In primary hyperparathyroidism, which calcium-phosphate pattern best fits the laboratory findings?

• A. Elevated ionized calcium with decreased inorganic phosphate
• B. Elevated ionized calcium with increased inorganic phosphate
• C. Normal calcium with normal phosphate
• D. Decreased ionized calcium with decreased phosphate

Answer: (A)

Elevated ionized calcium with decreased inorganic phosphate is the hallmark pattern in primary hyperparathyroidism. Excess parathyroid hormone (PTH) raises calcium in three ways: it stimulates bone resorption, increases renal calcium reabsorption, and enhances intestinal absorption of calcium via active vitamin D. At the same time, PTH promotes phosphate wasting in the kidneys, so serum inorganic phosphate falls. Because the ionized portion of calcium is the active form, it is this fraction that is typically elevated, and phosphate is lowered, producing the characteristic pattern.

Other patterns don’t fit this physiology. If calcium is high but phosphate is also high or normal, that wouldn’t reflect PTH’s phosphaturic effect. Normal calcium and phosphate would not indicate hyperparathyroidism. A pattern with low calcium (with or without low phosphate) would not reflect the excess PTH activity seen in primary hyperparathyroidism.