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Primary Care Hacks

Hyponatraemia in Primary Care

Guidelines presents Primary Care Hacks, a series of clinical aide-memoires across a range of topics. Developed by Dr Kevin Fernando, Primary Care Hacks aim to provide a quick and easy resource for primary healthcare professionals and ultimately help improve patients' lives.

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Hyponatraemia in Primary Care

Abnormal sodium levels are commonly encountered in primary care. This Primary Care Hack on hyponatraemia, alongside the companion Primary Care Hack on hypernatraemia, will aid appropriate investigation of abnormal sodium levels to help elucidate possible underlying causes.

Click on the link below for a downloadable PDF of this Primary Care Hack:

Hyponatraemia in Primary Care

What is Hyponatraemia?
  • Hyponatraemia is defined as a serum sodium concentration of <135 mmol/l
  • Hyponatraemia is often asymptomatic and found incidentally
  • Clinical effects of hyponatraemia depend on speed of onset, severity, and underlying cause; acute hyponatraemia (onset <48 hours) is rare but often symptomatic and can cause confusion, coma, and even death
    • check for LADS signs and symptoms: Lethargy, Anorexia/apathy, Disorientation, and Seizures
  • An assessment of volume status is pivotal to the diagnosis and management of hyponatraemia
  • PAI (Addison’s disease) is an important diagnosis not to miss; it can be fatal if untreated
  • Severe hyperglycaemia can lead to hyponatraemia; always exclude new or poorly controlled diabetes as a cause of hyponatraemia.
Causes of Hyponatraemia
  • Diuretics
  • Severe hyperglycaemia and diabetes
  • PAI (Addison’s disease) 
  • Diarrhoea and vomiting (GI sodium loss)
  • Sweating and extensive skin burns (transdermal 
    sodium loss)
  • Third space losses e.g. bowel obstruction, pancreatitis, severe hypoalbuminemia, sepsis, or muscle trauma
  • Medication-induced or consequences of illicit drug use: amiodarone, antipsychotics, diuretics (especially thiazides), PPIs, SSRIs (especially citalopram), ACEi and ARBs, amloride, carbamazepine, phenytoin, valproate, sulfonylureas and insulin, NSAIDs, opioids, dopamine antagonists (e.g. metoclopramide) and illicit drugs such as MDMA
  • SIADH: excessive secretion of ADH causing water retention, dilution of plasma, and accumulation of intracellular fluid. It can lead to cerebral oedema, coma, and death. Can be a paraneoplastic phenomenon; many cancers result in SIADH but especially lung cancer. 
  • Severe hypothyroidism
  • Water excess (e.g. polydipsia)
  • Pseudohyponatraemia is an artificially low sodium level due to hyperproteinaemia (e.g. multiple myeloma) or hypertriglyceridaemia
  • AKI, CKD, and nephrotic syndrome
  • Liver cirrhosis with ascites
  • Chronic HF due to low cardiac output.
Investigations for Hyponatraemia
  • Serum osmolality is a measure of the concentration of different solutes in plasma and is primarily determined by sodium, glucose, and urea. NR is usually 275–295 mmol/kg and is tightly maintained by ADH, which regulates fluid balance. An increase in serum osmolality results in secretion of ADH, which increases water reabsorption in the kidneys to return serum osmolality to baseline
  • Urine osmolality is a measure of urine concentration and whether this is appropriate for the clinical state of the individual. It provides an estimate of ADH activity. NR is usually 300–900 mmol/kg water. If osmolality ≤100 mOsm/kg (dilute urine), ADH is not acting. If osmolality is >100 mOsm/kg (concentrated urine), ADH is acting. After 12–14 hours’ fluid restriction, urinary osmolality should be >850 mmol/kg water
  • Urinary sodium is a measure of the concentration of sodium in a litre of urine. It is useful for the differential diagnosis of hyponatraemia but must be interpreted alongside volume status, and is therefore difficult to interpret in those taking diuretics
  • Serum urea is a marker of extracellular fluid volume. A raised urea may suggest dehydration
  • Serum creatinine is useful as an assessment of renal impairment as a cause of hyponatraemia.
ACEi=angiotensin-converting enzyme inhibitor; ADH=antidiuretic hormone; AKI=acute kidney injury; ARB=angiotensin receptor blocker; CKD=chronic kidney disease; CNS=central nervous system; CXR=chest X-ray; GI=gastrointestinal; HbA1c=haemoglobin A1c; HF=heart failure; JVP=jugular venous pressure; LFT=liver function test; MDMA=methyl​enedioxy​methamphetamine; Na=sodium; NR=normal range; NSAID=non-steroidal anti-inflammatory drugs; PAI=primary adrenal insufficiency; PPI=proton pump inhibitor; SAI=secondary adrenal insufficiency; SIADH=syndrome of inappropriate antidiuretic hormone secretion; SSRI=selective serotonin reuptake inhibitor; TSH=thyroid-stimulating hormone; U/E=urea and electrolytes.
Primary Care Hacks are developed by Dr Kevin Fernando, GP Partner, North Berwick Health Centre; GP with special interest in CVRM and medical education; Content Advisor for WebMD Medscape Global and UK. This Primary Care Hack is based on the author's interpretation of relevant summaries of product characteristics. Primary Care Hacks are for information for primary healthcare professionals in the UK only. They bring together currently available recommendations and/or prescribing information and indications for therapeutics licensed within Great Britain. Licensed indications and/or prescribing information for Northern Ireland may differ. You are advised to review local licensed indications before prescribing any therapeutic. Primary Care Hacks are reviewed intermittently to ensure the information is up to date at the time of publication. Primary Care Hacks are independently produced by WebMD, LLC and have not been created in conjunction with any guideline or prescribing body.