Type 1 Diabetes Mellitus
Autoimmune destruction of pancreatic beta cells leading to absolute insulin deficiency.
Also known as: T1DM, type 1 diabetes, insulin-dependent diabetes, juvenile diabetes, IDDM
Overview
Chronic autoimmune disease characterized by T-cell mediated destruction of pancreatic islet beta cells, resulting in absolute insulin deficiency and lifelong dependence on exogenous insulin to prevent ketoacidosis and death.
Epidemiology
Accounts for ~5-10% of all diabetes. Bimodal peak incidence at age 4-7 and 10-14 years, but can present at any age (including LADA in adults). Higher prevalence in non-Hispanic whites, Scandinavian populations. Strong HLA association (HLA-DR3, HLA-DR4).
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Risk factors
- Genetic: HLA-DR3/DR4-DQ8 haplotypes (highest risk), family history (5-10% if first-degree relative)
- Environmental triggers: enteroviral infection (coxsackie B), early cow's milk exposure, vitamin D deficiency
- Other autoimmune disease: Hashimoto thyroiditis, celiac disease, Addison disease, pernicious anemia (polyglandular syndrome type 2)
Pathophysiology
Autoreactive T cells infiltrate pancreatic islets (insulitis) and selectively destroy insulin-producing beta cells. Clinical hyperglycemia emerges when ~80-90% of beta-cell mass is lost. Absolute insulin deficiency leads to unopposed glucagon action, hepatic gluconeogenesis, lipolysis, and ketogenesis. Without insulin, patients cannot suppress ketone production and are at risk for DKA.
Clinical presentation
Symptoms
- Classic triad: polyuria, polydipsia, polyphagia
- Unintentional weight loss despite normal or increased intake
- Fatigue, weakness, blurred vision
- May present with DKA as initial manifestation (~30% of pediatric cases): nausea, vomiting, abdominal pain, Kussmaul respirations, altered mental status
Signs / physical exam
- Often thin or recent weight loss; not obese
- Dehydration: dry mucous membranes, decreased skin turgor, tachycardia
- Fruity (acetone) breath in DKA
- Kussmaul (deep, rapid) respirations in DKA
Classic findings
Lean young patient with rapid onset of polyuria, polydipsia, weight loss, sometimes presenting in DKA.
Differential diagnosis
- Type 2 diabetes mellitus — Older onset, obesity, acanthosis nigricans, family history, gradual onset; insulin resistance with relative deficiency; negative autoantibodies; C-peptide preserved
- Latent autoimmune diabetes of adults (LADA) — Adult onset (>30), initial response to oral agents then rapid progression to insulin; positive GAD-65 antibodies; slow autoimmune destruction
- MODY (maturity-onset diabetes of the young) — Autosomal dominant family history across 3 generations; onset <25; non-obese; no autoantibodies; preserved C-peptide; HNF1A/glucokinase mutations
- Secondary diabetes (pancreatic) — History of chronic pancreatitis, pancreatectomy, hemochromatosis, cystic fibrosis; absent C-peptide; pancreatic exocrine insufficiency
- Drug-induced diabetes — Glucocorticoids, tacrolimus, atypical antipsychotics (olanzapine, clozapine), HIV protease inhibitors, thiazides
- Stress hyperglycemia — Transient elevation during acute illness, sepsis, or trauma in patient without prior diabetes; resolves with recovery
Diagnostic workup
Diagnostic criteria
ADA criteria: A1c ≥6.5%, fasting glucose ≥126 mg/dL, 2-hr OGTT ≥200 mg/dL, or random ≥200 mg/dL with symptoms. T1DM confirmed by positive autoantibodies and/or low C-peptide.
