Calcium levels are maintained by the parathyroid hormone called parathormone. It increases calcium levels in the bloodstream. Another endocrine gland--the thyroid, secretes caclcitonin, which lowers calcium levels in the blood. PTH causes bone resorption and the release of stored calcium from inside the bones. Disorders occur when the body has too little or too much calcium. If the parathyroids are not functioning, or a person lacks vitamin D in the diet, this can lead to a lack of calcium absorption, and muscle cramps, numbness in the digits and irritability. If calcium levels are too high, over-active parathyroids, malgnancy and even too much vitamin D, can be the culprits. Symptoms include nausea, vomiting, anorexia, depression, confusion, aches and pains among others.