Parathyroid hormone and calcitonin are two hormones in the body with opposing effects. Calcium levels in the blood are increased by parathyroid hormone, while calcitonin decreases calcium in the blood. Taken together, parathyroid hormone and calcitonin help regulate calcium homeostasis.
Produced in the parathyroid glands, parathyroid hormone increases blood calcium concentrations in three ways. It promotes the action of osteoclasts, cells that reabsorb bone, thus freeing up calcium. Parathyroid hormone indirectly promotes calcium absorption by the small intestine, where it is released into the blood. Ultimately, this hormone prevents the loss of calcium through urine.
Made in the thyroid gland, Calcitonin decreases calcium concentrations in the blood through two mechanisms. It decreases the activity of osteoclasts, so that calcium remains in the bone. Additionally, it prevents kidney tubules from reabsorbing calcium, so that it leaves the body through the urine. Essentially, parathyroid hormone and calcitonin accomplish their tasks through complementary means.
Different blood concentrations of calcium trigger the secretion of parathyroid hormone and calcitonin. When calcium blood levels fall below a 1 millimolar concentration, receptors on the parathyroid gland are activated. In response, this gland produces higher levels of parathyroid hormone. High levels of blood calcium stimulate calcitonin secretion. Low levels of each hormone continue to be produced even when calcium levels do not favor their secretion.
Proper calcium blood levels are essential to healthy functioning. Excesses or deficiencies in parathyroid hormone and can therefore cause disease. Tumors of the parathyroid gland that increase parathyroid hormone levels lead to decalcified bones and kidney stones. If the kidneys cannot absorb calcium well, parathyroid hormone is produced continuously, causing weak, decalcified bones that break easily. Insufficient levels of parathyroid hormone can cause convulsions that can be lethal.
Although calcitonin levels have been associated with certain diseases, there is not enough evidence to say that calcitonin levels cause these disease states. In fact, thyroid tumors that lead to calcitonin excess do not cause lower blood calcium concentrations. This may be because humans seem to have other methods of decreasing blood calcium. In other animals, however, calcitonin plays a much more important role.
Parathyroid hormone and calcitonin both can serve therapeutic purposes. Pulse injections of parathyroid hormone can paradoxically increase bone mass. Continuous administration of parathyroid hormone would lead to decreased bone mass, however. Calcitonin can be used to treat excess levels of blood calcium, as well as osteoporosis, to some extent.