Diabetes Mellitus

Slide 1 · Diabetes Mellitus

Pathogenesis · diagnostic classification · metabolic physiology

Dr Abu Suraih Sakhri

MBBS, MD (Pathology)

www.abusuraihsakhri.com

Slide 2 · Learning outcomes

Explain how defects of insulin secretion and action produce hyperglycemia.

Describe pancreatic islet hormones and normal fasting–fed glucose regulation.

Apply Robbins diagnostic thresholds to diabetes and prediabetes.

Classify the major diabetes phenotypes and important secondary forms.

Integrated emphasis: Mechanism → laboratory recognition → disease classification.

Slide 3 · Diabetes is a group of metabolic disorders

Persistent hyperglycemia is the shared biochemical feature.

The underlying defect involves insulin secretion, insulin action, or both.

Distinct etiologies can therefore produce a similar elevated plasma glucose measurement.

Hyperglycemia defines the phenotype; Pathogenesis defines the subtype.

Slide 4 · How insulin dysfunction produces hyperglycemia

Deficient insulin secretion

Reduced effective insulin action

Inadequate glucose disposal / regulation

Hyperglycemia

Both impaired secretion and diminished tissue responsiveness can coexist.

Slide 5 · Why chronic hyperglycemia matters

Diabetes is not confined to the endocrine pancreas.

Sustained hyperglycemia and metabolic dysregulation cause secondary organ injury.

The kidney, retina, peripheral nerves, and blood vessels are particularly vulnerable.

Organ complications provide the central clinicopathologic significance.

Slide 6 · The endocrine pancreas: Islets of Langerhans

Approximately one million endocrine islet clusters are distributed through the pancreas.

Each cluster contains several specialized hormone-secreting cell types.

Local insulin–glucagon coordination contributes to systemic glucose homeostasis.

Islet-cell identity provides a basis for understanding diabetes.

Slide 7 · Principal pancreatic islet-cell populations

Cell type

Major product

Glucose-related function

β cells

Insulin

Promotes glucose utilization; Lowers plasma glucose

α cells

Glucagon

Stimulates hepatic glucose production

δ cells

Somatostatin

Suppresses insulin and glucagon secretion

PP cells

Pancreatic polypeptide

Gastrointestinal regulatory effects

The endocrine response is coordinated rather than a single-hormone switch.

Slide 8 · β cells regulate metabolic storage and utilization

β cells synthesize insulin and release it in response to nutrient-associated stimuli.

Insulin favors uptake and utilization of circulating glucose.

Insulin deficiency is central to the hyperglycemia of type 1 diabetes.

The β cell is both a secretory organ and a metabolic glucose sensor.

Slide 9 · α cells protect against falling blood glucose

Pancreatic α cells synthesize glucagon.

Glucagon increases hepatic glycogen breakdown and glucose release.

Its physiological influence becomes prominent during fasting.

Fasting homeostasis requires an adequate counterregulatory response.

Slide 10 · Somatostatin modulates intraislet signaling

δ cells produce somatostatin.

Somatostatin inhibits the release of both insulin and glucagon.

This local endocrine brake shapes the balance of opposing glycemic signals.

Islet regulation depends on reciprocal control among cell populations.

Slide 11 · Insulin and glucagon: Opposing metabolic instructions

INSULIN

Higher after meals

Promotes glucose uptake and use

Supports anabolic metabolism

GLUCAGON

Relatively higher during fasting

Supports hepatic glucose output

Promotes glycogenolysis and gluconeogenesis

Their changing ratio links nutrient availability to circulating glucose.

Slide 12 · Three determinants of circulating glucose

Hepatic glucose production

Peripheral glucose uptake

Insulin / counterregulatory signals

Alterations in production, uptake, or hormone balance can raise glucose.

Slide 13 · Fasting physiology: Maintain glucose supply

Lower insulin / higher glucagon

Hepatic glycogenolysis

Hepatic gluconeogenesis

Maintained plasma glucose

Fasting plasma glucose predominantly reflects liver glucose output.

