Diabetes Mellitus: Diagnosis, Classification, and Pathophysiology¶

Chapter 415 | Part 12: Endocrinology · Part 12 – Endocrinology & Metabolism · Chapter 415

Key Clinical Points¶

Diabetes mellitus (DM) is a group of metabolic disorders characterized by hyperglycemia resulting from a combination of genetics and environmental factors.
Type 1 DM results from immune-mediated beta cell destruction leading to absolute insulin deficiency; Type 2 DM is a heterogeneous group involving insulin resistance, impaired secretion, and increased hepatic glucose production.
Diagnostic criteria for DM: HbA1c ≥6.5%, FPG ≥7.0 mmol/L (126 mg/dL), 2-h PG ≥11.1 mmol/L (200 mg/dL) after OGTT, or random glucose ≥11.1 mmol/L (200 mg/dL) with classic symptoms.
Prediabetes is defined as HbA1c 5.7–6.4% (42–47 mmol/mol), IFG (FPG 5.6–6.9 mmol/L), or IGT (2-h PG 7.8–11.0 mmol/L).
C-peptide serves as a marker of endogenous insulin secretion and is used to distinguish between endogenous and exogenous insulin sources in cases of hypoglycemia.
Incretin hormones (GLP-1, GIP) stimulate insulin secretion only when blood glucose exceeds fasting levels and concurrently suppress glucagon production.
Monogenic diabetes (MODY) is characterized by autosomal dominant inheritance, early onset (<25 years), and impaired insulin secretion due to specific gene mutations.
Gestational diabetes mellitus (GDM) occurs in the 2nd or 3rd trimester; diagnosis in the 1st trimester is classified as pregestational diabetes.
Bariatric surgery reduces T2DM incidence by 78% in obese patients, with 68.2% experiencing initial remission within 5 years (though one-third may redevelop the disease).
T1DM progresses through three stages: Stage 1 (autoantibodies present, normoglycemia), Stage 2 (autoantibodies, dysglycemia), and Stage 3 (overt hyperglycemia).

DEFINITION & OVERVIEW¶

• Definition: Diabetes mellitus (DM) refers to a group of common metabolic disorders that share the phenotype of hyperglycemia. • Pathophysiologic Drivers: • Reduced insulin secretion • Decreased glucose utilization • Increased glucose production • Clinical Impact: • Leading cause of end-stage renal disease (ESRD) • Leading cause of nontraumatic lower-extremity amputations • Leading cause of adult blindness • Major risk factor for cardiovascular disease (primary cause of morbidity/mortality). • Classification Framework: • Type 1 DM: Immune-mediated beta cell destruction → absolute insulin deficiency. • Type 2 DM: Heterogeneous group; includes insulin resistance, impaired secretion, and increased hepatic glucose production. • Prediabetes: Period of worsening glucose homeostasis before clinical diagnosis (includes IFG and IGT). • Monogenic Diabetes (MODY): Autosomal dominant inheritance, early onset (<25 years), and impaired insulin secretion. • Gestational Diabetes Mellitus (GDM): Glucose intolerance in 2nd or 3rd trimester of pregnancy.

