Hyponatremia is the most common disorder of body fluid and electrolyte balance encountered in clinical practice. It can lead to a broad spectrum of clinical symptoms and is associated with increased mortality, morbidity, and length of hospital stay. A 76-year-old female patient was referred to the nephrology department for kidney dysfunction and hyponatremia. She has an external biliary drainage catheter due to an inoperable Klatskin tumor. Initially, she was treated with isotonic fluid because of vomiting-related hypovolemic hyponatremia. Kidney dysfunction had recovered, but targeting sodium levels were not achieved and remained below 125 mM. Controlled urine sodium level remained below 20 mM. Thus, we considered a sodium loss to third spaces and the biliary drainage was the first presumption. Daily sodium wasting was calculated as 164 mmol/24 h (3.8 g sodium) via biliary drainage fluid. So, we added biliary sodium decrement count to the patient’s diet as oral salt supplementation; 10 g salt (sodium chloride) per day was equal to daily sodium wasting. The follow-up sodium levels remained around 135 mM, and the patient is asymptomatic since discharge.