What is the most common cause of secondary hyperparathyroidism?

Secondary hyperparathyroidism occurs when the parathyroid glands produce excess parathyroid hormone (PTH) due to an underlying condition that leads to low calcium levels. The most prevalent cause of this condition is vitamin D deficiency. When vitamin D levels are insufficient, the body struggles to absorb calcium from the diet. This results in decreased serum calcium levels, prompting the parathyroid glands to compensate by increasing PTH secretion. The elevated PTH helps mobilize calcium from the bones and increase renal calcium reabsorption, but in cases of vitamin D deficiency, this compensatory mechanism often is not enough to restore normal calcium levels.

In contrast, primary hyperparathyroidism is characterized by autonomous overproduction of PTH, usually due to a benign adenoma, and does not lead to secondary hyperparathyroidism. Bone metastases can cause bone resorption and potentially lead to elevated calcium levels, but they do not typically lead to secondary hyperparathyroidism, rather they often entail a different pathophysiology. Thyroid disorders can affect calcium metabolism indirectly, but they do not directly cause the hormone alterations that lead to secondary hyperparathyroidism. Thus, vitamin D deficiency stands out as the principal cause.