P>Objective Marked differences in pattern of thyroid dysfunction are seen in populations with different iodine intakes. We evaluated the influence of a higher iodine intake on thyroid hormone levels and the prevalence of thyroid dysfunction in the Danish population. Design Two cross-sectional studies matched on a group level according to sex and age. Participants In all, 8219 individuals were examined before (n = 4649) or after (n = 3570) the introduction of a mandatory iodization programme in 2000 in two regions with established mild and moderate iodine deficiency. Serum TSH, fT(4) and fT(3) were measured. An ultrasonography of the thyroid was performed. Results We found a higher median serum TSH after the introduction of mandatory iodization of salt: 1 center dot 51 mU/l (10-90th percentiles: 0 center dot 72-3 center dot 00) vs. 1 center dot 30 mU/l (10-90th percentiles: 0 center dot 59-2 center dot 66) before iodization. The difference was found in both regions and across age groups. There was a lower prevalence of mild hyperthyroidism and a tendency towards a lower prevalence of overt hyperthyroidism. The prevalence of mild hypothyroidism increased, most pronounced among young women after iodization. Conversely, there was a lower prevalence of undiagnosed overt hypothyroidism. However, when currently treated participants were included, the prevalence of hypothyroidism increased after iodization in the area with formerly mild iodine deficiency. Conclusion A change in pattern of thyroid dysfunction was seen in relation to mandatory iodization of salt. There was no rise in the prevalence of hyperthyroidism and the prevalence of mild hyperthyroidism was halved. Conversely, prevalence of hypothyroidism increased.