What is the most common pathologic cause of hypocalcemia?

The most common pathologic cause of hypocalcemia is hypoparathyroidism. This condition arises when there is insufficient secretion of parathyroid hormone (PTH) due to damage or removal of the parathyroid glands, leading to decreased calcium levels in the bloodstream. PTH plays a crucial role in regulating calcium homeostasis by promoting calcium release from bones, increasing renal tubular reabsorption of calcium, and enhancing intestinal absorption of calcium via its effects on vitamin D metabolism.

In contrast, hyperparathyroidism typically leads to hypercalcemia due to excess PTH causing increased bone resorption and renal calcium reabsorption. Hypomagnesemia can lead to hypocalcemia due to its impact on PTH secretion and function but is not as common as hypoparathyroidism. Vitamin D deficiency leads to decreased intestinal absorption of calcium, which can result in hypocalcemia as well, but this is often a secondary problem rather than a primary pathologic cause as seen in hypoparathyroidism.

Understanding the roles of hormones and nutrients in calcium regulation helps to clarify why hypoparathyroidism is a primary cause of hypocalcemia, distinguishing it from other conditions mentioned.