What is the PTH challenge response in PPHP?

• A. Urinary cAMP does not change
• B. Urinary cAMP decreases
• C. Urinary cAMP increases > 300 mmol/L, as in normal patients
• D. Urinary cAMP increases but only modestly

Answer: (C)

PTH challenge testing looks at how the kidney’s tubules respond to parathyroid hormone by measuring urinary cAMP production. In PPHP, the renal tissues are not resistant to PTH, so the PTH signal activates Gs and adenylate cyclase, driving a robust rise in urinary cAMP—similar to normal individuals. That means after the PTH challenge you see a marked increase in urinary cAMP, reflecting intact PTH receptor signaling in the kidney. If there were resistance to PTH, you’d expect little to no rise (or only a very small one), which is what happens in classic pseudohypoparathyroidism. So a strong, normal-like urinary cAMP increase after PTH confirms PPHP rather than a PTH-resistant form.