Hyponatremia is an electrolyte disorder defined by a serum sodium (Na⁺) concentration below 135 mEq/L, representing the most common electrolyte abnormality encountered in hospitalized patients. It is distinguished from hypernatremia (serum Na⁺ >145 mEq/L) and hypokalemia (low potassium), though these may coexist; the underlying pathophysiology involves either a net loss of sodium exceeding water loss, or — more commonly — a relative excess of free water that dilutes existing sodium stores. The condition is clinically classified by volume status into three physiologic subtypes: hypovolemic hyponatremia (loss of both sodium and water, with proportionally greater sodium loss — e.g., vomiting, diuretic use, adrenal insufficiency), euvolemic hyponatremia (total body water increased with near-normal total body sodium — e.g., SIADH, hypothyroidism, psychogenic polydipsia), and hypervolemic hyponatremia (excess total body water and sodium, with water retention disproportionately greater — e.g., heart failure, cirrhosis, nephrotic syndrome). At the cellular level, free water shifts down an osmotic gradient into cells, causing cerebral edema that produces the hallmark neurological symptoms — nausea, headache, confusion, and, in severe or rapidly evolving cases, seizures, herniation, and death. Clinically relevant subtypes coded under E87.1 include acute (onset <48 hours, higher seizure risk), chronic (onset ≥48 hours, greater risk of osmotic demyelination syndrome if corrected too rapidly), and severe symptomatic (Na⁺ <120 mEq/L with altered mental status); when hyponatremia results from SIADH, the etiology is coded separately as E22.2, which is excluded from E87.1 by a Type 1 Excludes note. It is commonly confused with pseudohyponatremia (artifactual low sodium from hyperlipidemia or hyperproteinemia, with normal osmolality) and translocational hyponatremia (sodium shift without true deficit, as seen with severe hyperglycemia), both of which do not map to E87.1.
Blood-condition suffix — “condition of (a substance) in the blood”; noun-forming suffix denoting the presence or level of a substance in the bloodstream
The word entered English in the 1930s (first known use 1935) as hyponatremia (noun), coined from New Latin elements as a clinical diagnostic term. The combining form derives from New Latin natrium, itself from Arabic natrūn (a natural sodium salt, used in ancient Egypt), which passed through Medieval Latin; the chemical symbol Na is a direct abbreviation of natrium. The root natr- (“sodium”) connects hyponatremia to the entire -natr- ROOT FAMILY: hypernatremia (hyper- + natr- + -emia → excess sodium in blood), natriuresis (natr + ur + -esis → urinary excretion of sodium), and natriuretic (pertaining to sodium excretion — as in natriuretic peptides BNP/ANP). The prefixhypo- is one of the most productive in medical vocabulary: hypoglycemia, hypotension, hypothyroidism, hypoxia, hypokalemia.
🔀 ALIASES / ALTERNATE TERMS
Hyponatremic(adjective form — used in collocations such as “hyponatremic encephalopathy,” “hyponatremic seizure,” “hyponatremic patient”)
Low sodium / Low blood sodium(lay term universally recognized by patients; triggers documentation review in nursing and discharge notes)
Salt depletion / Sodium deficiency(clinical synonym; coded under E87.1 per Applicable To note in ICD-10-CM Tabular; often used interchangeably in nephrology notes)
Hypo-osmolality(ICD-10-CM tabular title partner term; reflects the osmotic consequence of low sodium — serum osmolality <280 mOsm/kg; coded together with hyponatremia under E87.1)
Hypovolemic hyponatremia(subtype with net sodium loss exceeding water loss; causes include GI losses, diuretic use, third-spacing, adrenal insufficiency; volume-depleted exam findings)
Euvolemic hyponatremia(subtype with increased total body water but normal sodium stores; most commonly due to SIADH E22.2, hypothyroidism E03.9, or psychogenic polydipsia)
Hypervolemic hyponatremia(subtype with excess total body water and sodium; seen in CHF, cirrhosis, nephrotic syndrome; edematous states with dilutional sodium drop)
Dilutional hyponatremia(synonym for euvolemic or hypervolemic subtypes; caused by water retention rather than sodium loss; seen in SIADH and fluid-overloaded states)
Acute hyponatremia(onset <48 hours; higher risk of cerebral edema and seizures; requires urgent but controlled correction)
Chronic hyponatremia(onset ≥48 hours or unknown duration; brain has adapted; rapid correction risks osmotic demyelination syndrome/ODS, formerly called central pontine myelinolysis)
Hyponatremia of newborn(neonatal-specific form; coded separately as P74.22 — not E87.1; applicable to newborn records only)
🔗 RELATED TERMS
Hypernatremia — the opposite of hyponatremia; serum Na⁺ >145 mEq/L, typically reflecting free water deficit or excess sodium intake; coded under E87.0
