The development of the thyroid is beyond the scope of this chapter but is discussed elsewhere in this book. Anatomy and development of these glands also are discussed elsewhere in this journal. Under the middle layer of deep cervical fascia, the thyroid has an inner true capsule, which is thin and adheres closely to the gland with continuations as septa into the gland, dividing it into lobes and lobules. Extensions of this capsule within the substance of the gland form numerous septae, which divide it into lobes and lobules. Follicles vary in size, depending upon the degree of distention, and they are surrounded by dense plexuses of fenestrated capillaries, lymphatic vessels, and sympathetic nerves.
Parafollicular cells lie adjacent to the follicles within the basal lamina.
The anterior suspensory ligament extends from the superior-medial aspect of each thyroid lobe to the cricoid and thyroid cartilage. This firm attachment of the gland to the laryngoskeleton is responsible for movement of the thyroid gland and related structures during swallowing. On its way to the larynx, the recurrent laryngeal nerve usually passes deep to the Berry ligament, or between the main ligament and its lateral leaf. Deep to the ligament, but lateral to the nerve, is a posteromedial portion of the thyroid lobe, which might be overlooked during thyroidectomy. The lateral surface of the thyroid is covered by the sternothyroid muscle, whose attachment to the oblique line of the thyroid cartilage prevents the superior pole from extending superiorly under the thyrohyoid muscle.