In pseudohypoparathyroidism Type 1B, the response to a PTH challenge is characterized by which of the following?

• A. Increase in urinary cAMP
• B. Absence of normal increase in urinary cAMP
• C. Decrease in urinary cAMP
• D. No change in urinary cAMP

Answer: (B)

PTH works in the kidney by binding its receptor and activating Gs protein, which stimulates adenylyl cyclase to raise cAMP in tubular cells and drive processes like phosphate excretion. In pseudohypoparathyroidism Type 1B, the kidney is resistant to PTH because the signaling pathway is defective (often due to GNAS imprinting issues). When a PTH challenge is given, the renal tubules cannot generate the normal cAMP response, so there is no rise in urinary cAMP. This absence of the expected cAMP increase is the hallmark of Type 1B PTH resistance. In a person with normal PTH signaling, the challenge would produce a clear increase in urinary cAMP. A pattern showing decrease or no change beyond the anticipated lack of rise isn’t typical for this condition and would point away from the standard PTH resistance phenotype.