Which lab finding suggests calcium deficiency?

• A. Increased calcium
• B. Elevated phosphate
• C. Decreased PTH
• D. Increased PTH

Answer: (D)

Calcium deficiency triggers a compensatory rise in parathyroid hormone. When serum calcium drops, the parathyroids secrete more PTH to bring calcium back up. PTH acts in three ways: it stimulates bone resorption to release calcium, increases kidney reabsorption of calcium, and raises active vitamin D production, which boosts intestinal calcium absorption. This coordinated response is why an elevated PTH level is the best lab clue that calcium is deficient—the body is actively trying to correct the low calcium.

Other findings don’t fit as neatly with calcium deficiency. Increased calcium would not indicate a deficiency. Decreased PTH points to hypoparathyroidism, which would worsen low calcium rather than indicate compensation. Elevated phosphate can occur in contexts where PTH isn’t effectively lowering phosphate (like hypoparathyroidism or certain kidney conditions) and isn’t the direct signal of a calcium deficiency.