Incidence of different morphological types of thyroid cancer in the Nordic countries.

In order to determine the incidence of different morphological types of thyroid cancer in the Nordic countries, all cases of thyroid cancer reported to the national cancer registries of Finland, Iceland, Norway and Sweden over periods ranging from 1 to 14 years were re-examined (a total of 706 cases) by one or two pathologists from each country. The age-adjusted incidence rate for Iceland was clearly higher than those for the other Nordic countries. The rate for Finland was the lowest, and a five-fold difference was seen between Iceland and Finland. The differences in the incidence rates applied to all age groups and to both sexes. The differences also concerned all histological types and were more pronounced for papillary and medullary carcinomas. In papillary carcinoma, an almost 10-fold difference was found between Finland and Iceland. The causes of the differences are not known. There has been and probably still is some endemic goitre in Finland, Norway and Sweden, but no endemic goitre occurs in Iceland. Thus, no positive association was found between the presence of endemic goitre and the risk of follicular carcinoma. There was a marked negative association between endemic goitre and the risk of papillary carcinoma.