Calcium is involved in muscle contraction, bone health, nerve transmission and blood clotting. The levels of this important mineral are tightly controlled through the action of two major hormones, parathyroid hormone (PTH) and calcitonin.
The parathyroid glands are located on the posterior surface of the thyroid gland. Parathyroid hormone indirectly promotes the formation of the active form of vitamin D, which subsequently promotes the absorption of calcium from the intestines and the resorption of bone to release calcium into the extracellular fluid. PTH thereby raises the concentration of calcium in the extracellular fluid.
Calcitonin has a much smaller role in the regulation of calcium in the body and is produced by C cells of the thyroid gland. It has a larger role to play in calcium homeostasis in children, whose bones are remodelled at a much faster rate than adults. This hormone reduces the resorption of bone to reduce calcium concentration in the extracellular fluid.
To see how these hormones elicit their effects, please click the link below for a flowchart on calcium regulation.