Slide 14 · The liver controls fasting plasma glucose

During fasting, hepatic glycogen stores undergo breakdown.

Gluconeogenesis generates glucose from noncarbohydrate precursors.

Reduced glycogen synthesis and increased output protect against hypoglycemia.

Fasting hyperglycemia points toward failure to restrain hepatic glucose output.

Slide 15 · Fed-state physiology: Dispose of the glucose load

Meal-derived glucose

Insulin rises; Glucagon falls

Skeletal muscle glucose uptake

Return toward homeostasis

Postprandial regulation depends heavily on insulin-responsive tissues.

Slide 16 · Skeletal muscle is the major postprandial sink

Skeletal muscle is an important insulin-responsive site for glucose utilization.

Inadequate skeletal-muscle uptake promotes postprandial hyperglycemia.

The metabolic defect may reflect insulin resistance despite available insulin.

Muscle insulin responsiveness is central to glucose tolerance.

Slide 17 · Normal fasting and fed states

Feature

Fasting state

Fed state

Insulin

Relatively low

Rises

Glucagon

Relatively high

Falls

Hepatic output

Maintains blood glucose

Relatively suppressed

Muscle uptake

Less meal-driven

Major glucose disposal pathway

Diabetes may disturb one or more of these normal transitions.

Slide 18 · Etiologic framework: Three points of failure

Too little insulin is secreted for metabolic needs.

Target tissues respond inadequately to available insulin.

Combined resistance and inadequate β-cell compensation sustain hyperglycemia.

Measure the phenotype; Investigate the mechanism.

Slide 19 · Pathogenesis-based classification

Type 1: β-cell destruction

Type 2: Resistance + dysfunction

Monogenic / secondary types

Mechanistic categories are more informative than age-based labels.

Slide 20 · Type 1 diabetes: Absolute insulin deficiency

The typical form is immune-mediated destruction of pancreatic β cells.

Progressive β-cell loss leads to an absolute insulin deficit.

Insulin deficiency predisposes to ketoacidosis when untreated.

T1D is defined by pathobiology, not simply childhood onset.

Slide 21 · Type 2 diabetes: Failed β-cell compensation

Peripheral tissues resist insulin action.

β cells initially attempt to offset resistance through greater secretion.

When the response is insufficient, persistent hyperglycemia develops.

T2D requires consideration of both insulin resistance and β-cell function.

Slide 22 · Type 1 versus type 2: Clinical comparison

Characteristic

Type 1 diabetes

Type 2 diabetes

Typical onset

Often childhood/adolescence

Often adulthood; Increasingly youth

Weight phenotype

Normal weight or weight loss common

Overweight/obesity common

Insulin status

Progressive severe deficiency

Initially high, later declines

Acute crisis tendency

Ketoacidosis prominent

Hyperosmolar state more typical

Typical patterns are not absolute diagnostic rules.

Slide 23 · Type 1 versus type 2: Biological comparison

Characteristic

Type 1 diabetes

Type 2 diabetes

Immune markers

Islet autoantibodies often present

Generally absent

Major genetic pattern

MHC class II association

Polygenic / obesity-linked

Islet pathology

Insulitis, β-cell depletion

Islet amyloid, mild β-cell loss

Primary lesion

Autoimmune β-cell destruction

Insulin resistance + secretory failure

A pathology-led classification guards against misleading demographic shortcuts.

Slide 24 · Age of presentation does not establish diabetes type

Type 1 diabetes is common in the young but is not confined to children.

Type 2 diabetes increasingly affects children and adolescents.

Clinical phenotype, immune markers, insulin secretion, and context must be integrated.

Avoid equating type 1 with “juvenile” and type 2 with “adult-onset.”

Slide 25 · Body mass is a clue, not a classification test

Many individuals with T2D have excess adiposity.

Weight loss may accompany untreated marked insulin deficiency.

The body-habitus distinction overlaps and cannot replace mechanistic diagnosis.

Weight contributes context but does not establish subtype.

