In pseudohypoparathyroidism type Ia, what laboratory pattern is expected?

• A. Hypercalcemia with hypophosphatemia and elevated PTH
• B. Hypocalcemia with hyperphosphatemia and elevated PTH
• C. Hypercalcemia with hyperphosphatemia and elevated PTH
• D. Hypocalcemia with normal phosphate and low PTH

Answer: (D)

End-organ resistance to parathyroid hormone is the key feature of pseudohypoparathyroidism type Ia. Because the kidneys and bones don’t respond to PTH, calcium cannot be adequately raised and phosphate cannot be excreted. The result is low calcium levels (hypocalcemia) and high phosphate levels (hyperphosphatemia). The parathyroid glands sense the dropping calcium and respond by increasing PTH secretion, so PTH is elevated even though calcium is low. This combination—hypocalcemia with hyperphosphatemia and high PTH—is the expected laboratory pattern. In contrast, true hypoparathyroidism would show low or inappropriately normal PTH with hypocalcemia.