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THYROID GLAND Embryology

The thyroid gland is formed from an ectodermal down-growth from the first and second pharyngeal pouches and descends to its final level just below the cricoid cartilage. The line of descent is known as the thyroglossal tract and, by the time of birth, this has normally involuted and disappeared. During descent, the thyroid gland comes in contact with the ultimo-branchial bodies from the fourth pharyngeal pouches and cells from these are thought to form the parafollicular, calci-tonin producing or 'C-cells' of the thyroid. On reaching its final position, the thyroid divides into two lobes with a central bridge or isthmus. The site of origin forms the foramen caecum of the tongue. Embryological abnormalities are:

• failure of descent - this may result in a lingual thyroid, an upper midline neck swelling, or a pyramidal lobe.

• cell nests in a persisting thyroglossal tract - these may result in a thyroglossal cyst.

• over descent resulting in a retrosternal goitre.

• agensis of one lobe - this may cause confusion on an isotope scan with a solitary nodule in one lobe and a cold area on the contralateral side.

Anatomy

The normal thyroid gland consists of two lateral lobes joined in the midline by a bridge of thyroid tissue or isthmus. The lobes extend from the level of the thyroid cartilage superiorly to the 5th or 6th tracheal rings inferiorly. The isthmus is variable in width and rests on the 2nd to 4th tracheal rings. Occasionally a pyramidal lobe extends from the isthmus superiorly and may reach as high as the hyoid bone (Figs 16.1 and 16.2).

The gland is encased in a sheath of pre-tracheal fascia which is attached to the arch of the cricoid cartilage and the oblique line of the thyroid cartilage. The thyroid therefore moves upwards on closure of the larynx on swallowing.

The anatomical relations of the thyroid are as follows: • Medially (from below upwards)

- trachea, oesophagus, and recurrent laryngeal nerve;

- cricoid cartilage;

- thyroid cartilage;

- cricothyroid and inferior constrictor muscles;

- external branch of the superior laryngeal nerve.

Levator Superior Common glandulae thyroideae thyroid artery carotid artery

Internal jugular vein

Omohyoid

Anterior jugular vein

Levator Superior Common glandulae thyroideae thyroid artery carotid artery

Internal jugular vein

Omohyoid

Anterior jugular vein

Isthmus, thyroid gland

Figure 16.1. Dissection of the front of the neck showing the position of the thyroid gland.

trunk thyroid vein

Figure 16.2. Anterior surface of the thyroid gland.

trunk thyroid vein

Figure 16.2. Anterior surface of the thyroid gland.

• Anteriorly

- sternohyoid and sternothyroid;

- anterior border of sternocleidomastoid.

• Postero-laterally

- the carotid sheath containing the common carotid artery, internal jugular vein and vagus nerve.

• Posteriorly

- cervical pre-vertebral muscles.

The relation to the parathyroid glands is very variable. The superior glands most commonly are adjacent to the intersection of the recurrent laryngeal nerve and the inferior thyroid artery, and the inferior glands are at the lower pole of the lobe or inferior to this.

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