Thyroid Gland Anatomy

Thyroid gland (Glandula thyroidea) is an endocrine organ located along the midline of the anterior neck responsible for producing hormones that regulate metabolism growth and serum calcium concentrations. In adults the gland typically weighs about 25 grams and has a volume of approximately 6.6 ml. It is situated between the C5 and T1 vertebral levels. Shaped like a butterfly the gland lies anterior to the trachea beneath the skin and is not visible externally in healthy individuals. As part of the endocrine system it secretes thyroxine (T4) and triiodothyronine (T3) through follicular cells and calcitonin through parafollicular cells (C cells).

The thyroid gland consists of two symmetrical lobes the right (lobus dexter) and left (lobus sinister) connected by a midline structure called the isthmus. The isthmus lies anterior to the second and third tracheal cartilaginous rings. Each lobe is conical in shape extending from its base at the level of the fourth or fifth tracheal ring to its apex at the oblique line of the thyroid cartilage. The dimensions of each lobe are approximately 5 cm in length 3 cm in width and 2 cm in anteroposterior diameter. In approximately 28 to 55 percent of individuals a variation called the lobus pyramidalis extends superiorly from the left side of the isthmus; this structure is a remnant of the ductus thyroglossus. The gland is surrounded by a fibrous capsule (capsula fibrosa) that sends septa inward dividing the gland into lobules. The pretracheal layer of the fascia cervicalis profunda covers the outer surface of the capsule and anchors the thyroid to the cricoid and thyroid cartilages; due to this anatomical connection the thyroid gland moves with the larynx during swallowing.

The thyroid gland is located in the visceral compartment of the neck posterior to the sternothyroidandsternohyoid muscles(strap muscles). The anterior surface of the gland is covered by these muscles while the medial surfaces of the lobes are adjacent to the larynxtracheapharynx and esophagus. The posterolateral surface is in close relation with the vagina carotica (carotid sheath) which contains the arteria carotis communis the vena jugularis interna and the nervus vagus. Of surgical importance the nervus laryngeus recurrens (RLN) runs in the groove between the trachea and esophagus and has a close relationship with the posteromedial surface of the thyroid gland. Additionally the parathyroid glands are located on the posterior surface of the thyroid between the capsule and the fascia.

_{Representative image showing the thyroid gland and its relationships. (Generated by artificial intelligence.)}

Histologically the thyroid tissue consists of thousands of follicles lined by a single layer of epithelium and filled with a gelatinous substance called colloid. Follicular cells (thyrocytes) are the primary functional units responsible for synthesizing and storing the thyroid hormones T3 and T4. The shape of the follicular epithelium varies from squamous to cuboidal or columnar depending on the functional activity of the gland; in active states the epithelial height increases. In addition to follicular cells parafollicular cells (C cells) are found either interspersed among the follicular epithelium or in clusters between follicles. These cells are larger and stain more lightly than follicular cells and synthesize calcitonin which lowers blood calcium levels.

_{Image showing the histological structure of the thyroid gland. (Generated by artificial intelligence.)}

The thyroid gland is one of the most highly vascularized organs in the body and receives its arterial supply primarily from two pairs of major vessels. The arteria thyroidea superior arises as the first branch of the arteria carotis externa and supplies the superior pole of the gland. The arteria thyroidea inferior originates from the truncus thyrocervicalis a branch of the arteria subclavia and ascends to supply the inferior portion of the gland. In approximately 10 percent of the population a variant artery called the arteria thyroidea ima may arise from the brachiocephalic trunk or the aortic arch and supply the isthmus region.

Venous drainage occurs through three pairs of veins: the vena thyroidea superior and media drain into the internal jugular vein while the vena thyroidea inferior typically drains into the brachiocephalic vein. Lymphatic drainage proceeds toward the prelaryngeal pretracheal paratracheal and deep cervical lymph nodes.

The innervation of the thyroid gland is provided by the autonomic nervous system. Sympathetic fibers originate from the superior middle and inferior cervical ganglia while parasympathetic fibers reach the gland via the nervus vagus. The primary function of these nerves is not to directly control hormone production or secretion but to regulate the tone of the gland’s blood vessels (vasomotor function). Of critical importance in surgical anatomy the nervus laryngeus recurrens (RLN) and nervus laryngeus superior run in close proximity to the arteries supplying the thyroid (inferior and superior thyroid arteries respectively).