Diabetes Mellitus — Type 2

Type 2 diabetes mellitus (T2DM) is a chronic metabolic disorder defined by hyperglycaemia arising from a combination of insulin resistance and progressive β-cell secretory failure. It is the commonest endocrine disorder in NEET PG, and questions cluster tightly around diagnostic cut-offs, drug mechanisms/contraindications, and the modern outcome-driven algorithm. Master the numbers and the drug classes and you secure easy marks.

Definition & classification

Diabetes mellitus is a group of metabolic diseases characterised by chronic hyperglycaemia with disturbances of carbohydrate, fat and protein metabolism resulting from defects in insulin secretion, insulin action, or both. T2DM accounts for ~90–95% of all diabetes.

Aetiological classification (ADA):

Type	Core defect	Key features
Type 1	Autoimmune β-cell destruction (anti-GAD65, IA-2, ZnT8)	Young, lean, ketosis-prone, insulinopenic, HLA-DR3/DR4
Type 2	Insulin resistance + relative insulin deficiency	Adult, obese (central), strong family history, ketosis-resistant
MODY	Monogenic β-cell dysfunction (autosomal dominant)	<25 yrs, non-obese, 3-generation history, negative antibodies
GDM	Insulin resistance of pregnancy	Diagnosed in 2nd/3rd trimester
Secondary	Pancreatic, endocrine, drug-induced	Cushing, acromegaly, haemochromatosis, steroids, thiazides

High-yield: MODY 3 (HNF1A) is the commonest MODY and is exquisitely sulfonylurea-sensitive. MODY 2 (glucokinase) causes mild, stable, non-progressive fasting hyperglycaemia needing no treatment. MODY is monogenic, autosomal dominant — not polygenic like ordinary T2DM.

LADA (Latent Autoimmune Diabetes in Adults) — "type 1.5": adult-onset, antibody-positive (anti-GAD), initially non-insulin-requiring but progresses to insulin dependence faster than T2DM. Suspect when a "lean type 2" fails oral agents early.

Aetiology & pathophysiology

T2DM is driven by an interplay of genetic susceptibility (concordance in identical twins ~70–90%, far higher than type 1) and environmental factors — obesity, physical inactivity, calorie excess.

The classic dual lesion:

1. Insulin resistance — reduced glucose uptake in skeletal muscle (the largest disposal site), failure to suppress hepatic gluconeogenesis, and impaired suppression of adipose lipolysis. Free fatty acids and inflammatory adipokines (TNF-α, resistin, low adiponectin) worsen resistance.
2. Progressive β-cell failure — hyperinsulinaemia compensates early (impaired glucose tolerance stage), but secretory capacity declines ~4% per year. Frank hyperglycaemia appears when the β-cell can no longer compensate. By diagnosis, ~50% of β-cell function is already lost.

The "ominous octet" (DeFronzo) — eight organ-level contributors to hyperglycaemia, frequently asked:

↓ Incretin effect (gut) → ↑ Glucagon (α-cell) → ↑ Hepatic glucose output → ↓ Muscle uptake → β-cell failure → ↑ Lipolysis (adipocyte) → ↑ Renal glucose reabsorption (kidney) → Neurotransmitter dysfunction (brain).

High-yield: The incretin effect = oral glucose evokes far more insulin than equivalent IV glucose, mediated by GLP-1 and GIP from gut L- and K-cells. This effect is blunted in T2DM and is the rationale for GLP-1 agonists and DPP-4 inhibitors.

Clinical features

• Classic osmotic symptoms: polyuria, polydipsia, polyphagia, weight loss — but T2DM is frequently asymptomatic, detected on screening.
• Recurrent infections — candidal balanitis/vulvovaginitis, skin abscesses, UTIs.
• Acanthosis nigricans — velvety hyperpigmentation of neck/axillae, a marker of insulin resistance.
• Blurred vision (osmotic lens changes), fatigue, poor wound healing.
• Presentation may be a complication: silent MI, neuropathic foot ulcer, retinopathy on routine eye exam, or HHS (hyperosmolar hyperglycaemic state).

High-yield: HHS (not DKA) is the classic hyperglycaemic emergency of T2DM — glucose often >600 mg/dL, serum osmolality >320 mOsm/kg, profound dehydration, minimal/no ketosis, altered sensorium. Residual insulin suppresses ketogenesis. Mortality is higher than DKA.

Diagnosis & investigation of choice

Any one of the following ADA criteria establishes diabetes (repeat to confirm unless unequivocal hyperglycaemia):

Test	Diabetes	Prediabetes
Fasting plasma glucose (FPG, ≥8 h fast)	≥126 mg/dL	IFG: 100–125
2-h plasma glucose on 75 g OGTT	≥200 mg/dL	IGT: 140–199
HbA1c	≥6.5%	5.7–6.4%
Random glucose + classic symptoms	≥200 mg/dL	—

High-yield: Memorise the trio — FPG ≥126, 2-h OGTT ≥200, HbA1c ≥6.5%. A random glucose ≥200 counts only with symptoms. In the absence of unequivocal hyperglycaemia, two abnormal results (same or different tests) are required.

