Living kidney donation is safe and established but can lead to short- and long-term complications. Hyponatremia is the most common disorder of body fluid and electrolyte balance in clinical practice, associated with increased morbidity, mortality, and the length of hospital stay. A correct diagnosis of the etiology of hyponatremia is critical, both to determine correct management and prognosis. Here, we present a case of a severe hyponatremia following left- sided donor nephrectomy with a physical examination suggestive of mild hypovolemia. Laboratory tests revealed high urine osmolality and sodium concentration mimicking syndrome of inappropriate antidiuretic hormone secretion (SIADH), in the setting of abnormally blunted response to Synacthen. The patient responded well to hydrocortisone replacement. Differentiating between primary adrenal insufficiency and SIADH as a cause of severe hyponatremia was the key to successfully treating this patient. Hyponatremia following donor nephrectomy is unusual and could be explained in this case by hypocortisolism.