Endemic goitre occurs when the prevalence of thyroid enlargement in the population of an area exceeds 10%. With few exceptions its cause is iodine deficiency superimposed on other goitrogenic factors normally present and responsible for sporadic goitre. Iodine deficiency causes significant health problems and so, the term iodine deficiency disorders (IDD) has been introduced. The earliest sign of IDD is goitre, but these disorders also include cretinism, neonatal hypothyroidism and congenital defects, as well as retardation of mental and physical development etc. IDD are a worldwide problem: WHO estimates that substantially more than 800 million people are at risk and more than 190 millions suffer from IDD; over 3 million people have cretinism and in the largest and worst affected areas many millions suffer from mental and physical developmental defects. IDD can be totally eliminated by prophylaxis using iodine administered in salt, oil or some other vehicle. Problems over preventing iodine deficiency relate to difficulties in the handling and distribution of the iodized vehicle in some parts of the world and on the political will to introduce preventive schemes. In only a very few areas does the presence of goitrogenic agents in the environment cause endemic goitre despite adequate iodine supply. In a limited number of places excessive iodine from seaweed used as staple food results in endemic goitre.