thyroid gland, endocrine gland, situated in the neck, that secretes hormones necessary for growth and proper metabolism. It consists of two lobes connected by a narrow segment called the isthmus. The lobes lie on either side of the trachea, the isthmus in front of it. Thyroid tissue is composed of millions of tiny saclike follicles, which store thyroid hormone in the form of thyroglobulin, a glycoprotein. Blood capillaries attached to the gland yield a constant supply of plasma. The protein thyroglobulin is the chief component of the jellylike substance, called colloid, that is secreted by the follicles. It attaches to the thyroid hormone for storage purposes; when the hormone is ready to be released, the protein detaches itself. Before it is released into the bloodstream, the thyroid hormone is converted into thyroxine and small quantities of the other closely related thyroid hormones. The amount of thyroxine production (and therefore the metabolic rate) is dependent on a sufficient intake of iodine and on stimulation by thyroid-stimulating hormone (TSH) from the pituitary gland. Metabolic disorders result when the thyroid secretes too little or too much thyroxine. Deficiencies in thyroid secretion (hypothyroidism) occur when there is insufficient iodine in the diet. A disease known as goiter results from the deficiency, although it has been virtually eliminated by the use of iodized salt. Hypothyroidism that results from glandular malfunction is known as myxedema in the adult and cretinism in infancy and childhood. Treatment is by administration of thyroxine. Excessive secretion of thyroxine, or hyperthyroidism, causes an increased metabolic rate, loss of weight despite good appetite, protrusion of the eyeballs, rapid pulse, and irritability. The condition, also known as Graves' disease, may be accompanied by enlargement of the thyroid. The thyroid gland also produces the hormone calcitonin, which is involved in the regulation of serum calcium in the body. See also endocrine system.