Etiologic Classification (Table 415-1)¶

• I. Type 1 DM: Immune-mediated beta cell destruction, leading to absolute insulin deficiency. • II. Type 2 DM: Range from predominantly insulin resistance with relative insulin deficiency to a predominantly insulin secretory defect with insulin resistance. • III. Other types of diabetes caused by: • A. Genetic defects (Monogenic/MODY): Mutations in HNF 4α, Glucokinase, HNF-1α, Insulin promoter factor-1, HNF-1β, NeuroD1, KLF11, PAX4, BLK, GATA4, GATA6, SLC2A2 (GLUT2), GLIS3, or Mitochondrial DNA. • B. Neonatal diabetes (<6 months): Mutations in subunits of ATP-sensitive potassium channel, RFX6, insulin. • C. Transient neonatal diabetes. • D. Diseases of the exocrine pancreas: Pancreatitis, pancreatectomy, neoplasia, cystic fibrosis, hemochromatosis, fibrocalculous pancreatopathy, mutations in carboxyl ester lipase. • E. Genetic defects in insulin action: Type A insulin resistance, leprechaunism, Rabson-Mendenhall syndrome, lipodystrophy syndromes. • F. Endocrinopathies: Acromegaly, Cushing's syndrome, glucagonoma, pheochromocytoma, hyperthyroidism, somatostatinoma, aldosteronoma. • G. Drug/Chemical induced: Glucocorticoids, calcineurin and mTOR inhibitors (after organ transplantation), pentamidine, nicotinic acid, statins, HIV therapies (antiretroviral, protease inhibitors, nucleoside/nucleotide reverse transcriptase inhibitors), diazoxide, β-adrenergic agonists, thiazides, proprotein convertase subtilisin/kexin type 9 (PCSK9) inhibitors, hydantoins, asparaginase, α-interferon, protease inhibitors, antipsychotics (second-generation atypicals and others), epinephrine, vacor (a rodenticide). • H. Infections: Congenital rubella, cytomegalovirus, coxsackievirus. • I. Uncommon immune-mediated: 'Stiff-person' syndrome, anti-insulin receptor antibodies, treatment of malignancy with immune checkpoint inhibitors. • J. Genetic Syndromes: Wolfram syndrome, Down's syndrome, Klinefelter's syndrome, Turner's syndrome, Friedreich's ataxia, Huntington's chorea, Laurence-Moon-Biedl syndrome, myotonic dystrophy, porphyria, Prader-Willi syndrome. • IV. Gestational diabetes mellitus (GDM).

EPIDEMIOLOGY¶

• Global Trends: • 1985: ~30 million cases • 2021: 537 million cases (10.5% prevalence) • 2030 Projection: 643 million (11.3%) • 2045 Projection: 783 million (12.2%) • Regional Burden: • China (140.9M), India (74.2M), Pakistan (33.0M), USA (32.2M), Indonesia (>19.5M), Brazil (15.7M), Mexico (14.1M). • Demographics: • 1/3 of deaths occur in individuals in their working years (65 years of age). • Type 2 DM is rising faster due to dietary changes, obesity, and aging. • Asian populations: Rapidly increasing T2DM at lower BMI and younger age, with greater visceral adiposity and reduced insulin secretory capacity. • T1DM Prevalence by Ethnicity (2019–2021): • Non-Hispanic whites: 10.3% • Asians: 9.2% • Hispanics: 10.3% • Non-Hispanic blacks: 12.5% • Native American/Alaskan native: 16%. • Mortality: • 8th leading cause of death in the United [Source truncated]. • Approximately 75% of individuals with diabetes live in low- or middle-income countries.

ETIOLOGY & PATHOPHYSIOLOGY¶

• Glucose Homeostasis Balance: • Balanced by: Energy intake, hepatic glucose production (gluconeogenesis/glycogenolysis), and peripheral tissue uptake. • Insulin: Primary regulator; promotes storage of carbohydrate, fat, and protein synthesis. • Glucagon: Increases in fasting state or during exercise → increases hepatic glucose production. • Beta Cell Physiology: • C-peptide: Co-secreted with insulin; serves as a marker of endogenous secretion. Used to distinguish between endogenous vs. exogenous insulin sources in hypoglycemia evaluation. • IAPP (Amylin): 37-amino-acid peptide co-secreted with insulin; major component of amyloid fibrils in T2DM. • Glucose Sensing: • Transported via GLUT1/GLUT2. • Glucokinase: Rate-limiting step for glucose-regulated insulin secretion. • Metabolism → ATP production → Inhibition of K^+ ATP-sensitive channels → Membrane depolarization → Calcium influx → Insulin release. • Incretins: GLP-1 and GIP bind receptors to stimulate insulin (only when glucose > fasting) and suppress glucagon. • T1DM Progression (Figure 5): • Stage 1: Autoantibodies present + normoglycemia. • Stage 2: Autoantibodies + dysglycemia. • Stage 3: Significant beta cell loss → Overt diabetes. • 'Honeymoon' phase: First 1-2 years after onset; reduced insulin requirements. • T2DM Progression (Figure 6): • NGT (Point A) → Insulin Resistance begins (Point B) → Failure to compensate → IGT (Point C) → T2DM (Point D). • Transition involves decreasing insulin sensitivity and eventually insufficient insulin secretion.

