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Thyroidectomy – Partial or Total
Goals/Objectives
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Anatomy
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Workup of Thyroid Nodule
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Technique
Thyroid
From Townsend CM: Sabiston Textbook of Surgery, 19th edition (Saunders 2012)
Historical Perspective
The name thyroid is derived from the Greek description of a shield-shaped gland in the anterior aspect of the neck ( thyroides ). Classic anatomic descriptions of the thyroid were available in the 16th and 17th centuries, but the function of the gland was not well understood. By the 19th century, pathologic enlargement of the thyroid, or goiter, was described. Iodine-rich seaweed was used to treat this condition. Direct surgical approaches to thyroid masses had frighteningly high complication and mortality rates.
In the late 19th century, two surgeon-physiologists revolutionized the understanding and treatment of thyroid diseases. Theodor Billroth and Emil Theodor Kocher established large clinics in Europe and, through the development of skilled surgical techniques combined with newer anesthetic and antiseptic principles, provided surgical results that proved the safety and efficacy of thyroid surgery for benign and malignant problems. As a result of his pioneering developments in the understanding of thyroid physiology, Kocher received the Nobel Prize in 1909.
The 20th century began with the contributions of Kocher and Billroth. In rapid succession, the understanding of altered physiology, including hypothyroidism, hyperthyroidism, and thyroid cancer, and advances in imaging, epidemiology and, most recently, minimally invasive diagnostic and surgical techniques, have taken place. These advances have allowed the diagnosis and treatment of thyroid diseases to become rapid, cost-effective, and low-morbidity procedures.
Anatomy
Embryology
The tissue bud that ultimately becomes the thyroid gland arises initially as a midline diverticulum in the floor of the pharynx. This tissue originates in the primitive alimentary tract and consists of cells of endodermal origin. The main portion of this cellular structure descends into the neck and develops into a bilobed solid organ. The original attachment in the pharynx is in the buccal cavity at the foramen cecum. This structure becomes the thyroglossal duct, which is usually reabsorbed after 6 weeks of age. The very distal end of this remnant may be retained and mature as a pyramidal lobe in the adult thyroid.
Microscopic thyroid follicles first appear as the lateral lobes develop. When the embryo is about 6 cm in length, these follicles begin to develop colloid. In the third month, the follicular cells first demonstrate iodine trapping and thyroid hormone secretion begins. Calcitonin-producing C cells arise from the fourth pharyngeal pouch and migrate from the neural crest into the lateral aspects of the thyroid lobes. These cells migrate into the lateral and posterior upper two thirds of the thyroid lobes and are distributed among the follicles. In adults, they remain limited to the upper and middle areas of the gland, usually in the posterior and medial aspects. These C cells are the only component of the adult gland that is not of endodermal origin.
Knowledge of basic embryology is essential for understanding certain embryologic congenital malformations, including thyroglossal duct cysts and fistulae, which result from retained tissue along the path of descent of the thyroid. A lingual thyroid is another anomaly that occurs when the median thyroid anlage does not descend in a normal fashion. In unusual circumstances, ectopic thyroid tissue can be found in the central compartment of the neck. Small amounts of ectopic tissue may be located under the lower poles of a normal thyroid and, occasionally, in the anterior mediastinum. Historically, the thyroid tissue described in lateral neck compartments was known as lateral aberrant thyroid tissue and was explained as an embryologic variation. This concept has essentially been disproved, and it is believed that thyroid tissue found in the neck lateral to the jugular vein represents metastatic deposits from well-differentiated thyroid carcinoma, typically papillary cancer, and may be the initial presentation of this disease. Small thyroid follicles located at the periphery of central neck lymph nodes may occasionally occur in the absence of thyroid cancer.
Adult Surgical Anatomy
A normally developed adult thyroid weighs 10 to 20 g; it is a bilobed structure that lies next to the thyroid cartilage in a position anterior and lateral to the junction of the larynx and trachea. In this position, the thyroid encircles approximately 75% of the diameter of the junction of the larynx and upper part of the trachea. The lobes lie lateral to the trachea and esophagus, anteromedial to the carotid sheath and posteromedial to the sternocleidomastoid, sternohyoid, and sternothyroid muscles. The two lateral lobes are joined at the midline by an isthmus, whose superior edge is situated at or just below the cricoid cartilage. A pyramidal lobe is present in about 30% of patients and represents the most distal portion of the thyroglossal duct. In an adult, it may be a prominent structure that can extend from the midline of the isthmus as far cephalad as the hyoid bone.
A thin layer of connective tissue surrounds the thyroid. This tissue is part of the fascial layer that invests the trachea. This fascia is different from the thyroid capsule and, during surgery, can easily be separated from the capsule, whereas the true capsule of the thyroid cannot. This fascia coalesces with the thyroid capsule posteriorly and laterally to form a suspensory ligament known as the ligament of Berry, which is the primary point of fixation of the thyroid to surrounding structures. The ligament of Berry is closely attached to the cricoid cartilage and has important surgical implications because of its relationship to the recurrent laryngeal nerve.
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