Urinary Sodium Concentration
Normal urine sodium values for adults are generally 20 mEq/L in a random urine sample and 40 to 220 mEq/L per day (mEq/L/day).
The result depends on how much fluid and salt you take in. Normal value ranges may vary slightly among different laboratories. The lab at my current hospital has a range of 30-90 for a random urine sample.
In a Hyponatremic Patient, What is the Role of a Urine Sodium?
Urine sodium tells us how the kidneys are doing. It helps distinguish between renal and extra-renal causes. In extra-renal causes of hyponatremia, the kidneys are working well. In the context of a low [Na+] in serum, what would you expect working kidneys to do? You would expect working kidneys to reabsorb as much sodium as possible leaving little sodium in the urine. As such for extra-renal causes, urine [Na+] < 10.
Note that both hypovolemic hyponatremia and hypervolemic hyponatremia can be divided into renal and extra-renal causes.
Notice that extra-renal causes of hyponatremia can also be called pre-renal causes.
Further Reading
medlineplus.gov/ency/article/003599.htm