Sodium is the main extracellular cation and plays a critical role in maintaining fluid balance, osmolarity, nerve conduction, and blood pressure. Disturbances in sodium balance are common in clinical practice and can lead to significant neurological and cardiovascular complications if not recognised and managed appropriately. This article discusses the causes, clinical features, and management of hypernatraemia and hyponatraemia, with a particular focus on drug-related causes and sodium containing intravenous fluid therapy. It also reviews the sodium content of commonly used medicines and fluids, the different types of intravenous fluids, and the role of syndrome of inappropriate antidiuretic hormone secretion (SIADH) in hyponatraemia. Sodium Requirements A normal serum sodium level is usually between 135mmol/L-145mmol/L. As per NICE Clinical Guideline 174, adults receiving IV fluids should receive 1mmol/kg/day of sodium, plus any additional losses. The World Health Organization (WHO) recommends that adults limit their salt intake to less than 5 grams (2000mg / 90mmol of sodium) daily, although average intake is often higher. Patients with heart failure or chronic kidney disease may require a sodium-restricted diet, to avoid fluid overload and protect organ function. Clinical example: An 80kg adult patient who is vomiting and cannot keep down oral fluids will require 80mmol of sodium plus approximately 20-60mmol of sodium per 1L of emesis, every 24 hours. If the patient is vomiting 2L per day their requirements would be 120mmol – 200mmol daily. 1L 0.9% saline would already provide the patient with 154mmol of sodium requirements, therefore care must be taken to replace fluid and electrolyte losses, whilst avoiding sodium overload. Medicines that contain <1mmol of sodium per single dose are classified as “sodium free”. Effervescent, dispersible and soluble formulations are often high in sodium content. One vial of piperacillin/tazobactam contains 9.4mmol of sodium 0.9% saline contains 154mmol/L sodium Hartmann’s solution contains 131mmol/L sodium Peptac liquid contains ~12mmol/20ml dose Types of Intravenous Fluids Many intravenous fluids contain sodium, such as crystalloids, which are solutions of small molecules in water, often used in fluid resuscitation and maintenance therapy. Isotonic Fluids These have a similar concentration of solutes compared with blood: 0.9% saline (sodium chloride) Hartmann’s solution Hypotonic Fluids These have a lower concentration of solutes compared with blood: 0.45% NaCl 0.33% NaCl 2.5% dextrose/glucose 5% dextrose/glucose Hypertonic Fluids These have a higher concentration of solutes compared with blood: 3% NaCl 5% NaCl 50% dextrose 20% dextrose Conversely, colloids contain large molecules that help retain water within the intravascular space, expanding blood volume rapidly and increasing blood pressure. Examples include: Human albumin 5% and 20% (used in burns, liver failure, and sepsis) Gelatin-based fluids Dextran Hypernatraemia The definition of hypernatraemia is a serum sodium level of >145mmol/L. Severe hyponatraemia is generally considered to occur at serum sodium concentrations >155–160 mmol/L. Serum hyperosmolality always accompanies hypernatraemia. Causes Hypovolaemic Hypernatraemia This is the most common presentation of hypernatraemia. Loss of water and/or inadequate water intake which can lead to hypovolaemia, which concentrates sodium in plasma and requires fluid correction. It can also occur in hyperosmolar hyperglycaemic state (HHS) due to a net loss of water Alongside a raised sodium level, signs and symptoms include Raised urea Thirst Decreased urinary output Orthostatic hypotension Correction of sodium includes correcting losses whilst carefully monitoring sodium levels, if severe hypernatraemia is corrected too rapidly with intravenous hypotonic fluids (>8-10mmol/L/24hrs), this can lead to cerebral oedema and permanent neurological damage. Ideally water should be replaced enterally, if not possible then IV hypotonic fluids may be