THYROID GLAND

THYROID GLAND, an endocrine, or ductless, gland found in vertebrate (backboned) animals. The hormones produced by the thyroid gland influence reproduction, growth, differentiation, and metabolism; they are also believed to be important in the activation of migration in salmon. In humans the major role of thyroid hormones is the regulation of the body's metabolic processes, including the consumption of calories and oxygen. Excessive amounts of thyroid hormone speed up the metabolic rate; deficient quantities lower it.

The thyroid gland differs structurally among the various vertebrate groups. In birds, for example, it consists of two small masses in the neck region, whereas in most fishes it consists of small cell clusters (follicles) scattered through the pharyngeal region behind the mouth. In humans the thyroid gland is a firm, butterfly-shaped mass situated in the neck just below the larynx (voice box). The two "wings," called lobes, rise upward on each side of the trachea (windpipe) and are usually the size of flattened peach pits. The lobes are connected by a narrow segment called the isthmus, which extends across the front of the trachea.

Hormone Production. The thyroid gland actively takes up iodine from the bloodstream and also synthesizes, or manufactures, a protein molecule called thyroglobulin, which contains numerous units (residues) of the amino acid tyrosine. It then combines the iodine with the tyrosine residues, and, finally, pairs molecules of the now-iodinated tyrosines to form the thyroid hormones, either triiodothyronine (T3) or tetraiodothyronine (T4), the latter more commonly known as thyroxine. Degradation of the thyroblogulin by enzymes releases the thyroid hormones into the bloodstream. T4 is the most abundant form of thyroid hormone in the blood. It is two-thirds iodine by weight and is produced only in the thyroid. T3 contains one less atom of iodine but is ten times more potent than T4. Although some T3 is released from the thyroid gland, it is mostly derived by the removal of an iodine atom from T4 by other body tissues, particularly the liver and kidneys.

The amount of hormone produced by the thyroid is normally regulated by a feedback system involving thyroid stimulating hormone (TSH), produced by the pituitary gland, and thyroid hormone. An increase in TSH stimulates the thyroid to make and release more hormone, while an increase in thyroid hormones suppresses the production and release of TSH by the pituitary.

A third thyroid hormone, calcitonin, plays a role in regulating the level of calcium in the blood.

Clinical Abnormalities. In much of the world the usual diet provides a supply of iodine adequate to maintain normal thyroid hormone production. In the early 1900's the central United States and the Swiss Alps were regions of iodine deficiency and goiter, but the use of iodized salt eliminated this problem. In some mountainous areas of the world iodine deficiency remains a public health problem.

Hypothyroidism, or myxedema, results from an inadequate production of thyroid hormone. The thyroid gland may be enlarged (goiter) or entirely absent. The condition occurs more frequently in women than in men, and it is often the consequence of damage to the thyroid gland by the body's immune system. Symptoms may include drowsiness and difficulty in tolerating cool temperatures. In severe cases coma and even death may occur. Hypothyroidism has been treated with dried preparations of animal thyroid glands and, more recently, with tablets of synthetic T4.

Hyperthyroidism, or thyrotoxicosis, results from an excessive release of thyroid hormone. The thyroid gland is usually enlarged to three or four times the normal size. The condition is most often the result of the production of antibodies, normally protective immune substances, that stimulate the thyroid to grow and produce hormone. Symptoms may include nervousness and difficulty in tolerating warm temperatures. Retraction of the eyelids makes the individual appear to be staring, and swelling of tissues behind the eyes may cause the eyes to protrude. Hyperthyroidism may be treated with radioactive iodine, which is taken up by the gland and destroys cells by irradiation; with drugs that suppress hormone production; or by surgical removal of most of the gland.

Thyroid cancer, or thyroid carcinoma, is usually cured by surgery, either by itself or in combination with radioactive iodine. Cancer of the thyroid is more common in individuals with a history of radiation to the head and neck.

Diagnostic Tests. In the past, tests of the basal metabolic rate (BMR) were used to diagnose thyroid disorders, but now diagnosis is usually based on the measurements of T3, T4, and TSH in the blood. In hypothyroidism, thyroid hormone levels are low and the TSH is usually elevated. In hyperthyroidism, thyroid hormone levels are high and the TSH is low. Further, the uptake of radioactive iodine is usually decreased in hypothyroidism, but increased in hyperthyroidism.

David Sarne