Iodine is a constituent of thyroxine, the active principle of the thyroid gland, weighing about 25 g in a normal adult, contains only about  10 mg of iodine. The adult body as a whole contains about 50 mg of iodine. The thyroid gland plays an important part in energy metabolism and in the growth of the body.


Iodine is present only in small amounts in common foods, the quantity of iodine present depending on the iodine content of the soil. The soil of mountainous regions usually contains less iodine than the soil of the plains near the sea. Crude common salt prepared from sea water and sea fish are good sources of iodine.


Iodine requirements for adults are about 0.15 to 02 mg and for infants and children 0.05 to 0.10 mg daily. This is normally supplied by an ordinary well-balanced diet and by drinking water except in mountainous regions where the food and water are deficient in iodine.


If sufficient iodine is not taken in the diet, enlargement of the thyroid takes place, resulting in the disease called goitre. The thyroid gland of the adult which normally weighs about 25g may weigh as much as 200 to 500 g or even more in goitre. Histological examination shows diffuse overgrowth of the glandular tissue, known as general hyperplasia of the glands. The vesicles contain little or no colloid. If  treatment with iodine is started very early the thyroid may become normal. But if treatment is delayed, the enlargement of the gland persists.  In children, severe iodine deficiency may result in serious retardation of growth. This condition is known as cretinism.


A survey of the incidence of goitre in different parts  of the world shows that the disease is related to deficiency of iodine in the water and food. Goitre  commonly occurs among people living in mountainous regions. In India, goitre occurs in the hilly districts along the foot of the Himalayas, e.g. in Kashmir, Xangra  Valley, etc. Goitre can be prevented by the regular use of iodised salt (1 g of sodium  iodate being added to 100,000 g of common salt).


There is evidence indicating many food such as cabbage, cauliflower and radish contain substances which react with the iodine present in the food and make it unavailable. These substances are known as ‘goitrogenic’ substances. Consumption of large quantities of these foods leads to the development of goitre by making the iodine present in the food not available to the body.