2.8 Iodine (I)
Iodide ions (I − ) derived, like all mineral elements, from the breakdown of rocks, is present in some soils, but much of it has been dissolved out by water over millions of years and washed down into the sea. It is concentrated by some marine organisms, and so can occur at quite high concentrations in edible seaweed, and in fish and other seafood. Thus people living near coasts often have sufficient iodine in their diet, whereas those living in mountainous areas, such as the Himalayas and Andes, where most of the iodine has been removed from the soil by millennia of high rain and snowfall, can suffer from iodine deficiency.
Iodine is an essential component of thyroid hormones, produced by the thyroid gland at the base of the neck. These hormones play a vital part in the regulation of metabolic processes, especially growth and energy expenditure. If there is insufficient iodine for the production of normal amounts of these hormones, the thyroid gland enlarges as the cells attempt to boost their hormone production. Ultimately, the swollen thyroid produces an enlargement of the throat called a goitre ( Figure 3 ). Not only does the swelling impede breathing and swallowing, but the lack of sufficient thyroid hormones also leads to weight gain, lethargy, intolerance to cold, increased blood cholesterol, mental slowness and reduced heart function.
Iodine deficiency has its greatest impact during pregnancy, since it has major effects on the developing brain and physical growth of the fetus. In the worst case, the child suffers from cretinism, in which there is mental retardation, stunted growth, apathy, and impairment of movement, speech and hearing. However, even minor iodine deficiency can lower a child's IQ by between 10 and 15 points, which, if it occurs in a large percentage of the population, can severely hamper the economic development of a country. Iodine deficiency is regarded as the greatest cause of preventable brain damage, putting almost a thousand million children at risk worldwide. A UN initiative aims to eliminate the problem by adding iodine to salt, since salt is consumed by almost everyone in the world, regardless of culture or socioeconomic group. In the UK, cows'; milk is a major source of iodine due to the use of iodine-containing supplements in cattle food.
Nuclear accidents can release radioactive isotopes of iodine into the environment, which can then contaminate water and food supplies. The iodine settles onto the grass in pasture land, and is then eaten by cattle, and appears in their milk – a major way in which it is taken in by people. Radioactive iodine can become concentrated in the thyroid gland and cause thyroid cancers. If a large amount of normal (non-radioactive) iodine is taken in, it can displace the radioactive iodine (which is then excreted) and reduce the chances of cancer developing. This non-toxic iodine can be supplied to those at risk in the form of potassium iodide tablets. After the explosion at the nuclear power plant at Chernobyl in 1986, such tablets were supplied to 10.5 million adults and 7 million children thought to be at risk.