Labs
- Fasting plasma glucose ≥126 mg/dL on 2 occasions, OR random glucose ≥200 mg/dL with symptoms, OR A1c ≥6.5%, OR 2-hour OGTT ≥200 mg/dL
- Islet autoantibodies (≥1 positive confirms T1DM): GAD-65, IA-2, insulin autoantibodies (IAA), ZnT8
- C-peptide (low or undetectable) — differentiates from T2DM
- TSH, TPO antibodies, tissue transglutaminase IgA (screen for associated autoimmune disease)
- BMP, urine ketones, blood gas if DKA suspected
Imaging
- Not routinely needed for diagnosis
- Dilated retinal exam at diagnosis and annually thereafter
Diagnostic algorithm
| Feature | Type 1 DM | Type 2 DM |
|---|---|---|
| Age of onset | Usually <30 (any age possible) | Usually >40 (increasing in youth) |
| Body habitus | Lean or normal weight | Overweight/obese (~80%) |
| Onset | Acute (weeks); may present with DKA | Insidious (months-years) |
| Pathophysiology | Autoimmune beta-cell destruction | Insulin resistance + relative deficiency |
| Autoantibodies | Positive (GAD-65, IA-2, IAA, ZnT8) | Negative |
| C-peptide | Low or undetectable | Normal or elevated (early) |
| Ketosis-prone | Yes (absolute insulin deficiency) | Rare (except HHS or severe stress) |
| First-line treatment | Insulin (lifelong) | Lifestyle + metformin |
| HLA association | HLA-DR3/DR4-DQ8 | None |
| Family history | 5-10% in first-degree relatives | Strong (40% lifetime risk if both parents) |
Treatment
First-line
- Lifelong exogenous insulin — basal-bolus regimen mimicking physiologic secretion
- Long-acting basal insulin: glargine (Lantus, Basaglar, Toujeo), detemir, degludec (Tresiba) — once daily
- Rapid-acting prandial insulin: lispro (Humalog), aspart (Novolog), glulisine (Apidra) — at meals based on carb counting
- Insulin pump therapy (CSII) with rapid-acting insulin — alternative to multiple daily injections
- Continuous glucose monitor (CGM): Dexcom G6/G7, Freestyle Libre — strongly recommended
- Carbohydrate counting and insulin-to-carb ratio education
Second-line / adjunct
- Closed-loop / hybrid closed-loop systems (Tandem Control-IQ, Medtronic 780G, Omnipod 5) — automated insulin delivery
- Pramlintide (amylin analog) — adjunct to mealtime insulin in selected patients
- Pancreas or islet cell transplant — reserved for refractory hypoglycemia unawareness or end-stage renal disease requiring kidney transplant
Complications
- Acute: DKA, hypoglycemia (especially with intensive insulin therapy), hypoglycemia unawareness
- Microvascular: retinopathy, nephropathy, neuropathy (peripheral, autonomic)
- Macrovascular: CAD, stroke, peripheral arterial disease
- Infections: foot ulcers, recurrent UTIs, mucormycosis (in DKA)
- Psychosocial: eating disorders (diabulimia — insulin omission for weight loss), depression
PANCE pearls
- Honeymoon phase: transient partial remission after diagnosis with reduced insulin requirements; insulin needs return as residual beta cells fail.
- Dawn phenomenon: early morning hyperglycemia from nocturnal growth hormone and cortisol surge; treat by adjusting basal insulin timing.
- Somogyi effect: rebound hyperglycemia after nocturnal hypoglycemia (controversial in modern practice with CGM data).
- Always check for DKA in any T1DM patient with acute illness, vomiting, or hyperglycemia >250 mg/dL.
- Sick-day rules: never stop insulin; check ketones; increase frequency of glucose monitoring; maintain hydration.
References
- ADA 2025 — American Diabetes Association Standards of Care in Diabetes—2025 (Diabetes Care 2025; 48 Suppl 1)
- DCCT/EDIC — The Effect of Intensive Treatment of Diabetes on the Development and Progression of Long-Term Complications in IDDM (DCCT Research Group, NEJM 1993)
- ISPAD 2022 — ISPAD Clinical Practice Consensus Guidelines 2022 — Type 1 Diabetes in Children and Adolescents
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