HbA1c caveats — falsely low in conditions of reduced RBC lifespan (haemolytic anaemia, recent transfusion, blood loss, pregnancy, erythropoietin therapy); falsely high in iron/B12 deficiency anaemia, splenectomy, uraemia, alcoholism. Haemoglobinopathies (HbS, HbC, thalassaemia) give unreliable values — use fructosamine instead. HbA1c reflects average glycaemia over the prior ~2–3 months (RBC lifespan ~120 days).

Screening (ADA): all adults ≥35 years, or younger adults who are overweight/obese with one additional risk factor (family history, hypertension, dyslipidaemia, PCOS, prior GDM, sedentary). Repeat every 3 years if normal.

Glycaemic targets

Parameter	General adult target
HbA1c	<7% (individualise)
Preprandial capillary glucose	80–130 mg/dL
Peak postprandial (1–2 h)	<180 mg/dL

Tighter (<6.5%) is reasonable in young, healthy, short-duration patients if achievable without hypoglycaemia. Looser (<8%) suits the elderly, those with limited life expectancy, advanced complications, or hypoglycaemia unawareness. The ACCORD trial showed intensive control (HbA1c ~6.4%) increased mortality in high-risk patients — overtreatment kills.

Management & drug of choice

Foundation = lifestyle therapy (medical nutrition therapy, ≥150 min/week activity, weight loss of 5–10%) applied at every stage. Pharmacotherapy is now driven by comorbidities, not just HbA1c.

Stepwise outcome-based approach (current ADA/EASD):

1. Metformin + comprehensive lifestyle change is first-line for most.
2. Independently assess for high-risk comorbidity regardless of HbA1c or metformin use:
   • Established ASCVD / high CV risk → GLP-1 RA or SGLT2 inhibitor with proven benefit.
   • Heart failure (HFrEF/HFpEF) → SGLT2 inhibitor.
   • CKD (albuminuria / reduced eGFR) → SGLT2 inhibitor (preferred); GLP-1 RA if SGLT2 not tolerated.
   • Obesity / weight priority → GLP-1 RA (semaglutide, tirzepatide) for greatest weight loss.
3. Intensify with additional agents to reach individualised HbA1c; add basal insulin when needed.

High-yield: Metformin is the drug of choice / first-line for uncomplicated T2DM. But in a patient with CKD or heart failure, an SGLT2 inhibitor is added/preferred irrespective of HbA1c; in obesity or ASCVD, a GLP-1 receptor agonist is favoured. This "comorbidity-first" logic is the single most tested 2024–26 concept.

Drug classes — mechanism, benefit, caution

Class (example)	Mechanism	Hypo?	Weight	Key caution
Biguanide (metformin)	↓ Hepatic gluconeogenesis (AMPK), ↑ peripheral uptake	No	Neutral/↓	Lactic acidosis; hold if eGFR <30; GI upset, ↓B12
Sulfonylurea (glimepiride)	Close K-ATP channel → ↑ insulin secretion	Yes	↑	Hypoglycaemia (esp. glibenclamide in elderly)
Meglitinide (repaglinide)	K-ATP closure, short-acting	Yes	↑	Postprandial control; usable in CKD
TZD (pioglitazone)	PPAR-γ agonist → ↑ insulin sensitivity	No	↑	Fluid retention/CHF, fractures, bladder Ca
DPP-4 inhibitor (sitagliptin)	↑ Endogenous GLP-1/GIP	No	Neutral	Saxagliptin → HF hospitalisation; pancreatitis
GLP-1 RA (semaglutide)	Incretin mimetic; ↑ insulin, ↓ glucagon, delays gastric emptying, satiety	No	↓↓	Nausea; MTC/MEN2 contraindication; pancreatitis
SGLT2 inhibitor (empagliflozin)	Block proximal tubule glucose reabsorption → glycosuria	No	↓	Euglycaemic DKA, genital mycotic infection, volume depletion
α-glucosidase inhibitor (acarbose)	↓ Intestinal carbohydrate absorption	No	Neutral	Flatulence, diarrhoea

High-yield: SGLT2 inhibitors uniquely provide cardiorenal protection — reduce HF hospitalisation and slow CKD progression even in non-diabetics. Their hazard is euglycaemic DKA (ketoacidosis with glucose <250 mg/dL) — stop before surgery/acute illness ("sick day rule").

High-yield: GLP-1 RAs and DPP-4 inhibitors are contraindicated in personal/family history of medullary thyroid carcinoma or MEN-2 (rodent C-cell tumour signal). GLP-1 RAs produce the greatest weight loss; tirzepatide (dual GIP/GLP-1) is the most potent.

High-yield: Pioglitazone is contraindicated in heart failure (fluid retention) and associated with osteoporotic fractures and bladder cancer — but is beneficial in NASH and insulin-resistant states.

