Iodine intake and incidence of thyroid cancer in Denmark

Iodine intake is suspected to be a risk factor for thyroid cancer. Eastern Denmark is characterized by mild and western Denmark by moderate iodine deficiency, and this difference is associated with a 50% difference in the occurrence of goitre and thyrotoxicosis. The objective of the study was to determine whether the incidence of thyroid cancer differs between these two regions, as any difference would have important safety implications for the national iodine supplementation programme.

We studied all thyroid cancers notified to the Danish Cancer Registry in the period 1973-1997, focusing on the four most frequent subtypes: papillary, follicular, anaplastic and medullary thyroid cancer. A Poisson regression model was used with models of goodness-of-fit for age, period, sex.

No regional difference was found in the overall incidence of follicular (0.3% 100 000 person-years) or papillary (0.7% 100 000 person-years) thyroid cancer. A slight but nonsignificant increase in total incidence, resulting mainly from a significant increase in the incidence for the papillary subtype, was observed in both regions.

The results suggest that modest differences in iodine intake do not affect thyroid cancer incidence or the distribution of subtypes.