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 middle 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 thyroid gland controls how quickly the body uses energy, makes proteins, and controls how sensitive the body is to otherhormones. It participates in these processes by producing thyroid hormones, the principal ones being triiodothyronine (T3) and thyroxine (sometimes referred to as tetraiodothyronine (T4)). These hormones regulate the growth and rate of function of many other systems in the body. T3 and T4 are synthesized from iodine and tyrosine. The thyroid also produces calcitonin, which plays a role in calcium homeostasis.
Hormonal output from the thyroid is regulated by thyroid-stimulating hormone (TSH) produced by the anterior pituitary, which itself is regulated by thyrotropin-releasing hormone (TRH) produced by the hypothalamus.
The lobes (lobuli gl. thyreoideæ) are conical in shape, the apex of each being directed upward and lateralward as far as the junction of the middle with the lower third of the thyroid cartilage; the base looks downward, 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 isthmus (isthmus gl. thyreoidea) 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 middle line, on either side, by the Sternothyreoideus. Across its upper border runs an anastomotic branch uniting the two superior thyroid arteries; at its lower border are the inferior thyroid veins. Sometimes the isthmus is altogether wanting.
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, above, to the body of the hyoid bone, and below to the isthmus of the gland, or its pyramidal lobe. When muscular, it is termed the Levator glandulæ thyreoideæ.
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 (glandulæ thyreoideæ accessoriæ).
Development.—The thyroid gland is developed from a median diverticulum, which appears about the fourth week on the summit of the tuberculum impar, but later is found in the furrow immediately behind the tuberculum. It grows downward and backward as a tubular duct, which bifurcates and subsequently subdivides into a series of cellular cords, from which the isthmus and lateral lobes of the thyroid gland are developed. The ultimo-branchial bodies from the fifth pharyngeal pouches are enveloped by the lateral lobes of the thyroid gland; they undergo atrophy and do not form true thyroid tissue. The connection of the diverticulum with the pharynx is termed the thyroglossal duct; its continuity is subsequently interrupted, and it undergoes degeneration, its upper end being represented by the foramen cecum of the tongue, and its lower by the pyramidal lobe of the thyroid gland.
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 animal are generally closed spherical sacs; but in some young animals, e. g., young dogs, 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, semifluid, 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, and is readily stained by eosin.
Vessels and Nerves: