In malignancy-associated hypercalcemia due to PTHrP, what is the typical pattern of serum calcium, phosphate, PTH, and vitamin D?

In malignancy-associated hypercalcemia due to PTHrP, what is the typical pattern of serum calcium, phosphate, PTH, and vitamin D?

• A. Ca high, Phosphate normal, PTH low, Vitamin D normal
• B. Ca high, Phosphate high, PTH high, Vitamin D high
• C. Ca high, Phosphate low, PTH low, Vitamin D normal
• D. Ca low, Phosphate low, PTH normal, Vitamin D low

Answer: (C)

In PTHrP-driven malignancy, the tumor makes a peptide that mimics PTH’s actions on bone and kidney. This drives calcium up through increased bone resorption and enhanced renal calcium reabsorption. At the same time, the PTH-like effect in the kidney increases phosphate excretion, so serum phosphate falls. The rising calcium then suppresses the parathyroid glands, so endogenous PTH becomes low. Since this process does not involve increased production of active vitamin D, vitamin D levels remain normal. Thus the typical pattern is high calcium, low phosphate, low PTH, and normal vitamin D.

In PTHrP-driven malignancy, the tumor makes a peptide that mimics PTH’s actions on bone and kidney. This drives calcium up through increased bone resorption and enhanced renal calcium reabsorption. At the same time, the PTH-like effect in the kidney increases phosphate excretion, so serum phosphate falls. The rising calcium then suppresses the parathyroid glands, so endogenous PTH becomes low. Since this process does not involve increased production of active vitamin D, vitamin D levels remain normal. Thus the typical pattern is high calcium, low phosphate, low PTH, and normal vitamin D.