Geographical variation is also seen in the distribution of histological types that are associated with iodine content of the diet (Fig. 16.23). In areas of iodine deficiency (present or past), follicular carcinomas are more predominant than papillary carcinomas, and the converse is true where iodine is readily available. Reports from Switzerland have shown a decrease in percentage of follicular carcinomas and an increase in papillary carcinomas following the iodisation of salt. In Iceland, where the dietary intake of iodine is unusually large, the high incidence of thyroid malignancy is due mainly to papillary carcinomas. An interesting finding has been the variation in geographical incidence of microcarcinomas in step-sectioned post-mortem glands in women ΓÇô from seven per cent in Colombia, to twenty-eight per cent in Japanese residents in Hawaii. The equivalent figures for men were five per cent and twenty per cent, respectively.
