Risk of Thyroid Cancer in People With Type 1 Diabetes by Autoimmune Thyroid Diseases and Tumor Histology

Context Thyroid cancer is the most common endocrine cancer, but little is known about it in type 1 diabetes (T1D) and its potential association with autoimmune diseases.Objective This study aims to assess the risk of thyroid cancer in adults with long-term T1D compared to individuals without diabetes and the proposed association of thyroid autoimmune diseases with thyroid cancer.Methods The study included 4758 individuals with T1D participating in the Finnish Diabetic Nephropathy Study and 12 710 controls. Thyroid cancers were obtained from the Finnish Care Registers for Health Care.Results 27 (0.57%) individuals with T1D had thyroid cancer compared to 27 (0.21%) in the controls (standardized incidence ratio 2.43; 95% confidence interval 1.59-3.56). The absolute increase in incidence was modest, with a 0.36%-unit rise. This translates to 17 additional cases among 4710 individuals with T1D. Cancer type was papillary in 81.5% of individuals with T1D and 88.9% of the controls; the rest were follicular. In T1D the distribution of hypothyreosis was similar between those with (n = 5, 18.5%) and without (18.1%) cancer, but hyperthyreosis was diagnosed more often with thyroid cancer (n = 3, 11.1%) than without (2.3%, P = .003). None of the thyroid cancers were invasive or had metastatic characteristics.Conclusion Although there is an excess risk of thyroid cancer, it is only marginally increased (0.36%-unit) in individuals with T1D compared to control individuals and was not associated with increased morbidity or mortality. An overdiagnosis effect due to regular health care contacts is the most likely explanation for the higher risk.