DIAGNOSTIC APPROACH¶

Screening for Type 2 DM:
Identify high-risk individuals:
Overweight/Obese (BMI ≥25 kg/m²; ≥23 in Asian Americans) + one of the following risk factors:
Family history of diabetes (parent or sibling with T2DM)
Race/ethnicity (African American, Latino, Native American, Asian American, Pacific Islander)
Hypertension (blood pressure ≥130/80 mmHg)
Lipid profile: HDL <35 mg/dL (0.90 mmol/L) and/or triglycerides >250 mg/dL (2.82 mmol/L)
Polycystic ovary syndrome or acanthos nigricans
History of cardiovascular disease
Physical inactivity
Other conditions associated with insulin resistance (severe obesity, acanthosis nigricans).
High-risk conditions requiring annual screening:
Known IFG, IGT, or HbA1c 5.7–6.4%.
Gestational history:
Women with a history of GDM should be screened at least every 3 years.
Specific clinical scenarios:
Pancreatitis: Screen within 3–6 months of episode.
Cystic Fibrosis (starting at age 10): Oral GTT recommended.
Post-transplant: Oral GTT recommended.
Diagnostic Criteria for DM:
HbA1c ≥6.5% (48 mmol/mol)
FPG ≥7.0 mmol/L (126 mg/dL)
2-h PG ≥11.1 mmol/L (200 mg/dL) after OGTT
Random glucose ≥11.1 mmol/L (200 mg/dL) with classic symptoms.
Prediabetes Criteria:
HbA1c 5.7–6.4% (42–47 mmol/mol)
IFG: FPG 5.6–6.9 mmol/L (100–125 mg/dL)
IGT: 2-h PG 7.8–11.0 mmol/L (140–190 mg/dL).
Specialized Testing:
OGTT is preferred for Cystic Fibrosis-related diabetes and post-transplant screening.

MANAGEMENT & TREATMENT¶

Bariatric Surgery:
Reduces T2DM incidence by 78% in obese patients.
68.2% experience initial remission within 5 years → Note: one-third redevelop disease.
GDM Follow-up:
Women with GDM history → Lifelong screening for DM/prediabetes at least every 3 years.
Clinical Monitoring:
Use C-peptide to distinguish endogenous vs. exogenous insulin in hypoglycemia evaluation.

SPECIAL CONSIDERATIONS¶

• Gestational Diabetes (GDM): - Defined as glucose intolerance in 2nd or 3rd trimester. - Diagnosis in 1st trimester → Classified as pregestational diabetes. - Postpartum: Most revert to normal tolerance, but 35–60% risk of developing DM in the next 10–20 years. - Offspring: Increased risk of metabolic syndrome and T2DM later in life. • Pediatric/Younger Patients: - MODY (Monogenic) identified by early onset (<25 years) and autosomal dominant inheritance. - Cystic Fibrosis related diabetes → Start screening at age 10 using OGTT.

KEY PEARLS & CLINICAL TRAPS¶

• HbA1c vs. FPG: HbA1c and FPG are the most reliable/convenient for asymptomatic individuals; however, factors like hemoglobinopathies can affect HbA1c. • Beta Cell Failure: In T2DM, the progression from NGT to IGT to DM is driven by a failure of beta cells to compensate for increasing insulin resistance. • Incretin Effect: GLP-1 and GIP are only effective when blood glucose is above fasting levels. • C-peptide Utility: Essential for determining if a patient's hypoglycemia is caused by too much exogenous insulin (low C-peptide) or endogenous overproduction/failure of other mechanisms.

Reference Tables¶

TABLE 415-1 Etiologic Classification of Diabetes Mellitus I. Type 1 DM (immune-mediated beta cell destruction, usually…¶

