The risk of type 1 diabetes mellitus (T1DM) combined with epilepsy is significantly higher in people with this disease than in people without it, which may indicate a genetic predisposition for this combination. The issue of the relationship between diabetes and epilepsy has not yet been fully understood. The article is concerned with a clinical case of type 1 diabetes mellitus combined with Graves' disease in a 37-year-old patient with epilepsy. The combination of two autoimmune diseases can be taken as part of type 2 or 3 autoimmune polyglandular syndrome (APS). APS affects multiple glands in the endocrine system. The chronic adrenal cortex insufficiency is a characteristic sign for the most studied type 1 or 2 APS. Type 3 APS is defined by the presence of an autoimmune thyroid disease and other autoimmune diseases, both endocrine (excluding adrenal and parathyroid gland insufficiency) and nonendocrine. The autoimmune nature of T1DM and Graves' disease is beyond dispute in the considered case; the possibility of considering epilepsy as part of the autoimmune process (like type 3 APS) is not to be irrelevant. The article presents the differential diagnosis of convulsive conditions and comparison of clinical manifestations of hypoglycemia and epileptic seizures, and focuses on the prescription of anticonvulsant therapy in patients with type 1 diabetes. Thyrotoxicosis, which developed against the background of diabetes, leads to high variability of glycemia, increased need for insulin, metabolic decompensation, which requires correction of insulin therapy. Patients with polymorbid conditions should be considered from the perspective of single pathogenesis, and they should be followed-up. If a patient has polyendocrinopathy, the variability of the clinical picture, and pharmacokinetic and pharmacodynamic interaction of the prescribed drugs needs to be taken into account due to mutual influence of pathological processes.