Histology, Thyroid Gland

Excerpt

The thyroid gland is a vital butterfly-shaped endocrine gland situated in the lower part of the neck. It is present in the front and sides of the trachea, inferior to the larynx. It plays an essential role in regulating 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. The lobes are 5 x 2.5 x 2.5 cm in dimension and weigh 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 ring. 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 the thyroidea ima artery. 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 comprises the pre-tracheal layer of the 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.