Urine sodium in hypovolaemic hyponatraemia
In hypovolaemic hyponatraemia (a low sodium with a low circulating volume), urine sodium below 20 mmol/L means loss outside the kidneys; above 20 mmol/L points to renal loss.
Remember
In hypovolaemic hyponatraemia, a urine sodium below 20 mmol/L means sodium was lost outside the kidneys, most often through the gut, whereas above 20 mmol/L points to the kidneys.
Memory aid
a dry patient with a low urine sodium has a leaking gut, and a dry patient with a high urine sodium has a leaking kidney
It helps you place the loss: a dry patient with a low urine sodium has lost sodium from the gut, and one with a high urine sodium has lost it through the kidney.
In hypovolaemic hyponatraemia the patient looks dry. The body's water is low, and its sodium is even lower.
The fall in circulating volume makes the body release ADH (antidiuretic hormone, also called vasopressin, which makes the kidneys hold on to water). The retained water restores some volume but dilutes the sodium that is left, so the blood sodium falls even though the body is short of salt.
The urine sodium then shows where the sodium went. Below 20 mmol/L means the loss is extra-renal (from outside the kidneys), and the kidneys are appropriately conserving sodium. Vomiting, diarrhoea, burns and heavy sweating are examples. Above 20 mmol/L points to a renal loss, in which the kidney is wasting sodium when it should be conserving it. Causes include thiazide diuretics (tablets that make the kidneys excrete extra sodium and water), which are the most common drug cause of hyponatraemia, and mineralocorticoid deficiency (too little of the adrenal hormone aldosterone, which normally makes the kidney keep sodium), as in Addison's disease (an underactive adrenal gland).
There is one caution in protracted vomiting. The blood can become too alkaline (metabolic alkalosis), and bicarbonate spilled in the urine carries sodium with it. The urine sodium can then mislead, and urine chloride is more reliable.
Key facts
- Type
- Hypovolaemic hyponatraemia means sodium has been lost in excess of water, and the patient looks dry.
- Low urine sodium
- Below 20 mmol/L means the loss is extra-renal and the kidneys are conserving sodium, as in vomiting, diarrhoea, burns or heavy sweating.
- High urine sodium
- Above 20 mmol/L points to a renal loss, where the kidney is wasting sodium, as with thiazide diuretics or mineralocorticoid deficiency.
- Why the sodium is diluted
- Volume depletion releases ADH, and the water that is retained dilutes the sodium.
- Thiazides
- They are the single most common drug-induced hyponatraemia, and older people are especially susceptible.
- Vomiting caveat
- With metabolic alkalosis, urine chloride is a more reliable guide than urine sodium.
Exam traps
- Addison's disease does not give a urine sodium below 20 mmol/L, because the lack of aldosterone, the hormone that makes the kidney retain sodium, causes renal salt wasting.
- A low urine sodium alone does not prove fluid loss, because hyponatraemia from fluid overload, as in heart failure or cirrhosis, usually also has a urine sodium below 20 mmol/L, and the patient has oedema (swelling from excess tissue fluid) rather than a dry look.
- A high urine sodium in a dry patient is not explained by the syndrome of inappropriate antidiuresis (ADH released without a proper trigger), because patients with that syndrome are not dehydrated and have a normal circulating volume.
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