Diabetes mellitus is a group of metabolic diseases characterized by a deficiency of the pancreatic hormone insulin, which result from defects in insulin secretion, or action, or both. Diabetes mellitus Diabetes is a chronic medical condition, meaning that although it can be controlled, it lasts a lifetime. There are three main types of diabetes mellitus: 1. Type 1 DM; 2. Type 2 DM; 3. Gestational diabetes.