Thyroid Gland (Gr. θνρεός, a shield, and είδος, form), one of the vascular or ductless glands, situated on the anterior and lower part of the larynx, in front of the upper rings of the trachea; so called from its being situated in front of the thyroid or shield-like cartilage of the larynx. It is composed of two elongated ovoid lobes, flattened from before backward, united or separate, but generally connected by a transverse portion; it is covered by the muscles of the front of the neck. The tissue is tolerably firm, brownish and yellowish red, formed of lobes and lobules, consisting essentially of an aggregation of closed gland follicles imbedded in condensed areolar tissue; these contain a small amount of a fatty albuminous fluid, and do not communicate with any common reservoir; among the follicles are nucleated corpuscles or epithelial cells; it receives four arteries from the subclavians and the carotids, nerves from the pneumogastric and the sympathetic, and lymphatics communicating with the glands of the neck.

The vascular supply is great, and forms a very minute capillary plexus on the membrane of the follicles; like other ductless glands (see Thymus Gland), it is relatively larger in intra-uterine existence and in infancy than in after life. Its products are probably discharged into the venous blood, and serve for the elaboration of the circulating fluid. It is usually larger in females than in males; it is found in all mammals, birds, and reptiles, probably in the batrachians, and perhaps also in fishes. The organ may be inflamed, with suppuration, and variously enlarged. In the adult it is sometimes abnormally enlarged, forming the disease known as goitre or bronchocele, which is itself often an accompaniment of cretinism. (See Goitre).