What is the primary cause of hyperparathyroidism in most cases?

The primary cause of hyperparathyroidism in most cases is an adenoma of the parathyroid gland. This benign tumor leads to the overproduction of parathyroid hormone (PTH), which results in increased calcium levels in the blood. This form of hyperparathyroidism, known as primary hyperparathyroidism, is the most common cause encountered and accounts for approximately 85-90% of cases. The mechanism involves the adenoma releasing excessive PTH, which in turn causes the bones to release calcium, the kidneys to reabsorb more calcium, and the intestines to absorb calcium from dietary sources, leading to hypercalcemia.

Other options such as excessive vitamin D intake can lead to hypercalcemia but are not the primary cause of hyperparathyroidism itself. Genetic predisposition contributes to conditions like multiple endocrine neoplasia, which can cause hyperparathyroidism, but these are less common causes compared to adenomas. Autoimmune disorders can affect parathyroid function but do not typically lead to hyperparathyroidism as the primary mechanism. Each of these alternatives has distinct mechanisms that do not rival the prevalence of adenomas in causing hyperparathyroidism.