The thyroid gland is an unpaired gland of internal secretion, located in the front of the neck and on both sides of the trachea below the thyroid cartilage. It has a horseshoe shape, consists of two unequal lobes, the right of which is larger. The left and right lobes are connected by an isthmus lying below the cricoid cartilage. In the absence of an isthmus, the lobes fit snugly against one another. Some people have a pyramidal lobe, going from the isthmus up. It can reach the upper notch of the thyroid cartilage and even the hyoid bone.
The front-side surfaces of the gland are covered with muscles – sternohyoid, sternum-thyroid, scapular-hyoid. On the border of the anterolateral and posterior medial surface of the gland, a neurovascular bundle of the neck adjoins to it on each side, and a recurrent laryngeal nerve passes along the posterior medial surface. The posteromedial surface of the gland is adjacent to the lateral surfaces of the upper rings of the trachea, to the pharynx and esophagus. The lower edges of both lobes of the thyroid gland are at the level of the V or VI tracheal ring, the isthmus of the gland lies at the level of I – III or II – IV tracheal rings.
The thyroid gland is very well provided with blood and in terms of blood supply is one of the first places in the body. Blood flows to it from the upper thyroid arteries, departing from the external carotid arteries, and the lower thyroid arteries, departing from the thyroid trunks. On the front surface of the gland, these arteries anastomose. Venous blood flows through the veins of the same name into the internal jugular vein.
Lymphatic circulation of the gland is provided through the vessels flowing into the paratracheal, deep cervical and mediastinal lymph nodes. The sympathetic innervation of the thyroid gland is carried out by the fibers going to it from the middle cervical sympathetic ganglion, parasympathetic – by the branches of the vagus nerve (upper larynx and return larynx).
The thyroid gland has a lobed structure. Its structural unit is a follicle – a round or oval closed vesicle, the walls of which are lined with secretory epithelium. The follicle cavity is filled with a homogeneous mass of yellow color, called a colloid, in which iodine-containing thyroglobulin protein is located.
In the thyroid parenchyma , 3 types of cells are distinguished: A, B, C. Cells A – thyrocytes, which make up the bulk, produce thyroid hormones. Biogenic amines, including serotonin, were detected in the cytoplasm of B cells. C-cells are parafollicular cells, they provide the synthesis of the hormone calcitonin, which is involved in the metabolism of calcium in the body, these cells are not able to absorb iodine.