Diabetes is an extremely polymorphic disorder. Having higher than normal levels of glucose in blood is called as hyperglycemia but diabetes is not just hyperglycemia. The presentation, its effects on body, possible complications, monitoring and treatment of patient depends on basic cause behind high levels of blood glucose. Based on the cause Diabetes has been classified into 4 main types of types of diabetes with numerous subtypes by American Diabetes Association as follows.

1. Type 1 diabetes (b-cell destruction, usually leading to absolute insulin deficiency). Type 1 diabetes is further divided into Immune mediated and Idiopathic

2. Type 2 diabetes (may range from predominantly insulin resistance with relative insulin deficiency to a predominantly secretory defect with insulin resistance)

3. Other specific types

4. Gestational diabetes mellitus

Type 2 diabetes is the most common type of diabetes and gestational diabetes is only seen in pregnant females. Earlier most of the elderly diabetics used to be labelled as type 2 diabetes and younger once used to be put on insulin for life presuming them to be type 1 diabetes. It has now been found that many of younger diabetics are either type 2 diabetics who can be treated well with oral drugs or rarely the belong to one of the other specific types. Similarly type 1 diabetes is not uncommon in elderly people. Only a thorough history and some investigations can clear the picture in such cases.

There is a long list of subtypes under Other Specific types as listed below

Genetic defects of b-cell function

1. MODY 3 (Chromosome 12, HNF-1a)

2. MODY 1 (Chromosome 20, HNF-4a)

3. MODY 2 (Chromosome 7, glucokinase)

4. Other very rare forms of MODY (e.g., MODY 4: Chromosome 13, insulin promoter factor-1; MODY 6: Chromosome 2, NeuroD1; MODY 7: Chromosome 9, carboxyl ester lipase)

5. Transient neonatal diabetes (most commonly ZAC/HYAMI imprinting defect on 6q24)

6. Permanent neonatal diabetes (most commonly KCNJ11 gene encoding Kir6.2 subunit of b-cell KATP channel)

7. Mitochondrial DNA

8. Others

Genetic defects in insulin action

1. Type A insulin resistance

2. Leprechaunism

Rabson-Mendenhall syndrome

4. Lipoatrophic diabetes

5. Others

Diseases of the exocrine pancreas

1. Pancreatitis

2. Trauma/pancreatectomy

3. Neoplasia

4. Cystic fibrosis

5. Hemochromatosis

6. Fibrocalculous pancreatopathy

7. Others

Endocrinopathies

1. Acromegaly

2. Cushing’s syndrome

3. Glucagonoma

4. Pheochromocytoma

5. Hyperthyroidism

6. Somatostatinoma

7. Aldosteronoma

8. Others

Drug or chemical induced

1. Vacor

2. Pentamidine

3. Nicotinic acid

4. Glucocorticoids

5. Thyroid hormone

6. Diazoxide

7. b-Adrenergic agonists

8. Thiazides

9. Dilantin

10. Interferon

11. Others

Infections

1. Congenital rubella

2. Cytomegalovirus

3. Others

Uncommon forms of immune-mediated diabetes

1. Stiff-man syndrome

2. Anti-insulin receptor antibodies

3. Others

Other genetic syndromes sometimes associated with diabetes

1. Down syndrome

2. Klinefelter syndrome

3. Turner syndrome

4. Wolfram syndrome

5. Friedreich ataxia

6. Huntington chorea

7. Laurence-Moon-Biedl syndrome

8. Myotonic dystrophy

9. Porphyria

10. Prader-Willi syndrome

11. Others