A 15-year-old girl ingested vitamin D in a suicide attempt. One month later, calcium is mildly elevated. What is the mechanism for the increased calcium?

• A. Increased bone resorption
• B. Decreased renal excretion
• C. Increased intestinal calcium absorption
• D. Hypoparathyroidism

Answer: (C)

Vitamin D toxicity boosts calcium absorption from the gut. The active form of vitamin D increases the intestinal expression of calcium-binding proteins and transport mechanisms in enterocytes, which enhances transcellular calcium uptake into the bloodstream. As more calcium enters circulation, serum calcium rises and PTH secretion is suppressed in response. This gut absorption effect is the primary reason for the mild hypercalcemia seen after vitamin D overdose, rather than changes in bone resorption or renal excretion.