Iodbasedow : the response in an iodine deficient population to an increase in iodine consumption.

Endemic goitre due mainly to iodine deficiency has been recorded in many areas of the world throughout history and its response to crude iodine preparations has been noted. The good effect of the pure element in the treatment of goitre was modified by severe thyrotoxic reactions, reported particularly from areas of endemic goitre. A rational basis for the use of iodine in prophylaxis against goitre, came with the discoveries linking iodine with thyroid metabolism and measurements of low iodine concentrations in water, soil and plants. Mountainous areas which are far from the sea and have been subjected to glaciation in the last Ice Age are particularly prone to iodine deficiency. The excellent response to iodine prophylaxis in preventing goitre in the children of Ohio, U.S.A. was followed by an equally good one in Switzerland. However there were further reports of thyrotoxicosis following iodine prophylaxis again occurring in adult goitrous females. It was thought by those who believed that iodine caused thyrotoxicosis in some people, that the size of' thedose given was important but examples suggest that this was not so. Indeed many authorities denied the existence of iodbasedow. Tasmania has been a very satisfactory area to observe these phenomena. Population shifts are minimal and it was a moderately iodine deficient island with a goitre incidence among adult females of approximately 30 per cent. Although goitregens were pro- posed as a contributing cause of goitre none such have been found. Iodation of bread gave universal prophylaxis to the population and an excellent response has been achieved. Goitre prevalence in children has fallen to that seen in countries where goitre is not endemic. The pathology of goitre in Tasmania is similar to that seen in all parts of the world. There does not seem to be any increased incidence of thyroid cancer, cretinism is rare and before iodation of bread the incidence of thyrotoxicosis was lower than that recorded in iodine sufficient communities. Several factors have combined to produce a change in iodine content in the environment of Tasmania, before iodation of bread. The importation of large amounts of food from areas of Australia which are not iodine deficient and the use of iodophors in the food industry provided extra iodine. Several other sources of iodine were brought to Tasmania resulting in a gradual increase in iodine consumption. A steady increase in the incidence of thyrotoxicosis was noticed from 1960, as shown in Fig. 10-2 with a sudden rise which has continued from 1966 when bread was iodated. It is suggested that the incidence of thyrotoxicosis has risen to that occurring in nonendemic goitre areas, previous iodine deficiency having protected those otherwise prone to develope the disease. If this is so, the incidence will remain similar to that of the average world incidence for non-endemic goitre areas, but the age group affected should become progressively younger. The addition of iodine to the diet of the population of Tasmania can be justified by the disappearance of goitre and consequent decrease in thyrocardiac disease which is associated with nodular goitre in middle age, the probable disappearance of . lethal cancer of the thyroid in the elderly, tracheal obstruction by retrosternal goitre and the cosmetic effect in females who formed the majority affected by the condition. It is predicted that the price for this will be a slight increase in the incidence of thyrotoxicosis in an age group which will become progressively younger and more able to cope with the disease. On balance at the present time this price is reasonable. Evidence to support these assumptions are presented in the body of the thesis.