Order matters: tonicity first, symptoms second, cause third.

1. Check serum osmolality. If normal → pseudohyponatremia (high lipids/protein). If high → hyperglycemia/mannitol (corrected Na: add ~1.6 mmol/L per 100 mg/dL glucose above 100; some use 2.4). If low (< 275 mOsm/kg) → true hypotonic hyponatremia, continue.
2. Grade symptoms. If severe (seizure, coma, vomiting, cardiorespiratory distress) → 3% saline 100–150 mL IV bolus over 10–20 min, repeat up to 2–3 times until Na rises ~4–6 mmol/L or symptoms stop. If mild/chronic → work up the cause first.
3. Correction limit: rise ≤ 8–10 mmol/L in 24 h (≤ 8 if high risk of osmotic demyelination: alcohol, malnutrition, hypokalemia, liver disease). If overcorrected → give D5W ± desmopressin to re-lower.
4. Urine osmolality. If < 100 mOsm/kg → primary polydipsia, beer potomania/low solute intake. If > 100 → ADH is acting, continue.
5. Volume status + urine Na. Hypovolemic + UNa < 20–30 → GI/skin losses → 0.9% saline. Hypovolemic + UNa > 30 → thiazide, primary adrenal insufficiency, salt wasting. Euvolemic + UNa > 30 → SIADH, hypothyroidism, glucocorticoid deficiency. Hypervolemic (HF, cirrhosis, nephrotic) → fluid and salt restriction ± loop diuretic.
6. Before labelling SIADH → check TSH and cortisol, and review drugs (SSRIs, carbamazepine, thiazides). SIADH treatment: fluid restriction first; then salt tablets/urea or tolvaptan; treat the cause (e.g. small-cell lung cancer).

1. Real or artefact? If unexpected, no ECG changes, and the sample is hemolyzed or there is marked thrombocytosis/leukocytosis → repeat (heparinized/non-hemolyzed sample) before treating.
2. 12-lead ECG now. Peaked T → flat P, long PR → wide QRS → sine wave/VF. If ECG changes or K ≥ 6.5 mmol/L → treat as an emergency on a monitor.
3. Stabilize the myocardium: 10% calcium gluconate 10 mL IV over 2–5 min (UK guidance uses 30 mL); repeat if ECG changes persist. Calcium chloride 10 mL 10% is an option via a large/central vein. It does not lower K.
4. Shift K into cells: regular insulin 10 units IV + 25 g glucose (50 mL of 50% dextrose); monitor glucose for hours. Add nebulized salbutamol 10–20 mg. Sodium bicarbonate only if significant metabolic acidosis.
5. Remove K from the body: stop ACEi/ARB, spironolactone, NSAIDs, trimethoprim, K supplements; loop diuretic if passing urine; potassium binder (sodium zirconium cyclosilicate or patiromer). If refractory, oliguric AKI or ESRD → urgent hemodialysis.
6. Recheck K at 1–2 h and again at 4–6 h (insulin effect wears off → rebound).

1. Confirm true hypercalcemia: corrected Ca = measured Ca + 0.8 × (4 − albumin g/dL), or measure ionized Ca.
2. Severe (> 14 mg/dL) or symptomatic (confusion, vomiting, dehydration, AKI, short QT) → IV 0.9% saline first (e.g. 200–300 mL/h, aim urine output 100–150 mL/h), then calcitonin 4 IU/kg (fast, wears off in ~48 h) + zoledronic acid 4 mg IV (works in 2–4 days). Loop diuretic only after volume is restored. Denosumab if renal failure/refractory; dialysis if severe with renal failure.
3. Measure PTH. If high or inappropriately normal → primary hyperparathyroidism (commonest in outpatients); check 24-h urine Ca (low Ca/Cr clearance ratio < 0.01 → familial hypocalciuric hypercalcemia, no surgery). Also lithium, thiazides.
4. If PTH suppressed → malignancy (commonest in inpatients): PTHrP, SPEP + serum free light chains (myeloma), imaging. Check 25-OH and 1,25-(OH)₂ vitamin D (granulomatous disease/lymphoma → high 1,25; vitamin D toxicity → high 25-OH), TSH, immobilization, vitamin A.
5. Steroids for granulomatous disease (sarcoid, TB), lymphoma and vitamin D toxicity.
6. Primary hyperparathyroidism → parathyroidectomy if any: symptoms or stones, Ca > 1 mg/dL above upper limit, osteoporosis (T ≤ −2.5) or vertebral fracture, eGFR < 60, hypercalciuria (> 400 mg/24 h), age < 50. Localize (sestamibi/US) only after deciding on surgery.