In vitamin D deficiency rickets, what happens to PTH levels as a compensatory response?

• A. Reduced
• B. Normal
• C. Variable
• D. Elevated

Answer: (D)

When vitamin D is deficient, calcium absorbed from the gut drops, leading to lower serum calcium. In response, the parathyroid glands increase secretion of parathyroid hormone to restore calcium levels. This rise in PTH is a compensatory mechanism known as secondary hyperparathyroidism. PTH acts to raise calcium by stimulating bone resorption, increasing renal calcium reabsorption, and enhancing activation of vitamin D (to boost intestinal calcium absorption). In the setting of ongoing vitamin D deficiency, however, the gut still isn’t able to absorb enough calcium, so PTH remains elevated. This elevated PTH helps explain the bone changes seen in rickets, where mineralization is impaired despite the body’s attempt to normalize calcium.