1.1 WHAT IODINE DEFICIENCY DISORDERS ARE

The best-known effect of iodine deficiency is endemic goitre, of which it is the chief primary etiological factor. Another major effect is endemic cretinism. In its commonest form this is characterized by mental deficiency, deaf-mutism and spastic diplegia.

Goitre was well known in the ancient world and has continued to be of interest over the centuries. Extensive reviews of its geographical occurrence have been published, notably by Kelly and Snedden (1960) and Stanbury and Hetzel (1980). Endemic cretinism in Alpine Europe was identified by the Sardinian Commission of 1848, and both goitre and cretinism in the northwest frontier region of India were described by McCarrison in 1908 for the first time in modern medical literature.

In the last 20 years endemic cretinism has been rediscovered in remote places such as certain areas of Papua New Guinea (McCullagh, 1963; Choufoer et al., 1965). It had been 'forgotten' because of the isolation of the iodine-deficient populations from modern investigative facilities.

The relation of iodine deficiency to goitre was defined in the first decade of the 20th century in Marine's studies on experimental goitre in rats (Marine and Lenhart, 1909). Marine showed that when the iodine content of the thyroid fell below 0.1 percent, hyperplasia occurred with thyroid enlargement and the production of goitre. Intermittent iodine deficiency produced alternating hyperplasia and involution with production of the familiar 'colloid goitre'. In 1921 the successful prevention of goitre by iodide supplementation was shown by Marine and Kimball (1921) in a controlled study in school-children in Akron, Ohio. Iodized salt was introduced into Switzerland in 1924 and has since been used in many other countries.

Stanbury et al. (1954) used radio-iodine for the first time in the field in Mendoza, in the Argentinian Andes, to demonstrate iodine deficiency.

Since 1960 the effects of iodine deficiency have been studied in many remote, predominantly mountainous areas around the world, including the Himalayas (India, Nepal, Burma), the Andes (Peru, Ecuador, Bolivia, Chile), and Indonesia, New Guinea and Zaire, as well as the more Isolated parts of some European countries (Bavaria, Sicily, Portugal, Greece).

It is now known that endemic cretinism is associated with high rates of goitre and with severe iodine deficiency; for example, with dietary iodine intakes of about or below 20 mcg (micrograms) per day compared with a normal daily intake of 80-150 mcg; while goitre alone is seen at intake levels below 50 mcg iodine per day. The manifestations of cretinism vary, but they are clearly an important iodine-related public health problem in the community in which they occur because they cause severe disabilities.

It is also now known that endemic cretinism occurs with other problems related to severe iodine deficiency such as high stillbirth rates and high perinatal and infant mortality. In addition, it is associated with hypothyroidism and consequent varying degrees of mental deficiency, with or without goitre, in infancy and childhood. All these conditions can be prevented by correction of the iodine deficiency.

Goitre itself can arise from causes other than primary iodine deficiency, due to a variety of agents (goitrogens). These, however, are in general of secondary importance as etiological factors.

Recent research (Bourdoux et al., 1978, 1980a, 1980b) has shown that staple foods in some developing countries, such as cassava, maize, bamboo shoots, sweet potatoes, lima beans and millets, contain cyanogenic glucosides capable of liberating large quantities of cyanide by hydrolysis. Not only is the cyanide toxic but the metabolite is predominantly thiocyanate, which is a goitrogen. With the exception of cassava, however, these glycosides occur in the inedible portions of the plants, or, if in the edible portions, in such small quantities that they cannot generally produce toxic effects.

The role of cassava in the etiology of endemic goitre and endemic cretinism has now been demonstrated by Delange et al. (1982) from their studies in Zaire, observations confirmed by Maberly et al. (1983) in Sarawak, Malaysia. Cassava is cultivated extensively in some developing countries and represents an essential source of calories for more than 200 million people living in the tropics. (Delange et al, 1982)

Apart from goitre itself, more recent work on iodine deficiency has revealed a great variety of effects on human growth and development. These disorders, which are best described in relation to four different phases of life (see Section 2), are listed in Section 2.4. (Table 1).