Differential diagnoses were systematically considered and ultimately excluded: 1) Secondary adrenal insufficiency (SAI): Elevated ACTH levels and a normal pituitary CT effectively ruled out SAI (28); 2) Chronic kidney disease (CKD)-related electrolyte disturbances: Normal serum creatinine, eGFR, and absence of proteinuria excluded CKD as a cause of hyponatremia (29); 3) Myxedema: Though the patient had a history of primary hypothyroidism, stable TSH levels under adequate levothyroxine replacement therapy excluded thyroid-related etiology. The gene discussed is POMC; the disease is Hyponatremia.