Thyroid cancer incidence has increased worldwide during the previous decades. In this nationwide study, we aimed to identify the overall incidence of
thyroid cancer in Denmark during 66 years (1943-2008) and incidences of the four main histological types of
thyroid cancer from 1978 to 2008. Data were obtained from the nationwide Danish
Cancer Registry, and we focused especially on the period after implementation of compulsory
iodine supplementation, which was established on a national level in 2000. We calculated age-standardized incidence rates per 100,000 person-years, and age-period-cohort models were fitted to describe trends in incidence. To quantify trends in incidence over time, log-linear Poisson models were used to estimate annual percentage change. From 1943 to 2008, 1,947 men (29%) and 4,682 women (71%) were diagnosed with
thyroid cancer. The age-standardized incidence increased in both sexes; in men from 0.41 to 1.57 per 100,000 and from 0.90 to 4.11 per 100,000 in women, corresponding to a significant average annual percentage change of 1.7 and 1.8%, respectively. The incidence increased with younger birth cohorts. The rise was almost exclusively caused by
papillary carcinomas, and it was particularly present during the last decades of the study period. It cannot be ruled out that
iodine supplementation may play a role for the risk of
thyroid cancer, but as the strongest increase in incidence began in the years before the implementation, it is likely that improvement in diagnostic modalities increased diagnostic activity, and/or new unknown risk factors are also important contributors to the increase.