a cricothyroidotomy.

Parathyroid gland

The parathyroid glands (typically four) are located posterior to the thyroid gland. It is important to note there is a great deal of variety in both the location and number of glands. 

They are responsible for the release of parathyroid hormone, a key part of the calcium homeostasis pathways. Due to their location they are frequently removed during thyroidectomy, typically resulting in a transient hypocalcaemia.

Arterial supply

A rich blood supply is received from the external carotid artery and the thyrocervical trunk.

Superior thyroid arteries

  • Supplies: Anterosuperior aspect of the thyroid gland.
  • Source: It is the first branch of the external carotid artery.
  • Branches
    • Hyoid artery
    • Sternocleidomastoid branches
    • Superior laryngeal artery
    • Cricothyroid branch

Inferior thyroid arteries

  • Supplies: Posteroinferior aspect of the thyroid gland.
  • Source: Thyrocervical trunk (third branch of the subclavian artery).
  • Branches
    • Inferior laryngeal
    • Oesophageal
    • Tracheal
    • Ascending cervical
    • Pharyngeal

Venous drainage

The thyroid gland is drained by three pairs of veins.

Superior thyroid veins

  • Drains from: Superior poles of the thyroid gland
  • Drains to: Internal jugular vein

Middle thyroid veins

  • Drains from: Middle poles of the thyroid gland
  • Drains to: Internal jugular vein

Inferior thyroid veins

  • Drains from: Lower poles of the thyroid gland
  • Drains to: Brachiocephalic vein

Recurrent laryngeal nerve

The recurrent laryngeal nerve is a branch of the vagus nerve (CN X).

Recurrent laryngeal nerve

The right and left recurrent laryngeal nerves branch off at different levels on the right and left sides. The right side branches of at the level of the subclavian artery whilst the left side branches from at arch of the aorta.

They follow a ‘recurrent’ path upwards through a groove between the trachea and oesophagus. This brings them close to the thyroid gland. The nerve may be damaged during thyroid surgery, typically resulting in a hoarse voice. 

The recurrent laryngeal nerve has a number of functions:

  • Motor:
    • Intrinsic muscles of the larynx (except cricothyroid muscle).
  • Sensory and secretomotor
    • Glottis
    • Subglottis
    • Trachea

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