Latest Classifications of Diabetes Mellitus

Latest Classifications of Diabetes Mellitus
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Latest Classifications of Diabetes Mellitus /Diabetes Mellitus (DM) is a metabolic disease, involving inappropriately elevated blood glucose levels. DM has several categories, including type 1, type 2, maturity-onset diabetes of the young (MODY), gestational diabetes, neonatal diabetes, and secondary causes due to endocrinopathies, steroid use, etc. The main subtypes of DM are Type 1 Diabetes Mellitus (T1DM) and Type 2 Diabetes Mellitus (T2DM), which classically result from defective insulin secretion (T1DM) and/or action (T2DM). T1DM presents in children or adolescents, while T2DM is thought to affect middle-aged and older adults who have prolonged hyperglycemia due to poor lifestyle and dietary choices. The pathogenesis for T1DM and T2DM is drastically different, and therefore each type has various etiologies, presentations, and treatments.

Classifications of Diabetes Mellitus

DM is broadly classified into three types by etiology and clinical presentation, type 1 diabetes, type 2 diabetes, and gestational diabetes (GDM). Some other less common types of diabetes include monogenic diabetes and secondary diabetes.

Type 1 Diabetes Mellitus (T1DM)

  • Type 1 diabetes mellitus (T1DM) accounts for 5% to 10% of DM and is characterized by autoimmune destruction of insulin-producing beta cells in the islets of the pancreas. As a result, there is an absolute deficiency of insulin. A combination of genetic susceptivity and environmental factors such as viral infection, toxins, or some dietary factors have been implicated as triggers for autoimmunity. T1DM is most commonly seen in children and adolescents though it can develop at any age.
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Type 2 Diabetes Mellitus

  • Type 2 diabetes mellitus (T2DM) accounts for around 90% of all cases of diabetes. In T2DM, the response to insulin is diminished, and this is defined as insulin resistance. During this state, insulin is ineffective and is initially countered by an increase in insulin production to maintain glucose homeostasis, but over time, insulin production decreases, resulting in T2DM. T2DM is most commonly seen in persons older than 45 years. Still, it is increasingly seen in children, adolescents, and younger adults due to rising levels of obesity, physical inactivity, and energy-dense diets.

Gestational Diabetes Mellitus

  • Hyperglycaemia, which is first detected during pregnancy, is classified as gestational diabetes mellitus (GDM), also known as hyperglycemia in pregnancy. Although it can occur anytime during pregnancy, GDM generally affects pregnant women during the second and third trimesters. According to the American Diabetes Association (ADA), GDM complicates 7% of all pregnancies. Women with GDM and their offspring have an increased risk of developing type 2 diabetes mellitus in the future.

Monogenic Diabetes

  • A single genetic mutation in an autosomal dominant gene causes this type of diabetes. Examples of monogenic diabetes include conditions like neonatal diabetes mellitus and maturity-onset diabetes of the young (MODY). Around 1% to 5% of all diabetes cases are due to monogenic diabetes. MODY is a familial disorder and usually presents under the age of 25 years.

Secondary Diabetes

  • Secondary diabetes is caused due to the complication of other diseases affecting pancreas (for example, pancreatitis), hormone disturbances (for example, Cushing’s disease), or due to drugs (for example, corticosteroids).
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Other specific types

Genetic defects of beta -cell function

  • Chromosome 20q, HNF-4 alpha (MODY1)
  • Chromosome 7p, glucokinase (MODY2)
  • Chromosome 12q, HNF-1 alpha (MODY3)
  • Chromosome 13q, insulin promoter factor-1 (MODY4)
  • Chromosome 17q, HNF-1 beta (MODY5)
  • Chromosome 2q, Neurogenic differentiation1 (MODY 6)
  • Chromosome 9, carboxyl ester lipase (MODY 7)
  • Transient Neonatal Diabetes (Chromosome 6p22 or 6p24, ZAC encoding zinc finger protein)
  • Permanent Neonatal Diabetes (Chromosome 11p15, usually KCNJ11 encoding for KIR6.2 subunit of the beta-cell KATP channel)
  • Mitochondrial DNA
  • Others

Genetic defects in insulin action

  • Type A insulin resistance
  • Leprechaunism
  • Rabson-Mendenhall syndrome
  • Lipoatrophic diabetes
  • Others

Diseases of the exocrine pancreas

  • Pancreatitis
  • Trauma/pancreatectomy
  • Neoplasia
  • Cystic fibrosis
  • Hemochromatosis
  • Fibrocalculous pancreatopathy
  • Others


  • Acromegaly
  • Cushing’s syndrome
  • Glucagonoma
  • Pheochromocytoma
  • Hyperthyroidism
  • Somatostatinoma
  • Aldosterone
  • Others

Drug- or chemical-induced

  • Vacor
  • Pentamidine
  • Nicotinic acid
  • Glucocorticoids
  • Thyroid hormone
  • Diazoxide
  • beta-adrenergic agonists
  • Thiazides
  • Dilantin(phenytoin)
  • alpha-interferon


  • Congenital rubella
  • Cytomegalovirus
  • Others

Uncommon forms of immune-mediated diabetes

  • “Stiff-man” syndrome
  • Anti-insulin receptor antibodies
  • Others

Other genetic syndromes sometimes associated with diabetes

  • Down’s syndrome
  • Klinefelter’s syndrome
  • Turner’s syndrome
  • Wolfram’s syndrome
  • Friedreich’s ataxia
  • Huntington’s chorea
  • Laurence-Moon-Bieldel syndrome
  • Myotonic dystrophy
  • Porphyria
  • Prader-Willi syndrome
  • Post-transplant

Types of According to the Diagnosis of GDM 

“One- Step” Diagnosis- 75 gram glucose tolerance test (IADPSG Consensus)
Fasting ≥92 mg/dl (5.1 mmol/L)
1-hour ≥180 mg/dl (10.0mmol/L)
2-hour ≥153 mg/dl (8.5 mmol/L)
Two-Step” Diagnosis- (NIH Consensus)
Step 1: Perform 50 gram glucose load test (nonfasting)
1-hour ≥140 mg/dl (7.8 mmol/L)
IF POSITIVE, STEP 2: 100 gram glucose tolerance test
Carpenter/Coustan NDDG
Fasting ≥95 mg/dl (5.3 mmol/L) Fasting ≥105 mg/dl (5.8 mmol/L)
1-hour ≥180 mg/dl (10.0 mmol/L) 1-hour ≥190 mg/dl (10.6 mmol/L)
2-hour ≥155 mg/dl (8.6 mmol/L) 2-hour ≥165 mg/dl (9.2 mmol/L)
3-hour ≥140 mg/dl (7.8 mmol/L) 3-hour ≥145 mg/dl (8.0 mmol/L)
*One abnormal value is sufficient to make the diagnosis of GDM. The test should be done in the morning after at least 8 hour fast. +Two abnormal values establishes the diagnosis of GDM. NDDG=National Diabetes Data Group.
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