A case of mediastinal teratoma with pancreatic islets accompanied by discontinuation of insulin treatment in insulin-dependent diabetes mellitus.

A 74-year-old woman of insulin-dependent diabetes mellitus presented with gradually improvement of blood glucose control and finally discontinuation of insulin therapy, for some unknown reason. During follow-up period, she was admitted with hemoptysis. CT imaging showed a heterogeneous enhancement mass in the middle mediastinum with cyst and calcification, suggesting the diagnosis of mediastinal teratoma. Immediately after excision of the tumor, her plasma glucose levels again increased, and she required insulin therapy for glycemic control. Immunohistochemical examination showed that the tumor contained pancreatic tissue with both exocrine and endocrine components consisted with the islet cells of Langerhans with insulin-positive cells. Accordingly, we diagnosed insulin-producing mediastinal teratoma. Although hypoglycemic agents are the commonest cause of hypoglycemia in diabetic patients, an insulin-producing tumor should be considered in the patients who have dramatic improvement of diabetes mellitus, particularly after withdrawal of all hypoglycemic treatment. Mediastinal teratomas should be considered in differential diagnosis as etiology in undiagnosed case of hypoglycemia or blood glucose fluctuations. © The Japan Diabetes Society 2019.