Overview.
Type 2 diabetes affects approximately 10% of US adults (37 million people) and is preceded by prediabetes in up to 38% of the population. It develops when cells become resistant to insulin and the pancreas can no longer compensate with sufficient insulin production. The result is chronically elevated blood glucose that silently damages blood vessels, nerves, and organs over years before symptoms appear. Early detection via HbA1c and fasting glucose enables reversal at the prediabetes stage and prevention of serious complications.
In type 2 diabetes, insulin resistance in muscle, liver, and fat cells impairs glucose uptake. The pancreas initially compensates by producing more insulin, but over time beta-cell function declines, resulting in sustained hyperglycaemia. Unlike type 1 diabetes, beta cells are not destroyed by autoimmunity — they are exhausted. The process begins with insulin resistance (often detectable via fasting insulin and HOMA-IR) years before diagnostic thresholds for prediabetes or diabetes are crossed.
Prevalence: ~37 million US adults have type 2 diabetes (~10%). An additional ~96 million (~38%) have prediabetes. Approximately 80% of people with prediabetes are unaware of their status. Globally, 537 million people live with diabetes (IDF 2021), projected to reach 783 million by 2045.
Medical name: Type 2 Diabetes Mellitus (T2DM)
Symptoms.
Early warnings
- Elevated fasting glucose on routine blood work (100-125 mg/dL = prediabetes range)
- HbA1c between 5.7% and 6.4% on any blood test
- Elevated fasting insulin (>10 μIU/mL) suggesting insulin resistance before glucose rises
- Skin tags or acanthosis nigricans (darkened skin in skin folds — sign of insulin resistance)
- Unintentional weight gain around the abdomen
- Increased fatigue, especially after carbohydrate-rich meals
Classic symptoms
- Polyuria (frequent urination) — glucose spills into urine, pulling water with it
- Polydipsia (excessive thirst) — from polyuria-driven dehydration
- Polyphagia (increased hunger) — cellular glucose starvation despite high blood glucose
- Unexplained weight loss (less common in T2D than T1D)
- Blurred vision — lens changes from hyperosmolarity
- Slow wound healing — impaired immune cell function and microvascular disease
- Recurrent infections — candidal vulvovaginitis, balanitis, skin infections
- Peripheral neuropathy — numbness, tingling, burning feet (late complication)
Progression
Type 2 diabetes typically develops over a decade: insulin resistance → compensatory hyperinsulinaemia (detectable on fasting insulin) → impaired fasting glucose / impaired glucose tolerance (prediabetes) → overt diabetes. Without intervention, ~5-10% of prediabetes cases progress to T2D per year; over a lifetime, ~70% will develop T2D. With lifestyle intervention (diet + exercise), the Diabetes Prevention Program showed 58% reduction in progression. With intensive lifestyle modification and weight loss ≥15%, remission of established T2D is achievable in a significant minority.
Risk factors.
- Overweight or obesity (BMI ≥25; BMI ≥23 in Asian populations)
- Physical inactivity
- Family history of type 2 diabetes (first-degree relative)
- History of gestational diabetes or polycystic ovary syndrome (PCOS)
- Prediabetes (HbA1c 5.7-6.4% or fasting glucose 100-125 mg/dL)
- Metabolic syndrome (hypertension, dyslipidaemia, central obesity)
- Age ≥45 years
- Non-white ethnicity (Black, Hispanic, Asian, Native American populations at higher risk at lower BMI)
- MASLD/NAFLD (fatty liver disease)
- Sleep apnoea
- Certain medications: corticosteroids, antipsychotics, thiazide diuretics
Lab interpretation.
Key biomarkers
- HbA1c (Glycated Haemoglobin) — Diabetes: ≥6.5%. Prediabetes: 5.7-6.4%. Normal: <5.7%. Reflects average glucose over ~90 days. First-line screening test per ADA 2024. (primary)
- Fasting Plasma Glucose — Diabetes: ≥126 mg/dL (7.0 mmol/L). Prediabetes IFG: 100-125 mg/dL. Normal: <100 mg/dL. Requires 8-hour fast. (primary)
- Fasting Insulin — Optimal: <5 μIU/mL. Insulin resistance: >10-15 μIU/mL. Lab 'normal' ranges (often up to 25) include metabolically unhealthy individuals. Rises 10-15 years before glucose does. (primary)
- HOMA-IR — Calculated: (fasting glucose mg/dL × fasting insulin μIU/mL) / 405. >2.5 indicates insulin resistance; >3.5 significant resistance. Not a direct blood test — derived from glucose and insulin results. (primary)
- C-Peptide — Preserved in T2D (0.5-2.0 nmol/L range); depleted in T1D. Used to distinguish diabetes types and assess residual beta-cell function. (secondary)
- eGFR (Estimated Glomerular Filtration Rate) — Normal ≥60 mL/min/1.73m². Annual screening for diabetic nephropathy. eGFR <60 = CKD; combination of eGFR + UACR determines CKD staging. (secondary)
- UACR (Urine Albumin-to-Creatinine Ratio) — Normal <30 mg/g. Microalbuminuria: 30-300 mg/g (early nephropathy). Macroalbuminuria: >300 mg/g. Annual screening recommended for all T2D patients from diagnosis. (secondary)
Diagnostic criteria
- HbA1c ≥6.5% (using NGSP-certified, DCCT-traceable assay) — ADA 2024 first-line criterion
- Fasting plasma glucose ≥126 mg/dL (7.0 mmol/L) after minimum 8-hour fast
- 2-hour plasma glucose ≥200 mg/dL during 75g oral glucose tolerance test (OGTT)
- Random plasma glucose ≥200 mg/dL with classic hyperglycaemia symptoms (polyuria, polydipsia, unexplained weight loss)
- Note: Two separate abnormal results required on different days if asymptomatic; one result sufficient if classic symptoms present
- Prediabetes (IFG): fasting glucose 100-125 mg/dL
- Prediabetes (IGT): 2-hour OGTT glucose 140-199 mg/dL
- Prediabetes (HbA1c): 5.7-6.4%
Recommended panels
When & next steps.
