Study guide
9.6 Hyponatremia, Hypernatremia & Water Balance
ABIM Internal Medicine Exam
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1 Chapter 0: The ABIM Exam, Blueprint & Study Strategy
0.1 Exam Format, Question Types & the 2026 Blueprint 0.2 Eligibility, Registration, Fees & Exam-Day Logistics 0.3 Scoring, the Passing Standard & Score Reports 0.4 Building a Blueprint-Weighted Study Plan
2 Chapter 1: Cardiovascular Disease (14% of Exam)
1.1 Ischemic Heart Disease & Acute Coronary Syndromes 1.2 Antithrombotic Therapy, Secondary Prevention & Mechanical Complications of MI 1.3 Atrial Fibrillation, Atrial Flutter & Stroke Prevention 1.4 Supraventricular Tachycardias, Ventricular Arrhythmias & Bradycardias 1.5 Heart Failure: Classification, Staging & Guideline-Directed Medical Therapy 1.6 Device Therapy, HFpEF & the Cardiomyopathies 1.7 Valvular Heart Disease & Endocarditis Prophylaxis 1.8 Pericardial Disease & Acute Aortic Syndromes 1.9 Hypertension: Classification, Diagnosis & Secondary Causes 1.10 Hypertensive Crises & Dyslipidemia 1.11 Adult Congenital Heart Disease & Coarctation of the Aorta 1.12 Syncope: Reflex, Orthostatic & Cardiac Causes 1.13 Cardiac Physical Examination, Murmurs & Antithrombotic Selection
3 Chapter 2: Endocrinology, Diabetes & Metabolism (9% of Exam)
2.1 Diabetes Mellitus: Diagnosis, Glycemic Targets & Pharmacotherapy 2.2 Insulin Therapy, Hyperglycemic Emergencies & Chronic Complications 2.3 Thyroid Function Testing & Hypothyroidism 2.4 Thyrotoxicosis, Thyroid Storm & Thyroid Nodules 2.5 Adrenal & Pituitary Disorders 2.6 Hypercalcemia, Hypocalcemia & Disorders of Mineral Metabolism 2.7 Osteoporosis & Paget Disease of Bone 2.8 Ovarian, Testicular & Reproductive Endocrine Disorders 2.9 Hypoglycemia, Endocrine Tumors, Polyglandular Syndromes & Severe Hypertriglyceridemia
4 Chapter 3: Gastroenterology & Hepatology (9% of Exam)
3.1 Esophageal Disease 3.2 Stomach & Duodenal Disease and Gastrointestinal Hemorrhage 3.3 Inflammatory Bowel Disease 3.4 Celiac Disease & Irritable Bowel Syndrome 3.5 Diverticular Disease & Colorectal Cancer Screening 3.6 Viral Hepatitis, Steatotic & Alcohol-Associated Liver Disease 3.7 Cirrhosis & the Complications of Portal Hypertension 3.8 Acute Pancreatitis 3.9 Chronic Pancreatitis & Biliary Tract Disease 3.10 Inherited, Autoimmune & Drug-Induced Liver Disease 3.11 Small Intestinal Disease, Ischemia & Gastrointestinal Complications of HIV
5 Chapter 4: Infectious Disease (9% of Exam)
4.1 Lower Respiratory Tract Infections & Sepsis Syndromes 4.2 Viral Respiratory Pathogens & Tuberculosis 4.3 Central Nervous System Infections 4.4 Skin, Soft Tissue, Bone & Joint Infections 4.5 Infective Endocarditis & Cardiovascular Infection 4.6 Clostridioides difficile & Catheter-Related Bloodstream Infection 4.7 HIV Diagnosis, Antiretroviral Therapy & Prevention 4.8 Opportunistic Infections & Sexually Transmitted Infections 4.9 Fever of Unknown Origin, Neutropenic Fever & Toxic Shock 4.10 Travel-Related Illness, Vector-Borne Disease & Outbreaks 4.11 Infection Prevention, Nosocomial Infection & Postexposure Prophylaxis 4.12 Rheumatic Fever, Poststreptococcal Disease, Enteric & Hepatic Infections
6 Chapter 5: Pulmonary Disease (9% of Exam)
