Objective: Denmark was an area of iodine deficiency, and mandatory iodine fortification of table salt and salt in bread (13 p.p.m. iodine) was initiated in 2000/2001. The Danish investigation on iodine intake and thyroid disease (DanThyr) is the monitoring of the iodine fortification program. Design and methods: DanThyr consists of three main parts: a study of population cohorts initialized before (nZ4649) and after (nZ3570) iodization of salt, a prospective identification of incident cases of overt hyper-and hypothyroidism in a population of around 550 000 people since 1997, and compilation of data from the national registers on the use of thyroid medication, thyroid surgery, and radioiodine therapy. Studies were carried-out in parallel in subcohorts living in areas with differences in iodine content of ground water. Results: The study showed profound effects of even small differences in iodine intake level on the prevalence of goiter, nodules, and thyroid dysfunction. Mild and moderate iodine deficiency was associated with a decrease in serum TSH with age. Other environmental factors were also important for goiter development (increase in risk, smoking and pregnancy; decrease in risk, oral contraception and alcohol consumption), and the individual risk depended on the genetic background. Environmental factors had only a minor influence on the prevalence of thyroid autoantibodies in the population. There were more cases of overt hypothyroidism in mild than in moderate iodine deficiency caused by a 53% higher incidence of spontaneous (presumably autoimmune) hypothyroidism. On the other hand, there were 49% more cases of overt hyperthyroidism in the area with moderate iodine deficiency. The cautious iodine fortification program, aiming at an average increase in iodine intake of 50 mg/day has been associated with a 50% increase in incidence of hyperthyroidism in the area with the most severe iodine deficiency. The incidence is expected to decrease in the future, but there may be more cases of Graves' hyperthyroidism in young people. Conclusion: A number of environmental factors influence the epidemiology of thyroid disorders, and even relatively small abnormalities and differences in the level of iodine intake of a population have profound effects on the occurrence of thyroid abnormalities. Monitoring and adjustment of iodine intake in the population is an important part of preventive medicine.European Journal of Endocrinology 155 219-228

In studies of iodine intake, the correct choice of the method for collecting urine and the format for expressing the results of urine iodine measurement is essential to avoid misinterpretation of data on the iodine status of a population or individuals.

The prevalence of both TPO-Ab and Tg-Ab was higher 4-5 years after a cautious iodine fortification of salt was introduced in Denmark. The increase was most pronounced in young women and in the low concentrations of antibody. Further studies are needed to evaluate the long-term effects of increased iodine intake on thyroid autoimmunity in the population.