What are the other classifications of diabetes besides Type 1 and Type 2?

Classifications of Diabetes Beyond Type 1 and Type 2

Beyond Type 1 and Type 2 diabetes, there are two additional major classifications: Gestational diabetes mellitus (GDM) and Specific types of diabetes due to other causes, which include monogenic diabetes syndromes, diseases of the exocrine pancreas, and drug or chemical-induced diabetes. 1

Complete Classification of Diabetes

According to the most recent American Diabetes Association guidelines, diabetes is classified into the following categories:

1. Type 1 diabetes

   • Due to autoimmune β-cell destruction
   • Usually leads to absolute insulin deficiency
   • Includes latent autoimmune diabetes of adulthood (LADA)

2. Type 2 diabetes

   • Due to non-autoimmune progressive loss of β-cell insulin secretion
   • Often occurs on the background of insulin resistance and metabolic syndrome

3. Gestational diabetes mellitus (GDM)

   • Diabetes diagnosed in the second or third trimester of pregnancy
   • Not clearly overt diabetes prior to gestation

4. Specific types of diabetes due to other causes:

   • Monogenic diabetes syndromes:
     • Neonatal diabetes
     • Maturity-onset diabetes of the young (MODY)
   • Diseases of the exocrine pancreas:
     • Cystic fibrosis
     • Pancreatitis
     • Pancreatic cancer
   • Drug or chemical-induced diabetes:
     • Glucocorticoid use
     • Treatment for HIV/AIDS
     • Post-organ transplantation medications 1, 2

Specific Types of Diabetes Due to Other Causes

Monogenic Diabetes Syndromes

• MODY (Maturity-Onset Diabetes of the Young):

  • Single gene mutations affecting β-cell function
  • Typically presents before age 25
  • Strong family history with autosomal dominant inheritance
  • Often misdiagnosed as Type 1 or Type 2 diabetes 2

• Neonatal Diabetes:

  • Diagnosed within first 6 months of life
  • Permanent or transient forms
  • Caused by specific genetic mutations 2

Pancreatic Diabetes

• Results from diseases affecting the exocrine pancreas:
  • Pancreatitis (acute and chronic)
  • Trauma/pancreatectomy
  • Neoplasia
  • Cystic fibrosis
  • Hemochromatosis
  • Fibrocalculous pancreatopathy 1, 3

Endocrine Disorders

• Conditions with excess hormones that antagonize insulin:
  • Cushing's syndrome
  • Acromegaly
  • Pheochromocytoma
  • Glucagonoma
  • Hyperthyroidism 2, 3

Drug or Chemical-Induced Diabetes

• Medications that can impair insulin secretion or action:
  • Glucocorticoids (including cutaneous and inhaled forms)
  • Thiazide diuretics
  • Beta-blockers
  • Atypical antipsychotics
  • Antiretroviral drugs
  • Anti-rejection medications after organ transplantation 1, 2

Clinical Implications of Proper Classification

• Diagnostic Challenges: Classification is not always straightforward at diagnosis, and misdiagnosis is common, occurring in up to 40% of adults with new-onset Type 1 diabetes 2

• Treatment Implications: Proper classification directs appropriate therapy:

  • Some MODY subtypes respond well to sulfonylureas rather than insulin
  • Treating underlying endocrine disorders may resolve diabetes
  • Pancreatic diabetes often requires insulin therapy due to absolute deficiency 2

• Monitoring Requirements: All forms of diabetes require vigilance for complications, but the progression and risk profiles may differ between types 2

Diagnostic Considerations

When the classification is unclear, additional testing may help:

• Islet autoantibody testing (GAD, IA-2, ZnT8)
• C-peptide measurement
• Genetic testing for suspected monogenic forms
• Imaging studies for pancreatic disease 2

Regular reassessment of diabetes classification is important, especially when treatment response is suboptimal, as the true diagnosis may become more evident over time 1, 2.