We report a 1.5-year-old boy with refractory hyponatremia related to pituitary stalk transection, which is a rare cause of hyponatremia after traumatic brain injury. The patient was referred to our hospital with a hyponatremic convulsion 6 days after head trauma. The patient's laboratory findings were compatible with syndrome of inappropriate antidiuretic hormone secretion (SIADH). The hyponatremic convulsion was treated with a hypertonic saline infusion and the SIADH was treated with fluid restriction, but serum levels of sodium did not increase. Acute post-traumatic hypopituitarism was diagnosed based on basal pituitary function tests and imaging. Hypophysis magnetic resonance imaging showed pituitary stalk transection. He was diagnosed with post-traumatic hypopituitarism due to pituitary stalk transection and given L-thyroxine and hydrocortisone. After the treatment, sodium and thyroid hormone levels returned to normal. Acute post-traumatic hypopituitarism is a potentially important cause of hyponatremia after traumatic brain injury, and can be mis-diagnosed as SIADH. Copyright (C) 2016, Taiwan Society of Emergency Medicine. Published by Elsevier Taiwan LLC.