Around 3-4 billion people in the world are covered by iodine supplementation programs to prevent developmental brain damage and other iodine deficiency (ID) disorders. Mild ID is associated with more hyperthyroidism and less hypothyroidism in the population than a high iodine intake. Knowledge of the iodine intake levels where the shifts in incidences occur is important for planning of iodine supplementation programs. A computer-based register linked to thyroid diagnostic laboratories was used to continuously identify all new cases of overt hyper- and hypothyroidism in two population cohorts with moderate and mild ID, respectively (Aalborg, n = 310,124; urinary iodine, 45 mug/liter; and Copenhagen, n = 225,707; urinary iodine, 61 mug/liter). The investigation was initiated before iodization of salt in Denmark and was part of the monitoring program. In 1997-1998, the incidence rate of overt hyperthyroidism was high in the area with the lowest iodine intake (92.9/100,000 per year) compared with the area with only mild ID (65.4/100,000 per year). Standardized rate ratio was 1.49, and 95% confidence interval was 1.22-1.81. The opposite relationship was present for overt hypothyroidism (moderate ID, 26.5/100,000 per year; mild ID, 40.1/100,000 per year; standardized rate ratio, 0.73; 95% confidence interval, 0.55-0.97). The different incidence rates were confirmed during each of the two following years. The results of this prospective investigation of the incidence of overt hyper- and hypothyroidism suggest that iodine supplementation of a population may increase the incidence of overt hypothyroidism, even if the population is moderately iodine-deficient. In such a population, the increase in risk of hypothyroidism should be weighed against the risk of ID disorders such as hyperthyroidism due to multinodular toxic goiter. The optimal level of iodine intake to prevent thyroid disease may be a relatively narrow range around the recommended daily iodine intake of 150 mug.