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This is a clinician-written, evidence-based summary aligned to the USMLE Step 2 CK Content Outline. It is intended for medical students preparing for USMLE Step 2 CK. Management reflects current ACC/AHA, USPSTF, and APA guidelines. Always cross-reference with UpToDate, institutional protocols, and clinical judgment.
The Bottom Line
- T1DM = autoimmune beta-cell destruction with absolute insulin deficiency; insulin is mandatory from diagnosis
- Classic presentation: polyuria, polydipsia, weight loss, ketosis, or DKA in a child/adolescent or lean adult
- Confirm with pancreatic autoantibodies and low/declining C-peptide when phenotype is uncertain
- Treatment: basal-bolus insulin or pump therapy, carbohydrate counting, CGM, sick-day rules, and glucagon rescue
- Screen for associated autoimmune disease: autoimmune thyroid disease and celiac disease are high-yield
Overview
Type 1 diabetes mellitus is caused by immune-mediated destruction of pancreatic beta cells, producing absolute insulin deficiency. It usually presents in childhood or adolescence but can occur at any age, including latent autoimmune diabetes in adults. Because endogenous insulin is absent or minimal, patients are prone to ketosis and DKA if insulin is omitted during illness, fasting, pump failure, or psychosocial stress.
Epidemiology
T1DM accounts for approximately 5-10% of diabetes in the United States. Incidence is highest in childhood and adolescence, but a substantial minority presents in adulthood. Risk is increased with family history and HLA-DR3/DR4. Associated autoimmune conditions include Hashimoto thyroiditis, Graves disease, celiac disease, autoimmune adrenal insufficiency, pernicious anemia, and vitiligo.
Clinical Features
Symptoms
Polyuria, polydipsia, nocturia, and weight loss despite preserved appetite
Fatigue, blurred vision, poor school/work performance
Abdominal pain, nausea, vomiting, and Kussmaul respirations suggest DKA
Recurrent hypoglycemia, tremor, diaphoresis, or confusion from insulin therapy
Symptoms of associated autoimmune thyroid disease or celiac disease
Signs
Lean habitus or recent weight loss; dehydration if severe hyperglycemia
Fruity breath, tachypnea, tachycardia, hypotension, altered mental status in DKA
Lipohypertrophy at insulin injection sites causing erratic absorption
Delayed puberty or growth failure in children with poor control
Neuropathy or retinopathy in long-standing disease
Investigations
First-line
Glucose and HbA1cSame diagnostic thresholds as other diabetes: A1c >=6.5%, fasting glucose >=126, 2-hour OGTT >=200, or random glucose >=200 with symptoms
Ketones and metabolic panelSerum beta-hydroxybutyrate, bicarbonate, anion gap, potassium, creatinine if acute illness or marked hyperglycemia
Pancreatic autoantibodiesGAD65 most common in adults; IA-2, ZnT8, and insulin autoantibody support autoimmune diabetes
Second-line
C-peptideLow or inappropriately normal C-peptide with hyperglycemia supports insulin deficiency; may be preserved early
TSH and thyroid peroxidase antibodiesScreen at diagnosis and periodically because autoimmune thyroid disease is common
Celiac screeningTissue transglutaminase IgA plus total IgA, especially in children or symptoms
Specialist
CGM metricsTime in range, time below range, and glycemic variability guide insulin adjustments beyond A1c
Annual complication screeningRetinal exam usually 5 years after diagnosis, annual UACR/eGFR after puberty/5 years duration, neuropathy screening
Management
ADA Standards of Care in Diabetes—20261
Insulin replacement
- Basal-bolus insulin is required: long-acting basal plus rapid-acting mealtime insulin
- Typical total daily dose 0.4-0.8 units/kg/day, with ~40-50% basal and remainder prandial/correction
- Insulin pump or automated insulin delivery can improve time-in-range when used safely
- Never stop basal insulin, even when fasting or ill
2
Education and safety
- Carbohydrate counting and correction factor education
- CGM recommended for most patients; fingerstick backup for sensor failure or rapid changes
- Sick-day rules: check glucose and ketones more often, continue insulin, hydrate, seek care for vomiting or persistent ketones
- Prescribe glucagon rescue and train family/contacts
3
Targets and follow-up
- A1c target often <7% for many nonpregnant adults; individualize for hypoglycemia risk and comorbidity
- Address psychosocial barriers, eating disorders, insulin affordability, and diabetes distress
- Screen autoimmune thyroid disease, celiac disease, nephropathy, retinopathy, neuropathy, and lipids
Complications
- Diabetic ketoacidosis: Most important acute life-threatening complication; triggered by infection, missed insulin, pump failure, MI, pregnancy
- Hypoglycemia: Insulin-related; seizures/coma possible
- Microvascular: Retinopathy, nephropathy, neuropathy
- Autoimmune comorbidity: Hashimoto thyroiditis, Graves disease, celiac disease, Addison disease, pernicious anemia
- Psychosocial: Diabetes distress, disordered eating, insulin omission
USMLE Step 2 CK Exam Tips
- 1Child or lean adult with polyuria, weight loss, abdominal pain, Kussmaul respirations = check ketones and treat DKA
- 2T1DM requires insulin from diagnosis; oral agents alone are wrong answers
- 3DKA can occur with only moderate glucose if SGLT2 inhibitor use, pregnancy, or partial treatment — euglycemic DKA
- 4T1DM is associated with autoimmune thyroid disease and celiac disease — screen for both
- 5Do not stop basal insulin when NPO; reduce dose if needed but continue basal coverage
- 6Low C-peptide + positive GAD65/IA-2/ZnT8 antibodies supports autoimmune diabetes
- 7Recurrent fasting hypoglycemia suggests excessive basal insulin; postprandial highs suggest mealtime dosing issue
practicetest your knowledge on type 1 diabetes mellitusApply what you've learnt with USMLE Step 2 CK-style questions from the iatroX Q-Bank — endocrine and beyond.
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