When to test
- All adults aged ≥35 (ADA 2024) — HbA1c or fasting glucose; repeat every 3 years if normal
- Any adult who is overweight (BMI ≥25) with one or more additional risk factors — screen at any age
- All adults aged ≥45 if previous test was normal
- Women with gestational diabetes history — screen every 3 years postpartum
- Anyone with prediabetes — annual HbA1c monitoring
- Any unexplained polyuria, polydipsia, or nocturia
- New peripheral neuropathy (numbness/tingling feet) — HbA1c to rule out occult diabetes
- Before starting antipsychotics or corticosteroids (baseline metabolic screen)
If suspected
- HbA1c (primary) — no fasting required; reflects 90-day average glucose
- Fasting plasma glucose — requires 8-hour fast; combined with HbA1c most informative
- Fasting insulin (add-on) — identifies insulin resistance before glucose becomes diagnostic
- Lipid panel — cardiovascular risk assessment at diagnosis
- CMP — kidney function (eGFR) and liver enzymes
- If T2D vs T1D uncertain: C-peptide + GAD65/IA-2 autoantibodies
If confirmed
- Establish care with primary care physician or endocrinologist
- HbA1c target: generally <7.0% (individualised — <6.5% if achievable without hypoglycaemia, <8.0% if frail/complex)
- Annual screening: eGFR + UACR (nephropathy), lipid panel, dilated eye exam (retinopathy), foot exam (neuropathy)
- Metformin: first-line pharmacological agent; monitor B12 annually (depleted by metformin)
- Lifestyle: Mediterranean or low-carbohydrate diet, 150 min/week moderate exercise
- Weight loss ≥10-15%: can achieve T2D remission in recently diagnosed patients
- Blood pressure target: <130/80 mmHg; ACE inhibitor or ARB if UACR elevated
- Statin: moderate-to-high intensity for all T2D aged 40-75 per AHA/ACC
FAQs.
What's the difference between type 1 and type 2 diabetes?
Type 1 diabetes is an autoimmune condition where the immune system destroys insulin-producing beta cells — it typically starts in childhood or young adulthood and requires insulin from day one. Type 2 diabetes is a metabolic condition driven by insulin resistance and progressive beta-cell exhaustion — it can often be managed initially with lifestyle changes and oral medications. C-peptide and autoantibody testing (GAD65, IA-2) can distinguish the two when there is uncertainty.
Can type 2 diabetes be reversed?
Remission (HbA1c <6.5% without diabetes medication) is achievable, particularly with significant weight loss (>15% of body weight) early in the disease. The DiRECT trial showed 46% of participants achieved remission at 1 year with a low-calorie dietary approach. Remission is not guaranteed to be permanent, but the longer it is maintained, the lower the long-term complication risk. 'Cured' is not the right framing — lifelong metabolic vigilance remains important.
My fasting glucose is normal — could I still have insulin resistance?
Yes. Fasting glucose is one of the last markers to become abnormal. Insulin resistance can exist for 10-15 years before fasting glucose crosses the prediabetes threshold. Fasting insulin (>10 μIU/mL) and HOMA-IR (>2.5) can detect insulin resistance when fasting glucose is completely normal. HbA1c captures post-meal glucose exposure that fasting glucose misses.
I'm on metformin — what else should I monitor?
Annual vitamin B12 levels are essential for anyone on metformin. Metformin reduces B12 absorption through a calcium-dependent mechanism; deficiency is dose-dependent and develops over years. B12 deficiency on metformin can cause peripheral neuropathy that is easily misattributed to diabetic neuropathy — making it critical to check. Also monitor: HbA1c every 3-6 months until stable, annual eGFR/UACR, lipids annually.