Iodine, the fourth member of the halogen family and a blue-black solid at room temperature is a chemically reactive element of the periodic table that was first isolated from seaweed residues in 1811. The nature of the element was further established and the element so named in 1813. This lustrous, blue-black, soft substance sublimes when heated, giving off a violet vapor with a stinging odor like that of chlorine. The element’s name comes from its color; as iodes is a Greek word for violet. The vapor rapidly condenses again on a cold surface. The only isotope that occurs in nature is stable, but several radioactive ones have been produced artificially. Iodine, like all halogens, is chemically active and in pure form, it is poisonous. Though it is only slightly soluble in water and soluble in some organic reagents, it dissolves readily in an aqueous solution of potassium iodide.

 With seven electrons in the outer shell of its atom, iodine has several oxidation states, of which the main ones are -1, +1, +3, +5, and +7. It combines readily with most metals to form iodides, and it also combines with other halides. Reactions with oxygen, nitrogen, and carbon are accomplished with more difficulty. Iodine is a relatively rare element, ranking about 62nd in abundance on Earth, but its compounds are widespread in seawater, soil, and rocks. Iodine is obtained from brines and from Chilean nitrate ores in which it occurs as an impurity. To a lesser extent, iodine is also derived from sea organisms, such as brown seaweeds, that concentrate iodine in their tissues. Iodine is medicinally very important because it is an essential trace element; present in the hormone that is involved in growth-controlling and other metabolic functions. Without iodine, stunted and awful growth can result. Thus in areas where iodine is not sufficiently abundant naturally, iodine-containing salt serves to make up the deficit. In medicine, iodine-alcohol solutions and iodine complexes have been used as antiseptics and disinfectants. Radioisotopes of iodine are used in medical and other fields of research. More broadly, various iodine compounds find use in photography, the making of dyes, and cloud-seeding operations. In chemistry, various iodine compounds serve as strong oxidizing agents, among other uses.

 

Thyroid Gland is the endocrine gland found in almost all vertebrate animals and so called because it is located in front of and on each side of the thyroid cartilage of the larynx. The thyroid gland in human beings is a brownish-red organ having two lobes connected by an isthmus; it normally weighs about 28 g (about 1 oz) and consists of cuboidal epithelial cells arranged to form small sacs known as vesicles or follicles. The vesicles are supported by connective tissue that forms a framework for the entire gland. In the normal thyroid gland, the vesicles are usually filled with a colloid substance containing the protein thyroglobulin in combination with the two thyroid hormones tetraiodothyronine (T4), and triiodothyronine (T3). These hormones are composed of the amino acid tyrosine, containing four and three iodine atoms, respectively.

The amount of thyroglobulin secreted by the thyroid is controlled by the thyroid-stimulating hormone (TSH which, in turn, is regulated by a substance called thyroid-stimulating hormone releasing factor (TRF) that is secreted by the hypothalamus. Thyroglobulin is especially rich in iodine. Although the thyroid gland constitutes about 0.5 percent of the total human body weight, it holds about 25 percent of the total iodine in the body, which is obtained from food and water in the diet. Iodine usually circulates in the blood as an inorganic iodide and is concentrated in the thyroid to as much as 500 times the iodide level of the blood.

Many different laboratory tests, including direct measurement of thyroxine and triiodothyronine, are used to test the activity of the thyroid gland. Thyroid scanning with radioiodine or technetium-99m is especially useful for detecting or ruling out cancer of the thyroid in persons who have a palpable nodule, or thyroid lump. In most cases thyroid cancers are slow-growing and not fatal. The thyroid gland appears to be quite sensitive to irradiation: during the 1970s an increased incidence of thyroid cancer was found among people who had been treated early in life with X-rays for such conditions as acne, ringworm, and tonsillitis.

Excessive production of thyroid hormones, called hyperthyroidism or Graves’ disease, results in elevated metabolism and activity. Sometimes this condition is associated with abnormalities of the eye, including bulging eyes. The usual treatment is to administer an antithyroid drug, such as propylthiouracil, or a dose of radioactive iodine, which is concentrated in the thyroid gland and destroys some of the tissue. Apparent hyperthyroidism can result from destruction of thyroid cells with release of large amounts of hormone. In one condition, called Hashimoto’s thyroiditis, this destruction results from production of an antibody against thyroid tissue.

Deficiency of thyroid hormones, or hypothyroidism, is characterized by lethargy and lowering of metabolism. This condition can result from disorders of the pituitary or of the thyroid gland itself. Iodide is now added to table salt in order to avoid a deficiency of iodine in the diet. The condition called cretinism, more properly known as congenital hypothyroidism, is an inherited deficiency to thyroid function that occurs in about one in every 6000 births. In most instances, but not all, these infants grow up to be mentally retarded. However, its early treatment can prevent retardation.

 

Simple goiter is characterized by an enlargement of the entire thyroid gland or one of its two lobes. It is associated with hypothyroidism, a condition caused by insufficient production of thyroid hormone. As the body needs iodine to produce thyroid hormone, inadequate amounts of iodine in the diet may result in simple goiters. Simple goiters may be classified as either endemic or nontoxic. Endemic goiters are caused by a deficiency of iodine in the diet and usually occur in populations living in areas with iodine-depleted soil. Ingestion of iodine supplements prevents development of the diseases related to the deficiency of iodine.

The cause of most nontoxic goiters is unknown, but researchers suspect that environmental factors and heredity play a role. In some areas of the world, certain chemical compounds in food or water may block the body’s production of thyroid hormones and lead to nontoxic goiter formation. These compounds, known as goitrogens, also include certain drugs, such as aminoglutethimide. Most simple goiters do not produce any symptoms. The administration of thyroid hormone will lead to a reduction in size of simple goiters in most cases. Surgical removal of a simple goiter is usually unnecessary unless the goiter is very large and is impingeing on vital body organs.

This disease, also called exophthalmic goiter, hyperthyroidism, thyrotoxicosis, or Graves’ disease, for the Irish physician Robert James Graves, is caused by an excess of thyroxine secretion. The cause of the excessive secretion is obscure. The symptoms of toxic goiter may include a rapid heartbeat, tremor, increased sweating, increased appetite, weight loss, weakness etc. Toxic goiter is commonly treated with radioactive iodine, which is taken up by the gland and destroys the cells by irradiation. Drugs also can be used to suppress hormone production, or most of the toxic goiter can be removed surgically.