Abstract
The thyroid gland comprises three distinct regions (Figure 1): • the isthmus, which overlies the second and third rings of the trachea • the lateral lobes, each of which extends from the side of the thyroid cartilage downwards to the sixth tracheal ring • an inconstant pyramidal lobe, which projects upwards from the isthmus, usually on the left side, and represents a remnant of the embryological descent of the thyroid. The gland is enclosed in the pretracheal fascia, covered by the strap muscles and overlapped by the sternocleidomastoids. The anterior Ellisjugular veins course over the isthmus. The strap muscles stretch and adhere to the thyroid when it enlarges so that, at operation, they may resemble thin layers of fascia. The larynx and trachea lie on the deep aspect of the thyroid, with the pharynx and oesophagus behind and the carotid sheath on either side. Two nerves are found in close relationship to the gland; the recurrent laryngeal nerve lies in the groove between the trachea and oesophagus, and the external branch of the superior laryngeal nerve lies deep to the upper pole and passes to the cricothyroid muscle.
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