Slide 26 · Insulin maturation yields insulin and C-peptide

Proinsulin synthesis in β cell

Proteolytic processing

Insulin + C-peptide

Equimolar secretion

C-peptide reports endogenous insulin-secretory activity.

Slide 27 · C-peptide is a window into β-cell function

Insulin and C-peptide arise from the same proinsulin precursor.

Both products are released together after secretory stimulation.

A low C-peptide can accompany severe loss of endogenous β-cell function.

C-peptide helps separate insulin secretory failure from resistance.

Slide 28 · C-peptide patterns follow pathophysiology

MARKED β-CELL LOSS

Endogenous insulin production diminishes

C-peptide tends to decrease

Characteristic of advanced absolute insulin deficiency

INSULIN RESISTANCE

Insulin demand increases

Compensatory hypersecretion may occur

C-peptide may initially be elevated

Interpret with prevailing glucose concentration and disease stage.

Slide 29 · Glucose-stimulated insulin release

Glucose enters β cell

Metabolism raises ATP

Membrane depolarization / Ca²⁺ influx

Insulin granule exocytosis

The coupling of nutrient metabolism to Ca²⁺ entry triggers secretion.

Slide 30 · Metabolic sensing initiates secretion

Rising blood glucose is the principal physiological stimulus for insulin release.

Glucose entry and metabolism increase intracellular ATP.

Changes in ATP-sensitive ion-channel activity initiate membrane depolarization.

Glucose detection is translated into an electrical signal.

Slide 31 · Calcium converts the signal into exocytosis

ATP-linked membrane changes lead to opening of voltage-dependent calcium channels.

Intracellular Ca²⁺ rises.

Ca²⁺ stimulates release of insulin from secretory granules.

Calcium influx is the immediate trigger for granule release.

Slide 32 · Biphasic β-cell insulin secretion

EARLY PHASE

Rapid response to rising glucose

Release of stored insulin granules

Initial immediate secretory burst

LATER PHASE

Develops with persistent stimulation

More sustained hormone output

Includes ongoing insulin synthesis

The temporal pattern matters as much as the total amount secreted.

Slide 33 · Oral nutrients produce an incretin effect

Intestinal hormones amplify glucose-stimulated insulin secretion.

The effect explains why oral glucose provokes more insulin release than an equivalent intravenous load.

The major incretins are GIP and GLP-1.

The intestine participates in endocrine regulation of blood glucose.

Slide 34 · Two major incretins

Hormone

Physiologic source

Key metabolic effect

GIP

Intestinal enteroendocrine cells

Augments meal-related insulin secretion

GLP-1

Intestinal enteroendocrine cells

Augments insulin secretion; Promotes satiety

Both exert effects through β-cell G-protein–coupled receptors.

Slide 35 · The enteroinsular axis

Oral nutrient intake

Intestinal GIP / GLP-1 release

β-cell incretin signaling

Amplified insulin release

An intravenous glucose load bypasses much of this gut hormonal contribution.

Slide 36 · Incretins coordinate multiple post-meal effects

Incretins stimulate glucose-dependent insulin secretion.

They attenuate glucagon secretion after nutrient intake.

Delayed gastric emptying and satiety-related effects modify nutrient appearance and appetite.

The response involves pancreas, gut, and central appetite regulation.

Slide 37 · DPP-4 terminates endogenous incretin signals

GIP and GLP-1 are degraded in the circulation.

Dipeptidyl peptidase-4 is a key enzyme mediating this breakdown.

Rapid hormone degradation limits the duration of the endogenous incretin response.

Incretin lifetime is a pharmacologically modifiable process.

Slide 38 · Incretin-related therapeutic principles

GLP-1 RECEPTOR AGONISTS

Activate GLP-1 receptor signaling

Enhance insulin secretion in a glucose-dependent manner

Promote satiety / weight reduction

DPP-4 INHIBITORS

Reduce breakdown of endogenous incretins

Extend native incretin signaling

Improve glycemic regulation

Mechanistic illustration only; Clinical prescribing guidance is outside this lecture.