Metformin and contrast/surgery: withhold around iodinated contrast if eGFR <30 or acute kidney injury risk, due to lactic acidosis. Metformin's only absolute contraindication historically is significant renal impairment (eGFR <30), hepatic failure, and tissue hypoxia states.

Insulin — needed in marked hyperglycaemia (HbA1c >10%, glucose >300, symptomatic catabolism), intercurrent illness, pregnancy, or after oral failure. Start basal insulin (glargine/detemir/degludec) titrated to fasting glucose; add prandial (basal-bolus) if postprandial targets unmet.

Complications

Microvascular (glycaemic-control dependent):

• Retinopathy — leading cause of adult blindness; non-proliferative → proliferative (neovascularisation). Annual dilated fundus screening from diagnosis in T2DM. Treat with anti-VEGF / laser photocoagulation.
• Nephropathy — leading cause of ESRD. Earliest marker = moderately increased albuminuria (urine albumin-to-creatinine ratio 30–300 mg/g, formerly "microalbuminuria"). Screen annually with UACR + eGFR. ACE-i/ARB ± SGLT2 inhibitor are renoprotective.
• Neuropathy — distal symmetric "glove-and-stocking" sensory loss; autonomic (gastroparesis, postural hypotension, erectile dysfunction). Foot care + monofilament testing prevent ulcers/amputation.

Macrovascular — coronary artery disease (commonest cause of death in T2DM), stroke, peripheral arterial disease. Aggressive statin therapy and BP control (<130/80) matter more than glucose for CV outcomes.

Acute — HHS (see above), DKA (less common, may occur with severe stress or SGLT2 use), and hypoglycaemia (sulfonylureas, insulin — Whipple's triad).

High-yield: Intensive glycaemic control reduces microvascular complications robustly (UKPDS, DCCT legacy effect — "metabolic memory") but has a weaker, slower effect on macrovascular disease. Statin and BP control dominate macrovascular risk reduction.

Key differentials

• Type 1 DM / LADA — younger, lean, ketosis-prone, antibody-positive, low C-peptide. Check anti-GAD and C-peptide when phenotype is atypical.
• MODY — strong 3-generation autosomal dominant history, young, non-obese, antibody-negative.
• Secondary diabetes — Cushing syndrome, acromegaly, phaeochromocytoma, haemochromatosis ("bronze diabetes"), chronic pancreatitis, glucocorticoid/thiazide/antipsychotic-induced.
• Stress hyperglycaemia — transient during acute illness; confirm with HbA1c after recovery.

C-peptide distinguishes endogenous from exogenous insulin and is low in type 1, normal/high in T2DM — useful in classification dilemmas.

Recently asked / exam angle

• Matching drug class to comorbidity: HF → SGLT2i, obesity/ASCVD → GLP-1 RA — the highest-yield modern stem.
• Euglycaemic DKA with SGLT2 inhibitors — recognise normal/near-normal glucose with ketoacidosis.
• Sulfonylurea-responsive MODY 3 (HNF1A) and treatment-free MODY 2 (glucokinase).
• HbA1c falsely low in haemolysis / falsely high in iron deficiency — interpretation pitfalls.
• Pioglitazone contraindicated in CHF, useful in NASH; GLP-1/DPP-4 contraindicated with medullary thyroid Ca / MEN-2.
• Earliest nephropathy marker = albuminuria (UACR 30–300).
• ACCORD — harm of overly tight control in high-risk patients.
• Diagnostic numbers as direct recall (FPG 126 / OGTT 200 / HbA1c 6.5).

Rapid revision

1. T2DM = insulin resistance + progressive β-cell failure; ~50% β-cell loss at diagnosis.
2. Diagnose: FPG ≥126, 2-h OGTT ≥200, HbA1c ≥6.5%; random ≥200 only with symptoms.
3. Prediabetes: IFG 100–125, IGT 140–199, HbA1c 5.7–6.4%.
4. HbA1c reflects ~2–3 months; falsely low in haemolysis, falsely high in iron deficiency.
5. Metformin = first-line; hold if eGFR <30 (lactic acidosis), causes B12 deficiency.
6. SGLT2 inhibitors for HF and CKD — cardiorenal protection; risk = euglycaemic DKA.
7. GLP-1 RAs for obesity and ASCVD — best weight loss; contraindicated with MTC/MEN-2.
8. Pioglitazone — avoid in heart failure; useful in NASH; risks fractures, bladder Ca.
9. Sulfonylureas and insulin cause hypoglycaemia; SUs cause weight gain.
10. HHS (glucose >600, osmolality >320, minimal ketosis) is the T2DM hyperglycaemic emergency.
11. Earliest nephropathy sign = albuminuria (UACR 30–300); treat with ACE-i/ARB + SGLT2i.
12. MODY 3 = sulfonylurea-sensitive; MODY 2 = no treatment; LADA = antibody-positive adult "type 1.5".