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 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.Thyroid cancer is the most frequent endocrine malignancy, and, during the previous decades, the incidence has increased worldwide.1-4 The etiology is not well understood, but suspected risk factors include a family history of thyroid cancer (primarily medullary cancer) and radiation exposure during childhood [both accidental radiation (nuclear accidents), natural radiation (volcanic areas) and therapeutic and diagnostic radiation (X-ray)]. The disease is primarily seen in women, and it is commonly grouped into four subtypes with decreasing survival: papillary, folliculary, medullary and anaplastic carcinoma. Together, they constitute $95% of the total number of thyroid cancer cases, but papillary cancer is by far the most prevalent.Iodine plays a crucial role in the formation of metabolic hormones in the thyroid gland, and a low iodine diet increases the risk of developing benign thyroid disorders. However, iodine as a risk factor for thyroid cancer is controversial. To assess the iodine status in a geografical area, the urinary excretion of iodine can be measured. According to criteria described by the World Health Organization (WHO), iodine intake is sufficient if the median iodine excretion is more than 100 lg/l, while iodine deficiency status is mild when the median iodine excretion is 50-99 lg/l, moderate when the median iodine excretion is 20-49 lg/l and severe when median iodine excretion is lower than 20 l...
