What is the hallmark laboratory pattern of hypoparathyroidism?

• A. Hypercalcemia with high PTH.
• B. Hypocalcemia with high PTH and hypophosphatemia.
• C. Hypercalcemia with low PTH.
• D. Hypocalcemia with low PTH and hyperphosphatemia.

Answer: (D)

PTH keeps calcium high and phosphate low, so when PTH is deficient, calcium falls and phosphate rises. In hypoparathyroidism, the PTH level is low, leading to hypocalcemia because there’s less calcium reabsorption in the kidney’s distal tubule and less activation of vitamin D to boost gut calcium absorption. At the same time, without PTH to promote phosphate excretion, phosphate reabsorption in the proximal tubule increases, causing hyperphosphatemia. This combination—low calcium with low PTH and high phosphate—best fits hypoparathyroidism. It differs from states with high calcium or high PTH (such as primary hyperparathyroidism) and from those with low calcium plus high PTH (secondary hyperparathyroidism), which would show a different phosphate pattern.