A nurse is providing teaching to a client who has hyponatremia. Which of the following instructions should the nurse include?

Sodium is the primary extracellular cation and plays a central role in regulating fluid distribution, osmolarity, and blood pressure. Teaching a client with hyponatremia that sodium helps regulate blood pressure is accurate and clinically relevant. Sodium attracts water; when sodium levels are low, fluid shifts can cause hypotension and other hemodynamic changes, which is why sodium balance is closely monitored in clients with cardiovascular and renal conditions. Stating that sodium levels are controlled by the thyroid gland is incorrect — sodium is regulated primarily by aldosterone (secreted by the adrenal cortex), antidiuretic hormone (from the posterior pituitary), and atrial natriuretic peptide. The thyroid has no direct role in sodium homeostasis. Stating that sodium increases when magnesium increases is inaccurate; these two electrolytes do not have a direct proportional relationship. Magnesium and sodium are independently regulated through different hormonal and renal mechanisms. Stating that sodium controls blood clotting is incorrect — blood clotting is primarily regulated by calcium (essential for the coagulation cascade), platelets, and clotting factors. Sodium has no meaningful role in hemostasis. Therefore, the correct teaching point is sodium's role in blood pressure regulation.