This episode reviews hyponatremia through a physiology-first lens, emphasizing that it is primarily a disorder of water balance rather than sodium itself. We discuss how urine sodium helps differentiate hypovolemic, hypervolemic, and euvolemic hyponatremia and why it often provides a more reliable assessment of effective volume status than the physical exam. The episode then focuses on SIADH, explaining how persistent ADH activity limits free water excretion, why salt tablets can worsen hyponatremia by increasing water intake, and how urea offers a more effective osmotic strategy. We also compare the roles and risks of vaptans versus hypertonic saline in acute, symptomatic cases and review the mechanism of “desalination,” where normal saline worsens sodium levels in unrecognized SIADH. The goal is to provide a clear, practical framework for evaluating and managing hyponatremia at the bedside.