THYROID GLAND (from Gk. OupeouS77r, thyreocides, shield-shaped, from OvE6s, thyrcos, large oblong shield, from 06,9a, thyru, door + clam., chins, form). One of the ductless or vas cular glands, lying at the upper part of the trachea, and consisting of two lateral lobes, placed one on each side of this canal, and con nected together by a narrow transverse portion at the lower third, called the isthmus. It is of a brownish red color, and its normal weight is about an ounce. Each lobe is somewhat conical, and is about two inches long and three-quarters of an inch broad. Ninnte examination of the gland shows it to consist of lobes and lobules held together by connective tissue. Each lobule is made up of numerous completely closed sacs 0.04 to 0.1 inch in diameter. The sacs contain a transparent, viscid, albuminous fluid. Each sac is surrounded by a plexus of capillary blood vessels which do not pierce the lining of the sacs.
Numerous lymphatic vessels are also present. At an early period the sacs dilate, their cellular lining atrophies, and their contents undergo col loid degeneration. The function of the thyroid gland is very obscure. It is possibly a blood forming gland; possibly it regulates the forma tion of mucus in the body. It is subject to sev eral varieties of enlargement known under the general name of goitre, and is also the seat of various benign and malignant tumors. Myx oedema (q.v.) is a condition of the system brought about through loss of function of the thyroid gland, either from atrophy or removal of the organ. The prepared extract of the thy roid gland of animals is used in the treatment of this condition. See CRETINISM; ORGANOTISERAPY.