Slide 39 · Dual incretin-receptor agonism

Tirzepatide as a dual GIP/GLP-1 receptor agonist.

Its endocrine action engages both incretin-receptor pathways.

This drug class demonstrates the translational significance of gut–islet physiology.

Drug mechanisms are included to illuminate pathology, not to prescribe treatment.

Slide 40 · From normal physiology to diabetic hyperglycemia

β-cell output becomes inadequate

Insulin effects fail to match metabolic load

Glucose disposal / hepatic regulation deteriorate

Sustained hyperglycemia

This common endpoint arises through different initiating lesions.

Slide 41 · Four Robbins diagnostic pathways

Test

Diabetes threshold

Fasting plasma glucose

≥126 mg/dL

Random plasma glucose + classic symptoms

≥200 mg/dL

2-hour plasma glucose (75-g OGTT)

≥200 mg/dL

HbA1c

≥6.5%

These are textbook criteria; Verify current local laboratory standards for clinical decisions.

Slide 42 · Fasting plasma glucose

≥126 mg/dL

Diabetes diagnostic threshold

Measure plasma glucose in the fasting state.

A threshold-level result generally requires confirmation if hyperglycemia is not unequivocal.

The fasting value is strongly influenced by hepatic glucose output.

Slide 43 · Random plasma glucose

≥200 mg/dL

With classic hyperglycemic manifestations

A random glucose threshold is diagnostic in the specified symptomatic context.

The textbook makes this the exception to its separate-day confirmatory rule.

Without that clinical context, do not interpret a single random result in isolation.

Slide 44 · Oral glucose tolerance test

≥200 mg/dL

2-hour value after a 75-g glucose load

The test directly challenges systemic glucose handling.

The diagnostic comparison uses the 2-hour plasma glucose measurement.

Impaired insulin secretion or disposal may become apparent after glucose loading.

Slide 45 · Glycated hemoglobin

≥6.5%

HbA1c diagnostic threshold

HbA1c provides an integrated marker of antecedent glycemic exposure.

It is an alternative laboratory pathway to diabetes diagnosis.

Interpret assay results in clinical context and confirm when indicated.

Slide 46 · Confirming hyperglycemia before labeling diabetes

The Robbins text recommends repeat confirmation on a separate day for most diagnostic pathways.

An unequivocal symptomatic random plasma glucose ≥200 mg/dL is its stated exception.

Clinical context and test concordance determine interpretive confidence.

Distinguish persistent dysglycemia from a single abnormal measurement.

Slide 47 · Discordant diagnostic assays

Fasting glucose, oral glucose tolerance, and HbA1c measure related but distinct manifestations of dysglycemia.

Discordance can occur between glucose-based and HbA1c-based tests.

The more abnormal assay as the diagnostic readout; Confirmation and clinical assessment remain important.

Laboratory disagreement should prompt interpretation—not arbitrary averaging.

Slide 48 · Acute stress may cause transient hyperglycemia

Severe infection, burns, or trauma may provoke transient glucose elevation.

Catecholamines and cortisol oppose insulin action during stress.

Evaluate whether hyperglycemia persists after the acute illness resolves.

Stress hyperglycemia is not automatically chronic diabetes.

Slide 49 · Diagnostic logic in a symptomatic patient

Classic hyperglycemic features?

Random glucose ≥200 mg/dL?

Diabetes criterion fulfilled

If symptomatic criteria are absent, use fasting glucose, OGTT, or HbA1c with appropriate confirmation.

Slide 50 · Prediabetes: Robbins laboratory intervals

Test

Prediabetes interval

Fasting plasma glucose

100–125 mg/dL

2-hour plasma glucose (75-g OGTT)

140–199 mg/dL

HbA1c

5.7–6.4%

Any qualifying interval indicates elevated future diabetes risk.

Slide 51 · Impaired fasting glucose

100–125 mg/dL

Fasting plasma glucose range

The value is elevated but does not meet the diabetes threshold.

It indicates abnormal basal glucose regulation.

Coexisting obesity and family history influence progression risk.

