Disorders involving excessive or deficient blood levels of parathyroid hormone caused by abnormal functioning of the parathyroid gland. Parathyroid hormone is responsible for keeping the concentration of calcium in blood within a narrow normal range. If the blood calcium concentration falls, the parathyroid glands respond by secreting hormone which tends to increase the concentration of calcium. Conversely, an increase in blood calcium concentration above the normal range normally suppresses parathyroid hormone secretion. Parathyroid hormone acts directly on bone and kidney, and indirectly on the intestine, to increase the concentration of calcium in blood. It also acts on the kidney to increase excretion of phosphate in the urine, causing a lowering of the concentration of phosphorus in blood.