Which of the following has an effect on plasma calcium levels?

• A. Sodium
• B. Inorganic phosphate
• C. Potassium
• D. Iron

Answer: (B)

Calcium in plasma is kept in balance not just by hormones but also by how much inorganic phosphate is present. When phosphate levels rise, more calcium binds to phosphate to form calcium phosphate, which lowers the fraction of free (ionized) calcium—the form that’s physiologically active. That drop triggers parathyroid hormone release, which works to restore calcium by pulling it from bone, increasing kidney reabsorption of calcium, and boosting vitamin D activation to raise intestinal calcium absorption. This interplay shows how phosphate directly influences calcium status.

In clinical contexts such as kidney disease, phosphate retention can drive lower ionized calcium and secondary hyperparathyroidism, underscoring the phosphate–calcium connection. Sodium, potassium, and iron don’t have the same immediate, direct effect on plasma calcium homeostasis, so inorganic phosphate is the factor that most clearly impacts calcium levels.