CONTEXT: Iodine-induced hyperthyroidism has been reported in the early phases of almost all iodine fortification programs, depending on prior iodine intake in the population, the amount of fortification, and the rate of change. OBJECTIVE: The aim of the study was to monitor the effect of the Danish iodine fortification program on incidence of hyperthyroidism as measured by the incident use of antithyroid medication. DESIGN: We conducted a register study. Using the unique identification number of all Danes, we linked data from the Register of Medicinal Product Statistics and the Civil Registration register on an individual level. All dispensing of antithyroid medication from 1995 to 2007 was studied. The place of residency was used to divide patients into mildly and moderately iodine-deficient groups. MAIN OUTCOME MEASURE: We measured the incident use of antithyroid medication. RESULTS: In the region with moderate iodine deficiency, the number of incident users of antithyroid medication increased 46% in the first 4 yr of iodine fortification. The use increased the most among the youngest age group (younger than 40 yr) and the oldest age group (older than 75 yr). In the mildly iodine-deficient region, the number of incident users increased only 18%, and only in the youngest age groups (below 40 and 40-59 yr). After 4 yr of fortification, the incidence rates started to fall and reached baseline, for most groups, 6 yr after onset of fortification. CONCLUSIONS: This study shows that iodine fortification induced a temporary, modest increase in the incidence of hyperthyroidism as measured by use of antithyroid medication. A new steady state has not yet evolved.