Geographic patterns and trends in the incidence of thyroid cancer in Norway 1970-1986.

Norway has a special geographic distribution of thyroid cancer. There are very high incidence rates in the most northern part and an average Scandinavian level in the south. The incidence began to decline in the 1980s after increasing for a long time. The decline affects females more than males and is mainly seen in the oldest age groups and in mid- and north Norway. The decline is in papillary carcinomas and stage 1 and has occurred in primary trade communities more than the others. Cohort analysis demonstrates a secular trend in females, a continuous risk shift from older to younger age groups as the cohorts become younger--which explains the current bimodular age-specific incidence curve. Etiologic aspects are discussed.