In PTHrP-mediated hypercalcemia, what happens to PTH?

• A. Elevated PTH
• B. Suppressed (low) PTH.
• C. Normal PTH
• D. Variable PTH depending on tumor

Answer: (B)

PTHrP acts like PTH on bone and kidney, driving calcium release from bone and increased renal calcium reabsorption. This raises serum calcium without needing the native parathyroid hormone. In response to the resulting high calcium, the parathyroid glands reduce their own PTH secretion, so endogenous PTH becomes suppressed. This is why PTH is low in PTHrP-mediated hypercalcemia. Phosphate often mirrors the PTH effect, so it can be low or normal.