5.1 COPD: Diagnosis, GOLD Classification & Initial Pharmacotherapy 5.2 Interventions That Reduce COPD Mortality & Exacerbation Management 5.3 Asthma: Diagnosis, Stepwise Therapy & Status Asthmaticus 5.4 Interstitial Lung Disease & Idiopathic Pulmonary Fibrosis 5.5 Sarcoidosis, Hypersensitivity Pneumonitis & Occupational Lung Disease 5.6 Pulmonary Embolism: Diagnosis, Risk Stratification & Anticoagulation 5.7 Pulmonary Hypertension & Sleep-Disordered Breathing 5.8 Acute Respiratory Distress Syndrome & Mechanical Ventilation 5.9 Pleural Disease, Empyema & Pneumothorax 5.10 Bronchiectasis, Cystic Fibrosis & Restrictive Lung Disease 5.11 Evaluation of Cough, Dyspnea, Hemoptysis & the Solitary Pulmonary Nodule
7 Chapter 6: Rheumatology & Orthopedics (9% of Exam)
6.1 Rheumatoid Arthritis: Diagnosis & Systemic Manifestations 6.2 Antirheumatic Pharmacotherapy & the Spondyloarthropathies 6.3 Gout: Pathophysiology & Acute Management 6.4 Urate-Lowering Therapy & Calcium Pyrophosphate Deposition Disease 6.5 Osteoarthritis 6.6 Systemic Lupus Erythematosus 6.7 Systemic Sclerosis, Sjogren Syndrome & Inflammatory Myopathies 6.8 The Vasculitides 6.9 Regional Musculoskeletal Syndromes, Back Pain & Sports Injuries 6.10 Other Primary Rheumatic Disorders & Mixed Connective Tissue Disease 6.11 Osteonecrosis & Metabolic Bone Disease
8 Chapter 7: Hematology (6% of Exam)
7.1 Approach to Anemia & the Microcytic Anemias 7.2 Macrocytic, Normocytic & Hemolytic Anemias 7.3 Hemoglobinopathies, Thalassemias & Paroxysmal Nocturnal Hemoglobinuria 7.4 Platelet Disorders, Thrombocytopenia & the Bleeding Patient 7.5 Coagulation Factor Disorders, Thrombophilia & Anticoagulant Reversal 7.6 Acute Leukemias, Chronic Leukemias & Myeloproliferative Neoplasms 7.7 Lymphomas & Plasma Cell Disorders 7.8 Myelodysplastic Syndrome, Myeloproliferative Neoplasms & Porphyria 7.9 Transfusion Medicine & Late Complications of Transplantation
9 Chapter 8: Medical Oncology (6% of Exam)
8.1 Breast, Lung, Colorectal & Prostate Cancer 8.2 Oncologic Emergencies 8.3 Genitourinary & Gynecologic Malignancies 8.4 Head and Neck, Bone & Soft Tissue Cancer and Cancer of Unknown Primary 8.5 Complications of Cancer & Its Treatment 8.6 Cancer Prevention, Survivorship & Psychosocial Care
10 Chapter 9: Nephrology & Urology (6% of Exam)
9.1 Acute Kidney Injury: Staging, Etiology & Urine Microscopy 9.2 Glomerular Disorders & Nephrotic Syndrome 9.3 Nephritic Syndromes, RPGN & Indications for Renal Replacement Therapy 9.4 Chronic Kidney Disease: Staging & Progression Retardation 9.5 Complications of Chronic Kidney Disease: Anemia, CKD-MBD & Acidosis 9.6 Hyponatremia, Hypernatremia & Water Balance 9.7 Potassium Disorders & Systematic Acid-Base Analysis 9.8 Renal Tubular Acidosis & Nephrolithiasis 9.9 Prostate Disorders, Urinary Incontinence & Other Urologic Conditions 9.10 Cystic Kidney Disease, Obstructive Uropathy, Tubulointerstitial Disease & Hematuria
11 Chapter 10: Neurology (4% of Exam)
10.1 Acute Ischemic Stroke: Syndromes & Reperfusion 10.2 Transient Ischemic Attack, Secondary Prevention & Hemorrhagic Stroke 10.3 Headache & Facial Pain 10.4 Seizures, Epilepsy & Movement Disorders 10.5 Multiple Sclerosis & Neuromuscular Junction Disorders 10.6 Motor Neuron Disease & Acute Peripheral Neuropathy 10.7 Cranial Neuropathies & Neuro-Ophthalmology 10.8 Peripheral Neuropathies, CNS Tumors & Other Neurologic Disorders
12 Chapter 11: Psychiatry & Behavioral Health (4% of Exam)
11.1 Major Depressive Disorder & Medical Mimics 11.2 Antidepressant Pharmacotherapy & Treatment-Resistant Depression 11.3 Bipolar Disorder, Mood Stabilizers & Lithium Toxicity 11.4 Anxiety, Stress-Related & Obsessive-Compulsive Disorders 11.5 Alcohol Use Disorder & Withdrawal 11.6 Pharmacotherapy for Alcohol & Opioid Use Disorder 11.7 Toxidromes, Psychiatric Emergencies & Suicide Risk Assessment 11.8 Psychotic, Somatic Symptom, Factitious & Personality Disorders 11.9 Eating Disorders, Sleep Disorders & Sexual Health
