How does the parathyroid gland detect changes in blood calcium concentrations

Can you elaborate on the physiological mechanisms underlying the regulation of blood calcium levels and the roles of parathyroid hormone (PTH), calcitonin, and vitamin D in calcium homeostasis? How does the parathyroid gland detect changes in blood calcium concentrations and secrete PTH to stimulate calcium release from bones, enhance calcium reabsorption in the kidneys, and activate vitamin D synthesis in the kidneys and skin? Additionally, how does calcitonin secretion by the thyroid gland inhibit bone resorption and decrease blood calcium levels in response to hypercalcemia? Furthermore, how do dysregulations in calcium metabolism contribute to bone disorders like osteoporosis, hyperparathyroidism, and hypocalcemia?