American Board of Internal Medicine (ABIM) Certification Practice Exam

What condition does an elevated serum calcium and phosphorus suggest after correcting 25-hydroxy vitamin D deficiency?

1. Decreased PTH secretion

2. Increased PTH secretion

3. Normal phosphate levels

4. Normal calcium levels

The correct answer is: Increased PTH secretion

An elevated serum calcium and phosphorus level, particularly after correcting for a deficiency in 25-hydroxy vitamin D, suggests an increased secretion of parathyroid hormone (PTH). When vitamin D levels are restored, the body is able to absorb calcium from the gut more effectively, which may lead to increased serum calcium levels. In response to high calcium levels, one would expect the body to decrease PTH secretion; however, if both calcium and phosphorus are elevated, it suggests an underlying condition that leads to increased PTH secretion despite the hypercalcemia. One important context is that in certain conditions like primary hyperparathyroidism or parathyroid hormone-related peptide (PTHrP) secretion, the levels of both calcium and phosphorus can be elevated due to increased PTH levels stimulating both bone resorption and renal phosphate reabsorption. This state could illustrate primary hyperparathyroidism, where the presence of elevated calcium and phosphorus can occur alongside increased PTH secretion. Therefore, the scenario outlined indicates that the appropriate response to the elevation in both serum calcium and phosphorus, following the correction of 25-hydroxy vitamin D deficiency, is increased secretion of PTH.