Which condition is primarily associated with PTH-dependent hypercalcemia?

• A. Malignancy
• B. Primary hyperparathyroidism
• C. Vitamin D toxicity
• D. Granulomatous disease

Answer: (B)

Primary hyperparathyroidism is primarily associated with PTH-dependent hypercalcemia because it involves the overproduction of parathyroid hormone (PTH) from the parathyroid glands. This excessive PTH stimulates the release of calcium from bones, increases renal tubular reabsorption of calcium, and enhances intestinal absorption of calcium through its effect on vitamin D metabolism. As a result, the serum calcium levels rise due to the direct action of increased PTH.

In contrast, conditions such as malignancy often lead to hypercalcemia through mechanisms that do not primarily involve PTH secretion, but instead result from bone metastasis or paraneoplastic syndromes that release substances mimicking PTH. Vitamin D toxicity elevates calcium levels by enhancing intestinal absorption of calcium and mobilizing bone calcium, but this mechanism is independent of PTH. Granulomatous disease, such as sarcoidosis, can lead to hypercalcemia due to ectopic production of calcitriol (the active form of vitamin D) without involving PTH directly. Thus, primary hyperparathyroidism stands out as the primary condition characterized by hypercalcemia that is directly driven by elevated levels of PTH.