Hyponatremia Explained | Pathophysiology, SIADH & Classification | by an MRCP Examiner

Hyponatremia is the most common electrolyte disturbance in internal medicine — and the one most often mismanaged. In Part 1 of this 3-part series, we build the complete conceptual framework: pathophysiology, classification by tonicity, SIADH diagnosis, and causes across all three volume states. 🎓 WHAT YOU'LL LEARN IN THIS VIDEO Why hyponatremia is a water problem, not a sodium problem Tonicity vs osmolality — the distinction that drives every diagnostic decision How to classify hyponatremia: hypotonic, isotonic (pseudohyponatremia) & hypertonic Volume status assessment — the 4 clinical pillars SIADH: Bartter & Schwartz criteria, causes, and why it's a diagnosis of exclusion Thiazide-induced hyponatremia and drug-induced causes (SSRIs, carbamazepine, MDMA) Examiner pearls: vomiting trap, cerebral salt wasting vs SIADH, Hillier's correction ⏱ CHAPTERS 0:00:00 Introduction & Series Overview 0:00:29 Learning Objectives — Parts 1, 2 & 3 0:01:04 Severity Classification — Mild, Moderate & Severe 0:01:53 Anchor Case: 78-Year-Old on Thiazide 0:02:49 Epidemiology & Why It's Missed 0:03:30 Osmolality vs Tonicity — The Core Distinction 0:05:40 Calculated Osmolality & Osmolar Gap 0:06:26 Pseudohyponatremia — Isotonic Hyponatremia Explained 0:07:41 Classification by Tonicity: 3 Buckets 0:08:33 Hypertonic Hyponatremia & Hillier's Correction 0:10:39 Volume Status Assessment — The 4 Pillars 0:12:59 Hypovolemic Hyponatremia — Renal vs Extrarenal Losses 0:14:12 Vomiting Trap & Urinary Chloride Discriminator 0:16:05 Renal Sodium Loss — 4 Mechanisms 0:17:20 Euvolemic Hyponatremia — SIADH & Differentials 0:18:53 Bartter & Schwartz Criteria for SIADH 0:20:06 Causes of SIADH — 6 Categories 0:21:51 Hypervolemic Hyponatremia — Heart Failure, Cirrhosis & More 0:24:10 Effective Circulating Volume (ECV) — The Unifying Concept 0:25:39 Thiazide-Induced Hyponatremia 0:27:28 Drug-Induced Hyponatremia — SSRIs, Carbamazepine, MDMA & More 0:29:58 Primary Polydipsia vs SIADH 0:33:13 Acute vs Chronic Hyponatremia — The 48-Hour Rule 0:35:25 Case Resolution & Key Takeaways 0:39:01 Examiner Pearls — Top 3 0:41:25 Series Outro — Parts 2 & 3 Preview 🩺 ABOUT THIS CHANNEL ClinicalMedEd is created by Dr. Raza Ali Akbar — Senior Consultant in Internal Medicine (Hamad General Hospital, Qatar), Clinical Associate Professor (Qatar University), Assistant Professor (Weill Cornell Medicine–Qatar), current MRCP UK PACES Examiner, former MRCP UK Question Author, and FRCP Edinburgh. Think. Learn. Heal. ⚖️ Disclaimer: Visuals: All images, graphics, audio and video assets are licensed stock media provided via Canva, unless otherwise attributed. Infographics/images: Some content created specifically for ClinicalMedEd. Medical: Strictly educational, not medical advice, no doctor-patient relationship. Practice: Follow local guidelines. 🔗 RELATED VIDEOS ▶ Hyperkalemia — Causes, Diagnosis & Management [HYPERKALEMIA: Causes, ECG Changes & Clinic...] ▶ Hyperkalemia Management: Stabilise, Shift & Remove | MRCP [Hyperkalemia Management: Stabilise, Shift ...] ▶ Hyponatremia Part 2 — Diagnostic Algorithm & SIADH Workup [coming soon] ▶ Hyponatremia Part 3 — Hypertonic Saline, Safe Correction & ODS [coming soon] 📌 Found this useful? Subscribe and hit the bell for Part 2 — the full diagnostic algorithm with case resolution. #hyponatremia #SIADH #internalmedicine #hyponatremiaeducation #usmle #MRCP #electrolyteimbalance #medicalstudent #clinicalmeded #hyponatremiaexplained #tonicity #osmolality #thiazide #paces #usmleStep2