Almost all the calcium in your body (more than 99 percent) is stored in your bones. Although the calcium circulating in your body comprises less than 1 percent of your total body calcium, maintaining proper levels is vital for bone health and normal muscle and nerve function. Three major hormones regulate blood calcium levels: parathyroid hormone, vitamin D, and calcitonin.
Your blood calcium levels are influenced by the regulatory processes that occur in your bones, intestines and kidneys. The bones undergo constant remodeling. They are broken down by cells called osteoclasts, which release calcium into the bloodstream. The calcium in blood can be taken by other bone cells, called osteoblasts, which use the mineral to produce new bones. Blood calcium levels are also influenced by how much dietary calcium your small intestine absorbs and how much of the mineral your kidneys excrete.
Located on the back of the thyroid gland in your throat, your parathyroid gland consists of four nodules the size of a pea. When calcium levels in the blood fall, the parathyroid gland secretes parathyroid hormone. This hormone works by raising calcium levels in the blood, stimulating the esteoclasts to break bones and release calcium into the bloodstream.
PTH also tells your kidneys to keep calcium by reducing the amount excreted in the urine. The hormone also stimulates the renal production of the active form of vitamin D, which activates the increased calcium absorption of the intestine. Low levels of PTH (due to autoimmune disease or another cause) are usually caused by a tumor in the parathyroid, causing excessively high levels of calcium in the blood.
Vitamin D acts as a hormone in your body to help regulate calcium in the blood. The steroid hormone is so vital in maintaining the calcium balance that its active form is sometimes known as calcitriol. Vitamin D is necessary for sufficient absorption of dietary calcium in your small intestine. A deficiency in vitamin D can cause a low level of calcium in the blood, due to a poor absorption of calcium. This can lead to weakening of the bones, as seen with the conditions of vitamin D deficiency, rickets and osteomalacia. Vitamin D is produced in your skin with sun exposure and in some foods or supplements.