Slide 52 · Impaired glucose tolerance

140–199 mg/dL

2-hour plasma glucose after 75-g OGTT

A glucose challenge reveals reduced tolerance to a carbohydrate load.

The value lies below the diabetes threshold of 200 mg/dL.

The state identifies increased risk of later type 2 diabetes.

Slide 53 · Prediabetes by HbA1c

5.7–6.4%

Glycated hemoglobin interval

This range signifies dysglycemia below the diabetes criterion.

HbA1c may identify people with elevated future diabetes risk.

Interpret alongside other risk factors and glucose-based tests.

Slide 54 · Prediabetes is clinically consequential

Prediabetes can precede overt type 2 diabetes.

The progression rate is influenced by factors such as obesity and family history.

The textbook also links prediabetes with cardiovascular risk.

Risk begins before the threshold for established diabetes is crossed.

Slide 55 · Interpreting the three glycemic categories

Measurement

Prediabetes

Diabetes

Fasting glucose

100–125 mg/dL

≥126 mg/dL

2-hour OGTT

140–199 mg/dL

≥200 mg/dL

HbA1c

5.7–6.4%

≥6.5%

Values below prediabetes intervals are not automatically normal in every clinical context.

Slide 56 · No single number establishes the etiologic subtype

Glucose and HbA1c tests establish the hyperglycemic phenotype.

They do not identify autoimmune β-cell destruction versus insulin resistance.

Classification uses history, phenotypic findings, and where relevant tests of islet immunity and endogenous secretion.

Diagnosis of diabetes and classification of diabetes are separate tasks.

Slide 57 · Islet autoantibodies support type 1 diabetes

GAD65, IA-2, and ZnT8 as characteristic islet-autoantibody targets.

Their presence supports autoimmune β-cell destruction in an appropriate setting.

Autoantibody-negative or atypical presentations require careful classification.

An immune biomarker is not interchangeable with a glucose diagnostic criterion.

Slide 58 · Genetic architecture distinguishes common types

Type 1 susceptibility is strongly linked to MHC class II loci.

Robbins also notes CTLA4, PTPN22, and insulin-gene regulatory polymorphisms.

Type 2 risk involves multiple metabolic and obesity-associated loci rather than HLA linkage.

Susceptibility alleles do not serve as standalone clinical diagnostic tests.

Slide 59 · Two characteristic islet pathology patterns

TYPE 1 DIABETES

Immune-cell insulitis may be present

Progressive β-cell depletion

Atrophy of affected islets

TYPE 2 DIABETES

Islet amyloid deposition may occur

Variable mild β-cell loss

No characteristic insulitis

Morphologic descriptions are textbook findings; No synthetic photomicrograph is shown.

Slide 60 · Early T2D can show compensatory hyperinsulinemia

Insulin resistance raises the amount of insulin needed for normal glucose handling.

β cells can initially compensate by increasing insulin secretion.

The eventual mismatch between secretion and demand promotes hyperglycemia.

High insulin levels do not exclude diabetes.

Slide 61 · Type 1 and type 2 share a metabolic endpoint

Type 1 starts predominantly with profound insulin secretory loss.

Type 2 combines peripheral resistance with relative β-cell secretory inadequacy.

Both result in sustained hyperglycemia and risk of multisystem injury.

Similar glucose values may conceal fundamentally different mechanisms.

Slide 62 · Beyond type 1 and type 2: Other diabetes groups

Single-gene defects of β-cell function or insulin action can cause diabetes.

Pancreatic exocrine disease, endocrine disorders, and medications may lead to secondary hyperglycemia.

Gestational diabetes is a distinct context classification.

Atypical clinical features should stimulate etiologic reconsideration.

Slide 63 · Selected MODY-associated genes

MODY designation

Gene

Encoded function / designation

MODY1

HNF4A

Hepatocyte nuclear factor 4α

MODY2

GCK

Glucokinase

MODY3

HNF1A

Hepatocyte nuclear factor 1α

MODY4

PDX1

Pancreatic / duodenal homeobox 1

MODY5

HNF1B

Hepatocyte nuclear factor 1β

These are examples from the textbook classification, not a comprehensive gene panel.

