Classification of Diabetes Mellitus

1. Type 1 diabetes mellitus (T1DM)
   • Caused by autoimmune destruction of pancreatic beta cells.
   • Produces absolute insulin deficiency.
   • May occur in children or adults.
   • Includes immune-mediated type 1 diabetes and the rarer idiopathic form.
2. Type 2 diabetes mellitus (T2DM)
   • Caused by progressive beta-cell dysfunction, usually with insulin resistance.
   • Most common form of diabetes.
   • Commonly associated with overweight/obesity, family history, and sedentary lifestyle, but can occur at any body weight.
3. Gestational diabetes mellitus (GDM)
   • Diabetes first diagnosed during pregnancy that was not clearly present as overt diabetes before pregnancy.
   • Usually identified during the second or third trimester.
4. Specific types of diabetes due to other causes
   • Monogenic diabetes syndromes: MODY and neonatal diabetes.
   • Diseases of the exocrine pancreas: chronic pancreatitis, pancreatic cancer, cystic fibrosis, hemochromatosis, pancreatectomy.
   • Endocrine disorders: Cushing syndrome, acromegaly, pheochromocytoma, glucagonoma.
   • Drug- or chemical-induced diabetes: glucocorticoids, some antipsychotics, antiretroviral therapy, calcineurin inhibitors after transplantation.
   • Genetic defects affecting insulin action or beta-cell function.
   • Diabetes associated with infections or uncommon immune-mediated syndromes.