13 Chapter 12: Dermatology (3% of Exam)
12.1 Dermatitis, Papulosquamous & Inflammatory Dermatoses 12.2 Acne, Rosacea & Cutaneous Infections 12.3 Herpes Zoster & Autoimmune Blistering Diseases 12.4 Nodules, Tumors & Cutaneous Oncology 12.5 Dermatologic Emergencies & Severe Cutaneous Adverse Reactions 12.6 Pigment Disorders, Alopecia, Nail Disease & Photosensitivity 12.7 Vascular Dermatoses, Environmental Injury & Cutaneous Signs of Systemic Disease
14 Chapter 13: Obstetrics & Gynecology / Women's Health (3% of Exam)
13.1 Contraception, Preconception Care & Teratogenicity 13.2 Hypertensive Disorders of Pregnancy & Gestational Diabetes 13.3 Thyroid Disease & Venous Thromboembolism in Pregnancy 13.4 Menopause & Menopausal Hormone Therapy 13.5 Abnormal Uterine Bleeding, Endometrial Disease & Gynecologic Cancer Screening 13.6 Breast Disorders & Diseases of the Vulva and Vagina 13.7 Menstrual Disorders, Fibroids, Endometriosis & Early Pregnancy Complications
15 Chapter 14: Geriatric Syndromes (3% of Exam)
14.1 Comprehensive Geriatric Assessment & Frailty 14.2 Falls, Gait Assessment & Polypharmacy 14.3 Urinary Incontinence & Pressure Injuries in Older Adults 14.4 Mild Cognitive Impairment, Dementia & Its Subtypes 14.5 Delirium & the Pharmacotherapy of Cognitive Disorders 14.6 Palliative Care, Hospice & Advance Care Planning 14.7 Chronic Wounds, Nutrition, Swallowing, Constipation & Hip Fracture 14.8 Sensory Impairment, Dizziness, Elder Mistreatment & Care Settings
16 Chapter 15: Allergy & Immunology (2% of Exam)
15.1 Anaphylaxis: Recognition & Emergency Management 15.2 Urticaria & Angioedema 15.3 Drug Allergy, Desensitization & Adult Immunodeficiency 15.4 Food Allergy, Insect Venom Hypersensitivity, Serum Sickness & Contrast Reactions 15.5 Rhinitis, Conjunctivitis, Eosinophilic & Mast Cell Disorders
17 Chapter 16: Preventive Medicine, Immunization & Clinical Epidemiology
16.1 The USPSTF Framework & Evidence-Based Cancer Screening 16.2 Cardiovascular & Metabolic Screening and Primary Prevention 16.3 Adult Immunization & Vaccines in Special Populations 16.4 Clinical Epidemiology, Biostatistics & Study Design
18 Chapter 17: Miscellaneous - Ethics, Patient Safety & Quality Improvement (2% of Exam)
17.1 Principles of Bioethics & Decisional Capacity 17.2 Informed Consent, Treatment Refusal & Special Scenarios 17.3 Surrogate Decision-Making, Advance Directives & Confidentiality 17.4 Patient Safety Science, Medical Error & Diagnostic Reasoning 17.5 Quality Improvement Methods, Health Disparities & Health Equity
19 Chapter 18: Perioperative Medicine & Medical Consultation
18.1 Preoperative Cardiovascular Risk Assessment 18.2 Preoperative Pulmonary Risk & Perioperative Medication Management 18.3 Perioperative Anticoagulation, Bridging & Postoperative Complications
20 Chapter 19: Ophthalmology (1% of Exam)
19.1 The Acute Red Eye & Ophthalmic Emergencies 19.2 Painless Vision Loss & Chronic Ophthalmic Disease 19.3 Uveitis, Scleritis & Orbital Disease
21 Chapter 20: Otolaryngology & Dental Medicine (1% of Exam)
20.1 Dizziness, Vertigo & Epistaxis 20.2 Ear Disorders, Hearing Loss & Upper Respiratory Infections 20.3 Oral Cavity, Salivary Gland, Laryngeal & Taste and Smell Disorders
9.6 Hyponatremia, Hypernatremia & Water Balance
Key Takeaways Explain This Topic AI
Hypernatremia and hyponatremia are enumerated under both water and electrolyte balance in nephrology and posterior pituitary disorders in endocrinology.
