When blood calcium levels get low (hypocalcemia), the bones release calcium to bring it back to a good blood level. When blood calcium levels get high (hypercalcemia), the extra calcium is stored in the bones or passed out of the body in urine and stool. The amount of calcium in the body depends on the amount of:
http://www.webmd.com/a-to-z-guides/calcium-ca-in-blood
If the blood calcium level is too low, the parathyroid glands release more PTH. This causes the bones to release more calcium into the blood and reduces the amount of calcium released by the kidneys into the urine.
http://www.webmd.com/a-to-z-guides/parathyroid-hormone
PTH levels that are too high or too low can cause problems with the kidneys and bones and cause changes in calcium and vitamin D levels. Tests for calcium and phosphorus levels in the blood may be done at the same time as a PTH test.
http://www.webmd.com/a-to-z-guides/parathyroid-hormone
Certain nutrients work together with calcium to keep the levels consistent. Too much or too little of those nutrients might affect how well your body absorbs calcium. Low levels of magnesium, vitamin D or albumin can affect your calcium levels. The same is true with high levels of phosphorus. Calcium also needs vitamin D to be absorbed properly. If you don’t spend enough time in the sun, you might be vitamin D-deficient and experience low calcium levels as a result.
http://www.livestrong.com/article/470800-why-is-my-calcium-level-low/
Low calcium levels, or hypocalcaemia, can happen for a number of reasons. Although some people might experience no symptoms, low calcium levels might cause muscle spasms, numbness in the extremities, depression and seizures. Low levels of calcium may eventually lead to osteoporosis.
http://www.livestrong.com/article/470800-why-is-my-calcium-level-low/
1 To see if your symptoms may be caused by a very low calcium level in the blood. 2  Such symptoms may include muscle cramps, spasms, and twitching and tingling in the fingers and around the mouth. 3  To see if your symptoms may be caused by a very high calcium level in the blood.
http://www.webmd.com/a-to-z-guides/calcium-ca-in-blood
Guide. A parathyroid hormone (PTH) blood test measures the level of parathyroid hormone in the blood. This test is used to help identify hyperparathyroidism, to find the cause of abnormal calcium levels, or to check the status of chronic kidney disease. PTH controls calcium and phosphorus levels in the blood.
http://www.webmd.com/a-to-z-guides/parathyroid-hormone
Diet. Probably the most common reason for low calcium is a diet lacking in it. If you don’t consume enough calcium regularly, your body will eventually start taking calcium away from your bones to make up for the difference. Over time, this can result in a deficiency.
http://www.livestrong.com/article/470800-why-is-my-calcium-level-low/
Vitamin D also is involved in regulating the amount of calcium in your blood. Normally, this balancing act works well. When calcium levels in your blood fall too low, your parathyroid glands secrete enough PTH to restore the balance. PTH raises calcium levels by releasing calcium from your bones and increasing the amount of calcium absorbed from your small intestine. When blood-calcium levels are too high, the parathyroid glands produce less PTH. But sometimes one or more of these glands produce too much hormone, leading to abnormally high levels of calcium (hypercalcemia) and low levels of phosphorus in your blood.
http://www.mayoclinic.org/diseases-conditions/hyperparathyroidism/basics/causes/CON-20022086
Severe vitamin D deficiency. Vitamin D helps maintain appropriate levels of calcium in the blood, and it helps your digestive system absorb calcium from your food. Your body produces vitamin D when your skin is exposed to sunlight, and you consume some vitamin D in food.
http://www.mayoclinic.org/diseases-conditions/hyperparathyroidism/basics/causes/CON-20022086
causes of low calcium levels
The low calcium levels are caused by Muscle spasms, numbness in the extremities, depression and seizures. The causes of low calcium levels are muscle spasms, numbness in the extremities, depression and seizures.