Parathyroid hormone

Parathyroid hormone (PTH) is a hormone secreted by the parathyroid glands, which regulates calcium levels in the blood. PTH is secreted in response to low blood calcium levels (hypocalcaemia). An increase in calcium concentration above the normal value inhibits PTH production (negative feedback). PTH stimulates mobilisation of calcium and phosphate from bone tissue, resulting in bone resorption. PTH also acts directly on the kidney: it inhibits reabsorption of phosphate from primary urine while increasing the reabsorption of calcium.
Hyperparathyroidism is an increase in blood parathyroid hormone levels.