provided such as 5% glucose and 0.45% NaCl. Drugs that can cause hypernatraemia include the following Diuretics due to fluid loss and altered volume status Corticosteroids Lithium in overdose due to nephrogenic diabetes insipidus Sodium overload Rarely, excessive intake of sodium may cause hypernatraemia. This may present as raised sodium level with a normal urea. Hyponatraemia Hyponatraemia is defined as serum sodium of <135 mmol/L. Risk factors include elderly, low BMI, drugs, hypothyroidism and surgery. Symptoms of mild hyponatraemia include: Nausea Headache Confusion In severe hyponatraemia when sodium falls <125mmol/L, agitation, psychosis and seizures may occur. Correction of sodium levels should be undertaken gradually and not exceed 8-10mmol/L/24 hours to to reduce the risk of osmotic demyelination syndrome. Causes Hyponatraemia can be categorised into the following types: Hypertonic hyponatraemia: serum osmolarity >290mOsm/kg Causes include: Severe hyperglycaemia (the high levels of glucose draw intracellular water into the extracellular space) Administration of an active osmolyte (such as mannitol) Isotonic hyponatraemia: serum osmolarity 275-290mOsm/kg Often due to pseudo-hyponatraemia, caused by: High lipids: hyperproteinaemia or hypertriglyceridaemia High proteins: hyperproteinemia Infusion of isotonic fluids e.g. glucose/mannitol Hypotonic hyponatraemia: serum osmolarity <275mOsm/kg Hypovolemic (volume depletion) hyponatraemia due to sodium and water losses Renal causes (urinary sodium >20mmol/L), e.g. diuretics, adrenal insufficiency Extrarenal causes (urinary sodium <20mmol/L), e.g. gastrointestinal losses – the body keeps hold of sodium therefore does not filter out via the kidneys Euvoleamic hyponatraemia – increase in water but sodium stays the same leading to diluted sodium Examples include hypothyroidism and syndrome of inappropriate antidiuretic hormone (SIADH) Hypervolaemic hyponatraemia due to an increase in sodium and water Causes include oedema in coronary heart failure, kidney disease (nephrotic syndrome, AKI or CKD), cirrhosis with ascites and nephrotic syndrome By TeachMeSeries Ltd (2026) Fig 2: Hyponatremia overview SIADH SIADH occurs when excessive vasopressin (antidiuretic hormone) release → impaired water excretion → water retention and dilution of blood → hyponatraemia Signs: low serum sodium, low plasma osmolality, high urinary sodium, high urine osmolality Symptoms: range from nausea and headache to seizures and coma Causes: brain injury, malignancy, infection, hypothyroidism, drugs When correcting hyponatraemia, osmotic demyelination syndrome may occur with rapid correction (>12mmol/L/day) Vasopressin receptor antagonists (e.g., tolvaptan) can be used in the treatment of SIADH. VRAs competitively bind at the vasopressin receptor (mainly V2 receptors), antagonising the effect of vasopressin. Drug Causes Drugs that increase the production, or potentiate the action, of antidiuretic hormone can cause hyponatraemia, such as: Opioids. ACE inhibitors and angiotensin-II receptor antagonists Proton pump inhibitors SSRIs, SNRIs and TCAs Carbamazepine Antipsychotics such as haloperidol and phenothiazines Amiodarone Theophylline Tetracyclines Non-steroidal anti-inflammatory drugs (NSAIDs) Drugs that cause loss of antidiuretic hormone (ADH) inhibition: NSAIDs – particularly in combination with thiazide diuretics or in heart failure Drugs that produce exogenous ADH: Desmopressin Oxytocin References https://bestpractice.bmj.com/topics/en-gb/1215 accessed 21.1.25 https://www.medicines.org.uk/emc/product/13162/smpc#gref accessed 22.10.24 https://cks.nice.org.uk/topics/hyponatraemia/background-information/causes/ accessed 22.1.25 Overview | Intravenous fluid therapy in adults in hospital | Guidance | NICE Accessed 14/5/26 Gandy, J. (2019). Manual of Dietetic Practice. John Wiley & Sons. Do you think you’re ready? Take the quiz below Pro Feature - Quiz Hypernatraemia and Hyponatraemia Question 1 of 3 Submitting... 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