Serum osmolality first separates true hypotonic hyponatremia from pseudohyponatremia and from hypertonic causes such as hyperglycemia.
Urine osmolality and urine sodium then separate primary polydipsia, appropriate ADH suppression, hypovolemia and the syndrome of inappropriate antidiuresis.
Correction should not exceed roughly 8 mEq/L in 24 hours in patients at risk, to avoid osmotic demyelination syndrome.
Severe symptomatic hyponatremia with seizures or coma is treated with hypertonic saline boluses regardless of chronicity.
Last updated: August 2026
Fluid, electrolyte, acid-base disorders, and nephrolithiasis represent some of the most analytically rigorous topics on the ABIM Internal Medicine board examination. Clinical competence requires a systematic, formula-driven approach to deciphering mixed acid-base disorders, navigating strict correction speed limits in dysnatremias to prevent catastrophic neurologic injury, managing hyperkalemic emergencies, and formulating evidence-based stone prevention regimens.
1. Stepwise Diagnostic Approach to Hyponatremia
Hyponatremia (Serum Sodium < 135 mEq/L) represents a relative excess of water in relation to sodium. Diagnostic evaluation must follow a strict three-step sequence:
Step 1: Determine Serum Osmolality
Hypertonic Hyponatremia (Serum Osm > 295 mOsm/kg): Caused by unmeasured effective osmoles drawing intracellular water into the extracellular space. Most commonly severe hyperglycemia or IV mannitol. Corrected Sodium = Measured Na + 1.6 × ( Serum Glucose − 100 100 ) \text{Corrected Sodium} = \text{Measured Na} + 1.6 \times \left(\frac{\text{Serum Glucose} - 100}{100}\right) Corrected Sodium = Measured Na + 1.6 × ( 100 Serum Glucose − 100 ) (Note: For glucose >400 mg/dL, use a correction factor of 2.0 or 2.4) .
Isotonic Hyponatremia / Pseudohyponatremia (Serum Osm 275–295 mOsm/kg): Laboratory artifact caused by excessive lipids or proteins displacing the aqueous phase of plasma when measured by indirect ion-selective electrodes. Seen in severe hypertriglyceridemia (triglycerides >1500 mg/dL) or paraproteinemia / multiple myeloma .
Hypotonic Hyponatremia (Serum Osm < 275 mOsm/kg): True hypotonic hyponatremia; proceed to Step 2.
Step 2: Assess Clinical Extracellular Fluid (ECF) Volume Status
Hypovolemic Hypotonic Hyponatremia (Total Body Na Loss > Total Body Water Loss):
Physical Exam: Orthostatic hypotension, tachycardia, dry mucous membranes, decreased skin turgor, flat neck veins.
Urine Sodium < 20 mEq/L (FeNa < 1%): Extrarenal sodium losses (vomiting, diarrhea, third-spacing, burns).
Urine Sodium > 20 mEq/L (FeNa > 1%): Renal sodium losses (thiazide diuretics, cerebral salt wasting, mineralocorticoid deficiency / primary adrenal insufficiency [Addison disease with high K+ and low cortisol]).
Treatment: Volume resuscitation with 0.9% Isotonic Saline .
Euvolemic Hypotonic Hyponatremia (Normal Total Body Na, Moderately Increased Water):
Physical Exam: No peripheral edema, no ascites, normal JVP, normal BP.
