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Thyroid gland

Other Terms: Glandula thyroidea, Glande thyroïde, Schilddrüsedrüse, Glándula de tiroides

Description

The thyroid gland is a highly vascular organ, situated at the front and sides of the neck; it consists of right and left lobes connected across the mid line by a narrow portion, the isthmus. Its weight is somewhat variable, but is usually about 30 grams. It is slightly heavier in the female, in whom it becomes enlarged during menstruation and pregnancy. The lobes are conical in shape, the apex of each being directed superior and lateral as far as the junction of the middle with the lower third of the thyroid cartilage; the base looks inferior, and is on a level with the fifth or sixth tracheal ring. Each lobe is about 5 cm. long; its greatest width is about 3 cm and its thickness about 2 cm. The lateral or superficial surface is convex, and covered by the skin, the superficial and deep fasciae, the Sternocleidomastoid, the superior belly of the Omohyoid, the Sternohyoid and Sternothyreoid, and beneath the last muscle by the pretracheal layer of the deep fascia, which forms a capsule for the gland. The deep or medial surface is molded over the underlying structures, thyroid and cricoids cartilages, the trachea, the inferior pharyngeal constrictor and posterior part of the Cricothyreoideus, the esophagus (particularly on the left side of the neck), the superior and inferior thyroid arteries, and the recurrent nerves. The anterior border is thin, and inclines obliquely from inferior toward the mid line of the neck while the posterior border is thick and overlaps the common carotid artery, and as a rule the parathyroids. The isthmus connects together the lower thirds of the lobes; it measures about 1.25 cm. in breadth, and the same in depth, and usually covers the second and third rings of the trachea. Its situation and size present , however, many variations. In the middle line of the neck it is covered by the skin and fascia, and close to the mid line, on either side, by the Sternothyreoid. Across its superior border runs an anastomotic branch uniting the two superior thyroid arteries; at its inferior border are the inferior thyroid veins. A third lobe of conical shape, called the pyramidal lobe, frequently arises from the upper part of the isthmus, or from the adjacent portion of either lobe, but most commonly the left, and ascends as far as the hyoid bone. It is occasionally quite detached, or may be divided into two or more parts. A fibrous or muscular band is sometimes found attached, superior, to the body of the hyoid bone, and inferior to the isthmus of the gland, or its pyramidal lobe. Small detached portions of thyroid tissue are sometimes found in the vicinity of the lateral lobes or above the isthmus; they are called accessory thyroid glands.

Structure

The thyroid gland is invested by a thin capsule of connective tissue, which projects into its substance and imperfectly divides it into masses of irregular form and size. When the organ is cut into, it is of a brownish-red color, and is seen to be made up of a number of closed vesicles, containing a yellow glairy fluid, and separated from each other by intermediate connective tissue. The vesicles of the thyroid of the adult are generally closed spherical sacs; but in some young, the vesicles are more or less tubular and branched. This appearance is supposed to be due to the mode of growth of the gland, and merely indicates that an increase in the number of vesicles is taking place. Each vesicle is lined by a single layer of cubical epithelium. There does not appear to be a basement membrane. So that the epithelial cells are in direct contact with the connective-tissue reticulum which supports the acini. The vesicles are of various sizes and shapes, and contain as a normal product a viscid, homogeneous, semi fluid, slightly yellowish, colloid material; red corpuscles are found in it in various stages of disintegration and decolorization, the yellow tinge being probably due to the hemoglobin, which is thus set free from the colored corpuscles. The colloid material contains an iodine compound, iodothyrin. The thyroid gland prepares and secretes into the vascular channels a substance, formed under normal conditions in the outer pole of the cell and excreted from it directly without passing by the indirect route through the follicular cavity. In addition to this direct mode of secretion there is an indirect mode which consists in the condensation of the secretion into the form of droplets, having high content of solids, and the extension of these droplets into the follicular cavity. These droplets are formed in the same zone of the cell as that in which the primary or direct secretion is formed. The hormones secreted are primarily thyroxine(T4), triiodothyronine(T3). These hormones are very important in the rate of metabolism.

Vessels and Nerves

The arteries supplying the thyroid gland are the superior and inferior thyroids from the arch of the aorta, which ascends upon the front of the trachea. The arteries are remarkable for their large size and frequent anastomoses. The veins form a plexus on the surface of the gland and on the front of the trachea; from this plexus the superior, middle, and inferior thyroid veins arise; the superior and middle end in the internal jugula. The capillary blood vessels form a dense plexus in the connective tissue around the vesicles, between the epithelium of the vesicles and the endothelium of the lymphatics, which surround a greater or smaller part of the circumference of the vesicle. The lymphatic vessels run in the interlobular connective tissue, not uncommonly surrounding the arteries which they accompany, and communicate with a net-work in the capsule of the gland; they may contain colloid material. They end in the thoracic and right lymphatic trunks.

Innervation

The nerves are derived from the middle and inferior cervical ganglia of the sympathetic.

Latin

Glandula thyroidea

French

Glande thyroïde

German

Schilddrüsedrüse

Spanish

Glándula de tiroides

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