Thyroid Anatomy

The thyroid gland is located in the neck and plays a vital role in regulating the body’s metabolism. It sits in front of both the larynx, or voice box, and the trachea, or windpipe. It usually cannot be felt when it is in a healthy state.

The two parts, or lobes, of the thyroid gland are called the isthmus lobe and the pyramidal lobe. One sits to the left and one to the right of the mid-line of the neck. These lobes contain two types of cells that secrete substances that affect metabolism.

Thyroid Cells

The follicular cells produce thyroxin, known as T4, and triiodothyronine, known as T3. T4 and T3 are both hormones that are released into the bloodstream in regulated amounts. Too much of the hormones produces a condition known as hyperthyroidism while too little causes hypothyroidism.

The second type of cell found in the thyroid gland is the parafollicular cell. Parafollicular cells release calcitonin, which specifically affects the body’s use of calcium.

Thyroid Anatomy UCI Head Neck 1 - Thyroid Anatomy
Thyroid Anatomy UCI Head Neck 3 - Thyroid Anatomy

Thyroid Problems

When the thyroid gland becomes diseased, it may be possible to palpate it through the neck. An enlargement due to goiter or a mass can often be seen as a lump that rises every time the person swallows.

Surgical procedures on the thyroid gland are complicated by the fact that the gland itself is highly vascularized. It is also surrounded by other glands and nerves that could be damaged during surgery. Care must be taken to identify and protect these structures.

Laryngeal Nerves

The laryngeal nerves are among the structures in the neck that must be protected. The recurrent laryngeal nerves are found just behind the thyroid or even attached to it. These nerves lead to the larynx. They control when the larynx opens to allow breathing and when it closes for swallowing. They also enable the production of sound for speech. If one recurrent laryngeal nerve is damaged, the result is usually a hoarse or breathy voice. If both nerves are damaged, the vocal cords may be unable to open, causing respiratory difficulties.

The right and left superior laryngeal nerves are found just above the thyroid gland and pass very close to it. The superior laryngeal nerve has an external branch that travels medial to the thyroid gland’s lobes from above, ending at the voice box. This external branch is responsible for tightening the vocal folds when a loud or high-pitched sound is desired. If this branch is damaged, the result can be an inability to sing at high pitches or to shout.

Parathyroid Glands

Besides nerves, there are also other glands adjacent to the thyroid. These are the four parathyroid glands, which are formed of two superior and two inferior divisions. The thyroid shares a blood supply with the parathyroids. If this delicate vascular structure is injured during surgery it can interfere with the production of parathyroid hormone. This causes a significant drop in calcium that must be urgently treated with the administration of calcium and perhaps vitamin D. This condition is usually temporary, but if all the parathyroid glands are damaged or removed during surgery, it can result in permanent hyperparathyroidism.

Thyroid Anatomy UCI Head Neck 2 - Thyroid Anatomy