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 resorptionThe re-uptake of substances from tissues, in particular bone, into the blood. It should not be confused with absorption, the initial uptake of substances into the body, for instance from food in the intestines, through the lungs and via the skin. Reabsorption, in contrast, occurs.... PTH also acts directly on the kidney: it inhibits reabsorption of phosphate from primary urineThe initial urine entering the nephron due to filtration of the blood at renal corpuscles. It is not yet concentrated and does not contain any proteins.... while increasing the reabsorption of calcium.
Hyperparathyroidism is an increase in blood parathyroid hormone levels.