Slide 64 · Neonatal and mitochondrial monogenic diabetes

Activating changes in KCNJ11 or ABCC8 are associated with certain neonatal diabetes forms.

The genes encode components of the ATP-sensitive potassium-channel system.

Robbins also lists maternally inherited diabetes and deafness due to mitochondrial DNA variation.

Molecular mechanisms can pinpoint rare etiologic subgroups.

Slide 65 · Pancreatogenic diabetes: Injury to pancreatic tissue

Chronic pancreatitis or pancreatic resection may compromise endocrine function.

Pancreatic malignancy and cystic fibrosis are additional listed causes.

Hemochromatosis and fibrocalculous pancreatopathy are also recognized.

Exocrine pancreatic pathology can secondarily impair islet-cell function.

Slide 66 · Endocrine and drug-related secondary diabetes

Acromegaly, Cushing syndrome, and hyperthyroidism are listed endocrine associations.

Glucocorticoids, certain antiretrovirals, thiazides, and other drugs may contribute.

A medication and endocrine history is part of etiologic classification.

Hormones antagonizing insulin can produce a diabetes phenotype.

Slide 67 · Diabetes and pregnancy in the classification

Gestational diabetes is a separately recognized category.

The physiological context of pregnancy matters when interpreting hyperglycemia.

Detailed pregnancy-specific diagnostic approaches belong to the dedicated topic.

Do not automatically classify hyperglycemia in pregnancy as ordinary T2D.

Slide 68 · Organ injury from sustained diabetes

Persistent hyperglycemia

Metabolic and vascular injury

Kidney · eye · nerve · vessel disease

These complications motivate early detection and etiologic management.

Slide 69 · Clinical impact: Kidneys, eyes, nerves, vessels

Renal failure and ocular, neural, and vascular consequences.

Chronic injury produces functional disease beyond elevated laboratory glucose.

Specific lesion morphology and chronic complications are developed in separate Robbins topics.

A diabetes lecture must link laboratory diagnosis to end-organ pathology.

Slide 70 · Case application: Symptomatic hyperglycemia

CLINICAL VIGNETTE

An adult has polyuria, polydipsia, and a random plasma glucose of 236 mg/dL.

Does the reported test meet the Robbins diabetes diagnostic criterion?

INTERPRETATION

Yes. Random plasma glucose ≥200 mg/dL with classic hyperglycemic manifestations meets the stated diagnostic pathway.

Slide 71 · Case application: Prediabetes classification

CLINICAL VIGNETTE

A fasting plasma glucose is 116 mg/dL and HbA1c is 6.1%.

How should these measurements be classified?

INTERPRETATION

Both values fall within the Robbins prediabetes intervals; They do not reach diagnostic thresholds for established diabetes.

Slide 72 · Case application: Mechanism versus phenotype

CLINICAL VIGNETTE

A person has hyperglycemia, high endogenous insulin, and a phenotype suggesting reduced peripheral response to insulin.

Which pathogenic process is most consistent with the early presentation?

INTERPRETATION

Compensatory hyperinsulinemia associated with insulin resistance, often an early feature of type 2 diabetes.

Slide 73 · Integrated summary: Four diagnostic decisions

Establish a persistent hyperglycemic phenotype using appropriate criteria.

Separate diabetes from prediabetes using the correct assay interval.

Classify the underlying mechanism, not merely age or weight.

Connect the biochemical disorder to long-term organ pathology.

Correct classification informs clinical interpretation and future evaluation.

Slide 74 · Principal source and scope

Robbins, Cotran & Kumar: Pathologic Basis of Disease

11th edition (2026) · Chapter 24: The Endocrine System

Primary sections: Endocrine Pancreas; Diabetes Mellitus; Diagnosis; Classification; Glucose Homeostasis; Regulation of Insulin Release.

Primary classification and comparison tables: 24.6 and 24.7.