Parathyroid hormone (PTH) is secreted by the four tiny parathyroid glands embedded in the thyroid, and it is the master regulator of calcium and phosphate balance. When blood calcium falls, PTH raises it by three coordinated actions: stimulating bone resorption to release calcium, increasing calcium reabsorption in the kidneys while dumping phosphate, and boosting activation of vitamin D, which then increases calcium absorption from the gut. This calcium-vitamin D-PTH axis is the core of bone nutrition: adequate calcium and vitamin D intakes keep PTH in its healthy range, while deficiency drives secondary hyperparathyroidism that slowly demineralises bone. Disorders at either extreme, hypoparathyroidism causing dangerous low calcium or hyperparathyroidism from tumours, illustrate how tightly the body guards blood calcium at the skeleton’s expense.