A comparative study was made of thyroid function in samples of 'well' subjects, from separate geographical areas with dissimilar dietary iodine levels. In the area with the higher iodine level, Iceland, the prevalence of positive thyroid antibody tests and serum thyrotrophin levels were lower, while the capacity of the thyroid to respond to exogenous thyrotrophin was higher than in NE Scotland, an area with lower iodine levels. In contrast the prevalence of positive tests for another organ-specific antibody, gastric parietal cell antibody, was higher in Iceland. These observations are consistent with the reported differences in the incidence rates for thyrotoxicosis and gastric carcinoma in the areas studied. The results are in agreement with our findings in two groups of post-thyroidectomy patients from the same populations. This study provides support for the view that differences in the prevalence of constitutional and environmental factors in different populations contribute to the variation in reported outcome of treatment for thyroid disease.