Hypokalemia — shares the hypo- prefix; low serum potassium (<3.5 mEq/L); frequently coexists with hyponatremia, especially in diuretic-induced electrolyte loss; coded under E87.6
Hypo-osmolality — the osmotic correlate of hyponatremia; serum osmolality <280 mOsm/kg; coded together with hyponatremia under E87.1 per ICD-10-CM Tabular
SIADH (Syndrome of Inappropriate Antidiuretic Hormone Secretion) — the most common cause of euvolemic hyponatremia; excess ADH leads to free water retention and dilutional sodium drop; coded separately under E22.2, which is Type 1 Excluded from E87.1
Osmotic demyelination syndrome (ODS) — a devastating neurological complication of overly rapid correction of chronic hyponatremia; formerly called central pontine myelinolysis; coded under G37.2
Cerebral edema — the primary mechanism of neurological symptoms in acute hyponatremia; free water enters brain cells down osmotic gradient; coded under G93.6 when documented
Hyponatremic encephalopathy — clinical syndrome of neurological dysfunction (confusion, seizures, coma) caused by severe or acute hyponatremia; coded as E87.1 + neurological manifestation code
Pseudohyponatremia — artifactually low sodium reading caused by severe hyperlipidemia or hyperproteinemia displacing plasma water; serum osmolality is normal; no ICD-10-CM code maps to this entity directly — code the underlying cause
Adrenal insufficiency — endocrine cause of hypovolemic hyponatremia via aldosterone deficiency leading to renal sodium wasting; coded under E27.40 or E27.1; always query when hyponatremia is paired with hyperkalemia
Heart failure — leading cause of hypervolemic hyponatremia via neurohormonal activation and renal water retention; coded under I50.x category; hyponatremia codes as secondary CC
Cirrhosis — hepatic cause of hypervolemic hyponatremia via portal hypertension, splanchnic vasodilation, and secondary hyperaldosteronism; coded under K74.x; hyponatremia frequently present as CC
Serum osmolality — primary diagnostic test for classifying type of hyponatremia (true vs. pseudo vs. translocational); osmolality <280 mOsm/kg confirms true hypo-osmolar hyponatremia
Electrolyte panel — includes Na⁺ (84295), K⁺ (84132), Cl⁻ (82435), CO₂ (82374); most direct panel for electrolyte surveillance
80048
Basic metabolic panel (BMP) — includes 80051 components plus calcium, creatinine, glucose, BUN; standard initial workup
80053
Comprehensive metabolic panel (CMP) — all BMP components plus hepatic markers; used when hepatic or protein cause suspected; cannot bill with 80048 same DOS
82340
Urine sodium; quantitative — essential for differentiating SIADH (urine Na⁺ >20 mEq/L) from hypovolemic states (urine Na⁺ <20 mEq/L)
82565
Creatinine; serum — renal function assessment; required when renal etiology or AKI is considered
82945
Glucose, body fluid other than blood — or 82947 for serum glucose; rules out translocational hyponatremia from hyperglycemia
84703
Osmolality; urine — paired with serum osmolality to classify hyponatremia type and assess ADH activity
IV infusion, each additional hour — reported with 96365 for extended hypertonic saline correction sessions
96360
IV hydration, initial, 31 minutes to 1 hour — isotonic fluid (0.9% NS) for hypovolemic hyponatremia; note: hydration code, not therapy/infusion
⚠️ Coding Note:E87.1 is a CC (Complication/Comorbidity) when coded as a secondary diagnosis under MS-DRG V43.0, grouping to MS-DRG 640 (Miscellaneous disorders of nutrition, metabolism, fluids and electrolytes with MCC) or MS-DRG 641 (without MCC); when hyponatremia is the principal diagnosis, it carries its own DRG weight — but when it drives secondary CC credit on a high-acuity admission (e.g., sepsis, CHF, pneumonia), it can tip a case from a lower-weighted DRG pair into a higher one. A critical Type 1 Excludes note governs sequencing: E87.1 and E22.2 (SIADH) cannot be coded together — when SIADH is confirmed as the etiology of hyponatremia, assign E22.2 only; query the physician if documentation says “hyponatremia due to SIADH” without a standalone SIADH diagnosis to clarify the appropriate principal diagnosis. An undercoding alert applies to the etiology-specific query: vague documentation of “low sodium” or “electrolyte abnormality” without qualification of volume status, acuity (acute vs. chronic), or underlying cause should prompt a physician query, as the documented etiology (e.g., adrenal insufficiency, cirrhosis, SIADH) may yield a more specific and higher-weighted principal or secondary code. For IV saline correction, use 96365/96366 (therapy/infusion codes) when hypertonic (3%) saline is administered — not the hydration code 96360, which applies to isotonic maintenance fluids only; conflating these is a common profee billing error on hyponatremia admissions.
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