This review concerns the relation between most frequent thyroid gland diseases and diabetes mellitus in adult patients. Special attention is paid to autoimmune thyroiditis, Graves' disease, thyroid autoimmunity in pregnant diabetic women, and iodine metabolism. We focused on mechanisms leading to coexistence of both endocrine disorders, and on distinctions in the prevalence, diagnosis, clinical course and treatment of thyroid diseases in diabetic patients. The prevalence of thyroid diseases in diabetic patients is 2-3 times higher than in nondiabetic subjects; it raises with age, and is strongly influenced by female gender and autoimmune diabetes. Clinical relevance of thyroid diseases, especially in diabetic patients, significantly increases if it is associated with deteriorated function, which always cause a number problems with metabolic compensation of diabetes. Most serious consequences are increased frequency of hypoglycaemia in hypothyroidism and development of potentially life-threatening ketoacidosis in thyrotoxicosis. In spite of that, little attention is paid to the diagnosis of thyroid diseases in diabetics, as they are diagnosed in only about half of the patients. At the end, we provide recommendations for the thyroid disease screening and diagnosis in patients with diabetes mellitus based on our experience.