Which anatomical structure is primarily responsible for increased calcium levels in the blood during hyperparathyroidism?

The parathyroid gland is primarily responsible for regulating calcium levels in the blood through the secretion of parathyroid hormone (PTH). In cases of hyperparathyroidism, the parathyroid glands become overactive and produce excessive PTH. This hormone plays a critical role in increasing calcium levels in the bloodstream by stimulating several processes.

Firstly, PTH increases the release of calcium from bones into the bloodstream by activating osteoclasts, the cells responsible for bone resorption. Secondly, it promotes the reabsorption of calcium in the kidneys, reducing its excretion in urine. Additionally, PTH enhances the conversion of vitamin D into its active form (calcitriol) in the kidneys, which increases intestinal absorption of calcium from food.

Due to these actions, elevated PTH levels in hyperparathyroidism lead to increased blood calcium levels, often resulting in associated symptoms and complications. This foundational understanding of the role of the parathyroid gland highlights why it is the correct answer in this context.