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The thyroid gland is a vital butterfly-shaped endocrine gland situated in the lower part of the neck. It is present in front and sides of the trachea, inferior to the larynx. It plays an essential role in the regulation of the basal metabolic rate (BMR), and stimulates somatic and psychic growth, besides having a vital role in calcium metabolism.
It is a gland consisting of two lobes, the right and the left lobes joined together by an intermediate structure, the isthmus. Sometimes a third lobe called the pyramidal lobe projects from the isthmus. It has a fibrous/fibromuscular band, i.e., levator glandulae thyroideae running from the body of the hyoid to the isthmus.<1> The lobes are 5 x 2.5 x 2.5 cm in dimension and weight around 25 gm. It extends from the fifth cervical to the first thoracic vertebrae. The lobes extend from the middle of the thyroid cartilage to the fifth tracheal rings. The isthmus is 1.2 x 1.2 cm in dimensions and extends from second to third tracheal rings. It grows larger in females during the period of menstruation and pregnancy.
The lobes are conical in shape and have an apex, a base, three surfaces – lateral, medial and posterolateral, and two borders – the anterior and posterior. The isthmus, however, has two surfaces – anterior and posterior and two borders – superior and inferior.
The lobes are related anteriorly to the skin, superficial and deep fascia, and platysma. Posteriorly, the lobes are associated with the laminae of the thyroid cartilage and tracheal rings, and laterally to the external carotid artery and internal jugular vein.
The thyroid gland is a richly vascular organ supplied by the superior and inferior thyroid arteries and sometimes by an additional artery known as thyroidea ima artery.<2> The venous drainage is by superior, middle, and inferior thyroid veins. Sometimes a fourth thyroid vein might be present called the vein of Kocher. The nerve supply is mainly from middle cervical ganglion, but also partly from superior and inferior cervical ganglions.
Two capsules completely cover the thyroid gland. The true capsule is made up of fibro-elastic connective tissue. The false capsule is made up of the pre-tracheal layer of deep cervical fascia. It consists of deep capillary plexus deep to the true capsule. Hence, it is crucial to remove the plexus with capsule during thyroidectomy.
Issues of Concern
Nerves Related to the Thyroid Gland
The thyroid gland is in very close proximity to two important nerves: the external branch of the superior laryngeal nerve and also the recurrent laryngeal nerve. Both are branches of the vagus nerve. During thyroidectomy, damage to these nerves leads to disability in phonation and/or difficulty in breathing. Injury to one of the branches of the superior laryngeal nerve leads to great difficulty in singing. Injury to the recurrent laryngeal nerve unilaterally may lead to hoarseness of the voice, and difficulty in breathing. Bilateral recurrent laryngeal nerve injury is serious and often necessitates the need for a tracheostomy.<3>
The thyroid gland is entirely covered by two capsules- a true and false. The thyroid gland consists of deep capillary plexus deeper to the true capsule. This physical arrangement makes it very crucial to remove the plexus with capsule during thyroidectomy.<4>
The thyroid gland is divided into lobules by the septae dipping from the capsule. The thyroid lobules consist of a large number of typical units called thyroid follicles.<5> The thyroid follicles are the structural and functional units of a thyroid gland. These are spherical, and the wall is made up of a large number of cuboidal cells, the follicular cells. These follicular cells are the derivates of the endoderm and secrete thyroid hormone. The circulating form of this hormone is thyroxine, which is tetraiodothyronine (T4) along with a small quantity of triiodothyronine (T3). Even though most of T4 later converts to the more active form T3, both affect the target cells with varying degrees of stimulation. These hormones help in regulating the BMR of the body. In between these thyroid follicles or within the wall of the thyroid follicles, we find the small C cells, also know as Parafollicular cells. These are derived from neural crest cells and secrete polypeptide hormone known as calcitonin. The calcitonin helps in depositing calcium and phosphate in skeletal and other tissues leading to hypocalcemia. This function is the opposite of the parathormone.
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These thyroid follicles act as storage compartments, filled with a substance called the colloid. This colloid is thyroglobulin, which is nothing but acidophilic secretory glycoprotein that is PAS-positive. These follicles are held together tightly within a delicate network of reticular fibers with an extensive capillary bed.