In pseudohypoparathyroidism, which laboratory pattern is observed?

• A. Hypocalcemia, hyperphosphatemia, with elevated PTH
• B. Hypercalcemia, hypophosphatemia, with elevated PTH
• C. Hypocalcemia, hypophosphatemia, with low PTH
• D. Hypercalcemia, hyperphosphatemia, with low PTH

Answer: (A)

Pseudohypoparathyroidism shows end-organ resistance to parathyroid hormone, so the body can’t respond to PTH even though it may be produced in high amounts. PTH’s normal actions raise serum calcium and lower serum phosphate by increasing calcium reabsorption and decreasing phosphate reabsorption in the kidney, stimulating bone resorption, and promoting activation of vitamin D. When target tissues resist PTH, calcium remains low and phosphate stays high, and the parathyroid glands respond by secreting more PTH. The result is a pattern of low calcium, high phosphate, with elevated PTH levels, which is why this option best fits pseudohypoparathyroidism.

In contrast, commonly seen patterns in other conditions help distinguish them: primary hyperparathyroidism typically has high calcium with low phosphate and high PTH; hypoparathyroidism has low calcium with high phosphate and low PTH; PTH-independent hypercalcemia shows high calcium with low PTH.