Sodium disorders

Hyponatremia = excess free water relative to sodium. Workup: osmolality, volume, Uosm, UNa.

Anishaa Sivakumar7 min6 questions1 trials

Hyponatremia — first split on tonicity

Hyponatremia is free-water excess relative to sodium ((3):280-291). Often asymptomatic; can present with AMS, headache, nausea/vomiting, weakness, seizures, signs of raised ICP, salt craving.

Work up if Na <130, and draw labs before you treat. History: symptoms, water intake, diet, urination frequency, etiology, chronicity, new meds, endocrine ROS, volume status, irrigation surgery (prostate / bladder / uterus). Serum: BMP, osm; consider uric acid, TSH, AM cortisol. Urine: UA, osm, Na, uric acid.

Tonicity first — then volume

Hypotonic
SOsm <300
True hypo-osmolar hyponatremia. Next question: is ADH on (Uosm >100)? Then volume: hypovolemic, euvolemic, or hypervolemic. This is the algorithm that follows.
Isotonic
SOsm ≈300 — “pseudo”
Hyperproteinemia (myeloma, IVIG) or lipemia. Consider a VBG with electrolytes for a more accurate Na.
Hypertonic
SOsm >300 — translocational
Hyperglycemia, or ↑ osmolal gap (mannitol, glycine, sorbitol, IVIG stabilizers). Correct the glucose, not just the lab Na (~1.6 per 100 glucose over 100).

Serum osm misleads when ineffective osmoles are present (BUN, ethanol) — tonicity is not the same as measured osm. Figure adapted from

Is ADH present? Uosm >100**. Approximate Uosm from UA: last two digits of SG × 30 (SG 1.010 ≈ Uosm 300).
SIADH clues: serum uric acid <4, BUN <5, FEUA ≥10–12% (small studies: 100% PPV for SIADH / thiazide hypoNa; FEUA ≤8% = 100% NPV) (Eur J Inter Med 2016;29:22; ; ; ).
↓EABV clues: serum uric acid >5, BUN ↑, FEUA <4%.
NS challenge: SIADH — NS initially ↑SNa then ↓SNa (Na excreted, water retained). Hypovolemia — NS ↑SNa.

Hypotonic: hypovolemic · euvolemic · hypervolemic

UNa is a surrogate for volume / RAAS. Treatment depends on acuity, severity, and etiology (JASN 2017;28:1340). Call renal if they need 3% NaCl and/or a DDAVP clamp.

Branch	Uosm / UNa	Physiology	Treatment
ADH absent (Uosm <100)	Kidney already maximally dilute	Primary polydipsia (psychogenic, endurance athletes, MDMA). ↓ solute intake (tea-and-toast, beer potomania) — overwhelming the kidney’s ability to dilute.	Polydipsia: restrict fluids. Low solute: SLOW introduction of solute — they correct RAPIDLY and are high ODS risk.
Hypovolemic (UNa <30)	RAAS on, Na-avid. ↓TBW and ↓EABV. ADH is “appropriate.”	True volume depletion. Also 3rd-spacing (pancreatitis, muscle injury) with ↑TBW but ↓EABV.	Replete volume. High overcorrection risk — watch UOP, may need a DDAVP clamp. Pancreatitis: replete volume.
Hypervolemic (UNa <30)	RAAS on, Na-avid. ↑TBW, ↓EABV. ADH appropriate.	CHF, cirrhosis, nephrotic syndrome.	CHF / nephrosis: diurese. Cirrhosis: treat only if Na <120 — fluid restrict, correct hypoK; vaptans / 3% only if severe or for OLT.
Euvolemic (UNa >30)	RAAS off, Na wasting. Kidney will not dilute; ADH is inappropriate.	SIADH: CNS disease, pain/nausea, lung infection/cancer, meds (), idiopathic. Also: thiazides, ACE inhibitors/ARBs, ESRD, cerebral salt wasting, adrenal insufficiency, severe hypothyroidism.	Correct the cause. Mild SIADH: fluid restrict 1–1.5 L/day; consider NaCl tabs 1 g TID, urea 15–60 g/day (KI 2015;87:268), or an SGLT2i ((2)322). If Uosm >2× SOsm or UNa+UK > SNa → NaCl tabs + lasix 10–20 mg BID (↓ corticomedullary gradient) or vaptans with renal input ().