Syndrome of Inappropriate ADH Secretion (SIADH):
Diagnostic Criteria: Hypotonic hyponatremia, high urine osmolality ( Urine Osm > 100 mOsm/kg , typically >300), high urine sodium ( Urine Na > 40 mEq/L ), low serum uric acid (<4.0 mg/dL), low BUN (<10 mg/dL), and normal thyroid/adrenal/renal function.
Etiologies: Malignancies (Small Cell Lung Cancer [SCLC], head/neck tumors), CNS pathology (stroke, subdural hematoma, meningitis), Pulmonary disease (pneumonia, TB), Drugs ( SSRIs, SNRIs, Carbamazepine, Oxcarbazepine, Cyclophosphamide, NSAIDs ).
Treatment: Fluid restriction (<800–1000 mL/day) is first-line; oral salt tablets + low-dose loop diuretic (Furosemide); oral urea; or Vasopressin V2 Receptor Antagonists (Vaptans: Tolvaptan 15–30 mg daily) for refractory euvolemic hyponatremia (avoid fluid restriction during vaptan therapy; do not exceed 30 days due to hepatotoxicity risk).
Low Solute Intake Syndromes (Urine Osm < 100 mOsm/kg): Psychogenic Polydipsia (massive water intake overwhelming normal renal diluting capacity), Beer Potomania , "Tea-and-Toast" diet.
Endocrine: Severe hypothyroidism, Secondary adrenal insufficiency (ACTH deficiency -> cortisol deficiency impairs free water excretion without mineralocorticoid loss).
Hypervolemic Hypotonic Hyponatremia (Total Body Na Excess < Total Body Water Excess):
Physical Exam: Peripheral pitting edema, ascites, elevated JVP, pulmonary crackles.
Urine Sodium < 20 mEq/L: Congestive Heart Failure, Cirrhosis, Nephrotic Syndrome (decreased effective arterial blood volume triggers non-osmotic ADH release and RAAS activation).
Urine Sodium > 20 mEq/L: Advanced Acute Kidney Injury or ESKD .
Treatment: Dietary sodium and fluid restriction, loop diuretics, neurohormonal blockade.
Loading diagram... Stepwise Diagnostic Algorithm for Hyponatremia & Osmotic Demyelination Safety Limits
- 2. Emergency Management of Dysnatremias & Osmotic Demyelination Syndrome
- Critical Correction Speed Limits in Hyponatremia
Acute Symptomatic Hyponatremia (Duration < 48 hours with seizures, coma, respiratory arrest, herniation):
Administer 3% Hypertonic Saline (513 mEq/L) 100–150 mL IV bolus over 10 to 20 minutes .
May repeat up to 2 times PRN to rapidly elevate serum sodium by 4 to 6 mEq/L within 1 to 2 hours , which rapidly reduces cerebral edema and halts active herniation.
Chronic Hyponatremia (Duration > 48 hours or unknown):
In chronic hyponatremia, brain cells extrude organic osmolytes (myo-inositol, taurine, glutamate) to adapt to hypotonicity. Rapid sodium correction causes sudden hypertonic shrinkage of brain parenchyma.
STRICT MAXIMUM LIMIT: <= 8 mEq/L total increase within 24 hours (and <= 14–16 mEq/L in 48 hours).
High-Risk Patients: In patients with advanced cirrhosis, chronic alcoholism, malnutrition, hypokalemia, or baseline sodium < 105 mEq/L, target 4 to 6 mEq/L per 24 hours (never exceed 8 mEq/L).
Osmotic Demyelination Syndrome (ODS / Central Pontine Myelinolysis)
Pathophysiology: Overly rapid correction (>8–10 mEq/L/24h) strips water from oligodendrocytes, triggering apoptosis and demyelination of the central pontine white matter tracts and extrapontine basal ganglia.
Clinical Presentation: Classically biphasic . The patient initial improves as sodium rises, but 2 to 6 days later develops progressive dysarthria, dysphagia, horizontal gaze paralysis, spastic quadriparesis, encephalopathy, and irreversible "Locked-in" Syndrome .
Rescue Therapy for Overcorrection: If the sodium rises by >8 mEq/L in 24 hours, immediately stop all saline infusions, administer IV Dextrose 5% in Water (D5W) at 3 mL/kg/h, and administer Subcutaneous Desmopressin (DDAVP) 1 to 2 µg every 6 to 8 hours to arrest free water diuresis and safely re-lower serum sodium back within target limits.