Thyroid dysgenesis is the term used to describe abnormalities of thyroid gland development and includes a spectrum of conditions including thyroid agenesis (complete absence of thyroid tissue), thyroid hypoplasia (less than the normal amount of thyroid tissue), and thyroid ectopia (thyroid tissue in abnormal locations). Agenesis of the thyroid gland is a very rare anomaly and like thyroid hypoplasia is a cause of congenital hypothyroidism that must be recognized and treated with thyroid hormone replacement in early infancy to prevent neurodevelopmental compromise. One form of hypoplasia is hemiagenesis when one lobe does not develop. In this rare condition the left lobe is more commonly absent.

A thyroglossal duct cyst is a remnant of the diverticulum that forms along the migration path of the thyroid primordium from the foramen caecum at the base of the tongue to the thyroid gland’s final position in front of the trachea. This diverticulum usually begins to involute during the 5th week of development but incomplete involution may result in ectopic thyroid tissue that is typically located in the midline or just off the midline between the hyoid bone and the thyroid isthmus (Fig. 1.2). Thyroglossal duct cyst is the most common congenital neck mass found in children and it can also present later in life in the same location. Thyroglossal duct cysts may enlarge, become infected or rarely undergo malignant transformation. The treatment of a thyroglossal duct cyst is surgical excision of the cyst and (to reduce the chance of recurrence) the entire course of the thyroglossal duct including the middle third of the hyoid bone. This is known as the Sistrunk procedure [5].

Failure of the thyroid migration results in a lingual thyroid which is the most common form of complete thyroid ectopia. Lingual thyroid is a relatively rare condition that is more prevalent in women and in the majority of cases it is associated with an absence of a normal cervical thyroid. It is generally presents incidentally as an asymptomatic mass at the back of the tongue (Fig. 1.2). If large, a lingual thyroid can produce obstructive symptoms such as dysphagia, dysphonia, or dyspnoea. When a lingual thyroid is suspected based by clinical findings the diagnosis can be confirmed by radioisotope scanning which also confirms the absence of other thyroid tissue in the neck.

The recurrent laryngeal nerve (RLN) is a branch of the vagus nerve going to the larynx and its final path is linked to developmental changes in the aortic arches. By the 6th–7th week, the lowest persisting aortic arch on each side pulls the respective RLN downwards. On the left, the RLN passes around the 6th aortic arch which in the foetus becomes the ductus arteriosus that normally closes postnatally and becomes the ligamentum arteriosum. It then travels upwards in the tracheo-oesophageal groove. On the right, the 5th and 6th aortic arches disappear and the RLN passes around the 4th aortic arch which becomes the right subclavian artery. From this position the right RLN then ascends more obliquely than the left RLN (Fig. 1.3). As a consequence, the left RLN is longer than the right RLN and, during intraoperative nerve monitoring there is a slightly longer latency of the signal obtained during stimulation of the vagus on the left side compared with the right side.

An embryological vascular anomaly (present in 0.5–1% of people) leads to a non-recurrent laryngeal nerve and illustrates the close connection between the developing RLN and aortic arches. When the 4th aortic arch on the right does not develop an aberrant right subclavian artery originates from the left side of the aortic arch, distal to the origins of the right carotid, left carotid and left subclavian artery. From this origin it passes upwards and to the right behind the oesophagus. Occasionally this retro-oesophageal location is associated with esophageal compression and dysphagia explaining its designation as arteria lusoria. Since there is no embryological right aortic arch to pull the right RLN downwards the nerve goes straight from the vagus into the larynx and is therefore “non-recurrent” (Fig. 1.3). This unusual vascular anomaly can be demonstrated on Doppler ultrasound, which in some centres is part of routine preoperative investigation before thyroid surgery. The use of intraoperative neuromonitoring may also identify patients with this anatomical anomaly [6] when stimulation of the distal vagus nerve does not produce a RLN signal while stimulation proximal to the origin of the non-recurrent RLN generates the correct signal [7]. The diagnosis can be confirmed postoperatively by ultrasound identification of the vascular anomaly or by a barium swallow showing the retroesophageal artery as an extrinsic compression of the posterior oesophagus. On the left side the non-recurrent RLN is exceptionally rare with only a few cases reported. A more common finding is a false non-recurrent RLN which is seen when a large anastomotic branch of the cervical sympathetic chain joins a normal RLN.