Diabetes mellitus: diagnosis and classification
- Topic
Diabetes means blood sugar stays too high, because the body lacks insulin or does not respond to it, or both.
What it is
Diabetes mellitus is a group of metabolic disorders with chronic hyperglycaemia from defective insulin secretion, insulin action, or both. It is diagnosed when fasting glucose, a two-hour glucose after an oral glucose load, HbA1c, or a random glucose with classic symptoms is above the diagnostic thresholds (the condition pages give the numbers). Type 2 accounts for the large majority of cases and is rising fast in Latin America with obesity.
Step by step
- Glucose enters the beta cell through GLUT2 and is phosphorylated by glucokinase, the glucose sensor.
- Rising ATP closes KATP channels (KCNJ11 and ABCC8 subunits) and depolarises the cell.
- Calcium enters and triggers release of stored insulin (first phase), then newly made insulin (second phase); incretins (GLP-1, GIP) amplify it.
- Insulin binds its tyrosine-kinase receptor; through IRS and PI3K-AKT it moves GLUT4 to the membrane of muscle and fat and stores glycogen, fat and protein.
- If insulin is missing or its signal is blunted, glucose stays high, lipolysis and gluconeogenesis rise, and diabetes follows.
Type 1 and type 2 diabetes compared
| Feature | Type 1 | Type 2 |
|---|---|---|
| Mechanism | Autoimmune beta-cell loss | Insulin resistance with beta-cell failure |
| Onset | Usually childhood or young adult; can be any age | Usually adult, now also adolescents |
| Body weight | Normal or thin | Usually obese |
| Insulin levels | Very low, C-peptide low | Normal or high early, falls later |
| Islet autoantibodies | Present | Absent |
| Genetics | HLA-DR3/DR4-DQ8, strong | Polygenic, stronger family concordance |
| Acute crisis | Ketoacidosis | Hyperosmolar hyperglycaemic state |
| Islet morphology | Insulitis, loss of beta cells | Islet amyloid, modest beta-cell loss |
Clinical notes
- Classic symptoms: polyuria, polydipsia, polyphagia, weight loss.
- HbA1c reflects average glucose over the preceding two to three months.
- In Colombia and Latin America, diabetes is a leading cause of kidney failure, blindness and amputation.
Test yourself
Think of the answer, then tap the question to see it.
Which tests can diagnose diabetes?
Fasting plasma glucose, two-hour glucose after an oral glucose load, HbA1c, or a random glucose with classic symptoms, each above its diagnostic threshold.
What closes the KATP channel in a beta cell after a meal?
A rise in intracellular ATP from glucose metabolism, after glucose enters through GLUT2 and is phosphorylated by glucokinase.
Which two endocrine diseases commonly cause secondary diabetes?
Cushing syndrome and acromegaly (also pheochromocytoma and glucagonoma).
Elsewhere in DurgiPedia
- Islets and insulin: secretion and actions
- Glucagon and the counter-regulatory hormones
- Insulin signalling
- Glycated haemoglobin (HbA1c)
- Islets of Langerhans: α, β, δ cells
Connections
Educational content on the mechanisms of disease; not a diagnostic or treatment guide and no substitute for a health professional's advice. No doses are given.
Sources: Diabetes (World Health Organization, 2025); Diabetes Type 2 (MedlinePlus, US National Library of Medicine, 2025).
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Representative model, for educational purposes only.