In fish, birds and other non-mammalian animals, calcitonin is produced by cells of the ultimobranchial body. Calcitonin opposes the effects of parathyroid hormone (PTH), which acts to increase the blood calcium level. Calcitonin lowers blood calcium levels by suppressing osteoclast activity in the bones and increasing the amount of calcium excreted in the urine. The hormone also prevents the absorption of calcium from the intestine, as well as increasing the calcium absorbed by the kidney. However, the production and increased activity of calcitonin is only short-lived and many patients with chronic hypercalcemia (raised blood calcium) do not have high blood calcitonin levels.