IODINE SUPPLEMENTATION IN SWEDEN AND REGIONAL TRENDS IN THYROID-CANCER INCIDENCE BY HISTOPATHOLOGIC TYPE

We studied regional patterns of thyroid cancer incidence in relation t o iodine intake and iodinization in Sweden using 5,838 incident cases diagnosed in the period 1958-1981. Region was defined either by iodine status, urban-rural status or health-care region (internal controls). Age, period and cohort models were fitted to evaluate trends and regi onal variation in incidence by histopathologic type. In iodine-deficie nt areas, the relative risk (RR) of developing thyroid cancer was 0.92 for all histologic types combined, 0.80 for papillary cancer and 0.87 for anaplastic carcinoma. Residence in iodine-deficient regions was a ssociated with a 2-fold increased risk of follicular cancer in men (RR 1.98) and a 17% increase in risk in women (RR 1.17). Regional differe nces in iodine intake fell after iodinization of the food supply, whic h was started in 1936 and enhanced in 1966. Nevertheless, incidence of both papillary and follicular carcinoma increased during the study pe riod, with largely similar trends in iodine-deficient and iodine-suffi cient areas. Overall, residence in urban or rural areas was not an imp ortant determinant of incidence, though trends in the incidence of pap illary, follicular and anaplastic cancer did vary between urban and ru ral areas. The occurrence of thyroid cancer differed only marginally b etween the 6 health-cave regions in Sweden, suggesting that the observ ations in regions defined by iodine intake were unlikely to be artifac tual. Our data suggest that iodinization of the food supply is not ass ociated with adverse trends in the occurrence of thyroid cancer. (C) 1 996 Wiley-Liss, Inc.