Final answer:
Hypercalcemia can be caused by hypersecretion of parathyroid hormone, vitamin D deficiency, or reduced parafollicular cell activity, with the most common cause being the overproduction of PTH leading to increased blood calcium levels and associated health issues.
Step-by-step explanation:
Hypercalcemia can be caused by various conditions, one of which is hypersecretion of parathyroid hormone (PTH). Hyperparathyroidism occurs when the parathyroid glands produce too much PTH, leading to excessive calcium reabsorption from the bone and increased blood calcium levels. This heightened blood calcium level can result in structural weakness of the bones, potentially causing deformation and fractures, as well as decreased nervous system responsiveness. In addition, conditions such as vitamin D deficiency or reduced parafollicular cell activity (which produces calcitonin, a hormone that lowers blood calcium levels) may also lead to hypercalcemia, though these are less common causes.