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Thyroid Anatomy

The thyroid gland is a bilobed gland located in the anterior neck in front of the trachea (windpipe), and larynx (voice box). The thyroid is divided into right and left lobes, isthmus, and pyramidal lobe. The gland is very vascular. The thyroid gland has two major classes of cells. The follicular cells produce thyroid hormones, and the parafollicular cells produce Calcitonin. Thyroxin (T4) and triiodothyronine (T3) are the two major hormones produced by the thyroid follicular cells. These hormones are excreted into the bloodstream and play critical roles in regulating body metabolism. Excess thyroid hormone production results in hyperthyroidism and decreased or absent thyroid hormone production results in hypothyroidism. Calcitonin, produced by the parafollicular cells plays a role in calcium metabolism.

The gland is often difficult to feel in its normal state, but enlargement of the gland (goiter) or a thyroid mass may be palpable in the neck as a mass that elevates with swallowing. The thyroid gland is located near a number of vital structures that must be identified and protected during surgical procedures on the thyroid gland,

The recurrent laryngeal nerves lie just behind or occasionally attached to the thyroid gland. These nerves innervate the larynx, and are responsible for producing sound for speaking, opening the larynx for breathing, and closing the larynx during swallowing. Injury to one of these nerves generally produces a breathy and hoarse voice. In the unlikely event that both recurrent nerves are injured, the vocal cords cannot open, and there is limitation of ability to breathe. On the upper aspect of the thyroid gland, right and left superior laryngeal nerves travel near the thyroid gland, and also innervate the voice box. The external branch of the superior laryngeal nerve passes medial to the lobes of the thyroid gland from above and enters the larynx. This nerve serves to tense the vocal folds to help produce high pitch or loud voice, and injury to this nerve can result in loss of
ability to produce high pitch during singing, and decreased ability to yell.

The other important structure lying adjacent to, and occasionally within the thyroid glands are the parathyroid glands. There are normally four parathyroid glands, divided into two superior and two inferior glands. These glands share blood supply with the thyroid gland. The blood supply is delicate, and can be injured during thyroid surgery. This results in decreased parathyroid hormone production, and subsequent drop in calcium, which requires urgent treatment with calcium and sometimes vitamin D. This problem is generally temporary, but there is a risk of permanent hyperparathyroidism if all the glands are injured or removed during thyroid surgery.