Objective: To investigate the prevalence of overt thyroid disease in children in The Netherlands with and without type 1 diabetes mellitus (T1DM). Study design: Nationwide, retrospective cohort study in The Netherlands. Using the national registry of both healthcare reimbursement and pharmaceutical care, data of all Dutch children (aged 0-14 years) with a diagnosis of T1DM, or a diagnosis of hypothyroidism or hyperthyroidism in the period 2009-2011. Results: The prevalence of thyroid disease was 0.15% in children without T1DM, and 3.43% in children with T1DM (rate ratio 23.59; 95% CI 19.92-27.93; P <.001). Both hyperthyroidism and hypothyroidism were 24 times more likely in children with T1DM than in those without. Thyroid disease was more common in girls than in boys, both in children with T1DM (rate ratio of girls vs boys 3.07; 95% CI 2.10-4.49) and in children without T1DM (rate ratio 1.59; 95% CI 1.49-1.69). This sex difference was more pronounced for hypothyroidism than for hyperthyroidism. Conclusions: Children with T1DM in The Netherlands are 24 times more likely to develop thyroid disease than their peers without diabetes. Girls with T1DM were more prone to thyroid disease, particularly hypothyroidism.