Harrison's 22e, p.3196

I. Type 1 DM (immune-mediated beta cell destruction, usually leading to absolute insulin deficiency) II. Type 2 DM (may range from predominantly insulin resistance with relative insulin deficiency to a predominantly insulin secretory defect with insulin resistance) III. Other types of diabetes caused by: A. Genetic defects of beta cell development or function characterized by mutations such as (e.g., monogenic, MODY): 1. Hepatocyte nuclear transcription factor (HNF) 4α 2. Glucokinase 3. HNF-1α 4. Insulin promoter factor-1, HNF-1β, NeuroD1, and other pancreatic islet regulators/proteins such as KLF11, PAX4, BLK, GATA4, GATA6, SLC2A2 (GLUT2), GLIS3 5. Mitochondrial DNA B. Neonatal diabetes (onset <6 months of age) due to mutations in subunits of ATP-sensitive potassium channel, RFX6, insulin C. Transient neonatal diabetes D. Diseases of the exocrine pancreas—pancreatitis, pancreatectomy, neoplasia, cystic fibrosis, hemochromatosis, fibrocalculous pancreatopathy, mutations in carboxyl ester lipase E. Genetic defects in insulin action, including type A insulin resistance, leprechaunism, Rabson-Mendenhall syndrome, lipodystrophy syndromes F. Endocrinopathies—acromegaly, Cushing’s syndrome, glucagonoma, pheochromocytoma, hyperthyroidism, somatostatinoma, aldosteronoma G. Drug- or chemical-induced—glucocorticoids, calcineurin and mTOR inhibitors (after organ transplantation), pentamidine, nicotinic acid, statins, HIV therapies (antiretroviral, protease inhibitors, nucleoside/ nucleotide reverse transcriptase inhibitors), diazoxide, β-adrenergic agonists, thiazides, proprotein convertase subtilisin/kexin type 9 (PCSK9) inhibitors, hydantoins, asparaginase, α-interferon, protease inhibitors, antipsychotics (second-generation atypicals and others), epinephrine, vacor (a rodenticide) H. Infections—congenital rubella, cytomegalovirus, coxsackievirus I. Uncommon forms of immune-mediated diabetes—”stiff-person” syndrome, anti-insulin receptor antibodies, treatment of malignancy with immune checkpoint inhibitors J. Other genetic syndromes sometimes associated with diabetes—Wolfram syndrome, Down’s syndrome, Klinefelter’s syndrome, Turner’s syndrome, Friedreich’s ataxia, Huntington’s chorea, Laurence-Moon-Biedl syndrome, myotonic dystrophy, porphyria, Prader-Willi syndrome IV. Gestational diabetes mellitus (GDM)
	Normal glucose tolerance	Hyperglycemia
		Prediabetes*	Diabetes Mellitus
		Impaired fasting glucose or impaired glucose tolerance	Insulin Insulin Not required required insulin for for requiring control survival
HbA1C FPG 2-h PG	<5.6% (<41 mmol/mol) <5.6 mmol/L (100 mg/dL) <7.8 mmol/L (140 mg/dL)	5.7–6.4% (42–47 mmol/mol) 5.6–6.9 mmol/L (100–125 mg/dL) 7.8–11.0 mmol/L (140–199 mg/dL)	Symptoms of diabetes + random blood glucose concentration ≥11.1 mmol/L (200 mg/dL)a ≥6.5%b (≥48 mmol/mol) ≥7.0 mmol/L (126 mg/dL)c ≥11.1 mmol/L (200 mg/dL)d

TABLE 415-2 Criteria for Screening for Type 2 DM in Adults 1. Consider testing in adults of any age who are overweight…¶

Harrison's 22e, p.3198

1. Consider testing in adults of any age who are overweight or obese (BMI
  ≥25 kg/m2 or ≥23 kg/m2 in Asian Americans) who have one of these risk factors:
  • Family history of diabetes (i.e., parent or sibling with type 2 DM)
  • Race/ethnicity (e.g., African American, Latino, Native American, Asian
  American, Pacific Islander)
  • Hypertension (blood pressure ≥130/80 mmHg)
  • HDL cholesterol level <35 mg/dL (0.90 mmol/L) and/or a triglyceride level
  >250 mg/dL (2.82 mmol/L)
  • Polycystic ovary syndrome or acanthosis nigricans
  • History of cardiovascular disease
  • Physical inactivity
  • Other condition associated with insulin resistance (severe obesity,
  acanthosis nigricans)
  2. Individuals with previously identified IFG, IGT, or a hemoglobin A of 5.7–6.4%
  1c
  should be screened annually.
  3. Women who had GDM should be screened at least every 3 years.
  4. For other individuals, initiate testing at 35 years of age and repeat every
  3 years.
  5. Individuals taking HIV medications or medications associated with diabetes
  (second-generation antipsychotic, glucocorticoids).
  6. History of pancreatitis (screen within 3–6 months of episode of pancreatitis).
  7. Cystic fibrosis (beginning at age 10); oral GTT recommended.
  8. After organ transplantation; oral GTT recommended.