Background: Function of the
What is the thyroid?
The thyroid gland is one of the body's most important endocrine organs. It is located in the neck just below the thyroid cartilage (Adam's apple) and cricoid cartilage. Although its size can vary, based on each individual's size and iodine intake, the thyroid generally weighs approximately 15-20 grams (1/2 - 3/4 oz). It is composed of a right and left lobe which lie on either side of the trachea (windpipe). Each lobe is normally approximately the size of a small orange slice. The right and left lobes are connected by a thin strip of thyroid tissue called the isthmusIsthmus - small piece of thyroid tissue that connects the right and left thyroid lobe. Adjacent structures include the esophagus (food pipe) and the carotid artery (within the carotid sheathCarotid sheath - comprised of carotid artery, internal jugular vein, and vagus nerve), which is the main blood supply to the head and neck.
Animation courtesy of coachsurgery.com
The thyroid gland has a rich blood supply made up of two main arteries on each side: the superior and the inferior thyroid arteries. The veins draining the thyroid gland tend to run along with the arteries. An additional major vein draining directly into the internal jugular vein is the middle thyroid vein. (Figure 1) The lymphatic drainage from the thyroid gland is to lymph nodes located near the trachea and esophagus. The lymphatic drainage carries extra fluid from the body back to the heart and are filtered through lymph nodes in the center part of the neck next to the thyroid (i.e. central neck nodes) and to the lymph node in the side of the neck along the jugular vein (i.e. lateral neck nodes). These lymph nodes become important in cases of thyroid cancer.
Figure 1: Thyroid vascular anatomy
Two nerves involved in speech run right behind each thyroid lobe on either side of the neck. The recurrent laryngeal nerves, which look like guitar strings, enter the voice box (larynx) near where the thyroid is closest to the trachea. These nerves move the vocal cords to control the voice. Injury to one nerve causes a whisper-type hoarseness. Injury to both recurrent laryngeal nerves can cause the airway to close down leading to difficulty breathing. The external branches of the superior laryngeal nerves cross above the top of the thyroid. They are hair-thin. They control the pitch and volume of the voice. Injury to both of these nerves would lower one's voice, and make raising the voice or yelling difficult.