Hypovolemic

UNa <30, Uosm >100, dry.
NS will raise SNa.
Water diuresis after volume repletion is the classic overcorrection setup (UOP >100 mL/h) — clamp early.

Euvolemic

UNa >40, Uosm usually much higher than 100, clinically euvolemic.
NS may raise then drop SNa.
SIADH vs adrenal vs severe hypoT — check cortisol and TSH before you lock in SIADH.

Hypervolemic

UNa <30, wet, ↓EABV (CHF, cirrhosis, nephrosis).
Restrict water; treat the parent disease.
Cirrhosis: only chase Na <120; 3% / vaptans are niche (severe symptoms or transplant).

Correction, ODS, and the clamp

Rate / goal: ↑Na 4–6 mEq/L per 24 h, short-term goal ≥125. Typically 4–6 mEq is enough to clear symptoms.
Severe symptomatic (seizure, AMS): 3% NaCl 100 mL over 10 min; up to 3 times until symptoms resolve or Na ↑5 (max 8–10 / 24 h).
Severe asymptomatic (Na <120): 3% at a correction-rate drip, usually 15–30 mL/h, until Na ≥125. Na <120 is associated with 6–10% in-hospital mortality (). Consider a proactive DDAVP clamp if ODS risk is high.
Mild / moderate (Na 130–134 / 120–129): treat the etiology (replete, restrict, diurese, or hold diuresis). Slow correction has been associated with higher mortality ((1):38-51).

Overcorrection: Na ↑ >8 in 24 h or >18 in 48 h. In high-ODS-risk patients some guidelines call ΔNa >4–6 mEq/24 h overcorrection. Rapid correction is more likely in hypovolemic and low-solute hyponatremia (). Concurrent hypokalemia: 1 mEq K = 1 mEq Na; repleting K can overshoot Na.

ODS: delayed (2–6 days) dysarthria/dysphagia, paresis, behavior change / AMS, locked-in. MRI ~4 weeks after symptom onset. Risk: starting Na ≤110 (ODS is unlikely if starting Na >120 — Hosp Pract 1995;37:128), malnutrition, EtOH, cirrhosis, ↓K.
If the trajectory predicts overcorrection: DDAVP clamp ((6):890). DDAVP 2–4 mcg IV/SC q6–8h, Na q4–6h, until Na ≥125. Add 3% at a correction rate if needed.
If already overcorrected: urgently re-lower Na into the prior goal (ΔNa <8 mEq/L/24 h, or <4–6 in high ODS risk). D5W 6 mL/kg IBW over 2 h (↓Na by ~2) with q2h Na, PLUS DDAVP 2–4 mcg IV/SC q6h for 24–48 h even after the re-lowering goal is first hit.

Hypernatremia

Free-water loss in excess of NaCl loss (; ). Almost always a water deficit: ↓ access to free water or ↓ thirst, plus DI. Nursing-home patients cannot get to the sink.

Renal losses: Uosm <800. ↓ADH release or kidney response — post-ATN, osmotic diuresis, DI, loop diuretic, hypercalcemia, elderly.
Extrarenal losses: Uosm >800. GI loss (NGT, vomiting, diarrhea), insensible losses, hypodipsia.
Workup: water and diet history, urinary history, meds (especially lithium), recent brain surgery, malignancy. Labs: BMP, serum osm, urine osm. Uosm <300 → consider AVP deficiency or resistance. Uosm <700 → consider extra-renal water loss.

Calculate current free-water deficit. Shortcut: FWD (L) = (current Na − goal Na) / 3. Target Na is the 24-hour goal.
Add ongoing losses. Insensible: 700–900 mL/day, more with burns, fever, diarrhea (you cannot measure it, so increase replacement if the Na is not falling). Urine electrolyte-free water: 24h UOP × (1 − (UNa+UK)/SNa); alternative = (0 × 1st L) + (0.5 × 2nd and 3rd L) + (1 × urine beyond 3 L).
Free water today = current FWD + ongoing losses. Give PO, tube-feed water, or IV D5W. If DI, see Pituitary disorders for DDAVP.

BMP q12–24 h, strict I/O. Aim to ↓SNa by 12 mEq/L per 24 h. Accidental overcorrection is unlikely to be dangerous in adults; cerebral edema from too-fast correction is rare ().